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Relative light intensity and electric field intensity of cloud to ground lightning

Material Information

Title:
Relative light intensity and electric field intensity of cloud to ground lightning
Creator:
Jordan, Douglas Max, 1949-
Publication Date:
Language:
English
Physical Description:
xx, 231 leaves : ill. ; 29 cm.

Subjects

Subjects / Keywords:
Antennas ( jstor )
Calibration ( jstor )
Cameras ( jstor )
Density ( jstor )
Digitized images ( jstor )
Electric fields ( jstor )
Lightning ( jstor )
Luminous intensity ( jstor )
Recording instruments ( jstor )
Signals ( jstor )
Dissertations, Academic -- Electrical Engineering -- UF ( lcsh )
Electric fields -- Measurement ( lcsh )
Electrical Engineering thesis Ph. D ( lcsh )
Light -- Measurement ( lcsh )
Lightning -- Measurement ( lcsh )
City of Tampa ( local )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1990.
Bibliography:
Includes bibliographical references (leaves 225-229).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Douglas Max Jordan.

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Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright Douglas Max Jordan. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Resource Identifier:
023644564 ( ALEPH )
23110415 ( OCLC )

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RELATIVE LIGHT INTENSITY AND
ELECTRIC FIELD INTENSITY
OF CLOUD TO GROUND LIGHTNING















By

DOUGLAS MAX JORDAN


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1990
















ACKNOWLEDGEMENTS

I wish to begin by stating that without the support of my wife Beth I would have never completed this effort. Her understanding and compassion have been a comfort and an inspiration. I am grateful to Ross, Clifford, Julia, and Kim for their sacrifices in support of my work.

Dr. Martin Uman is the type of person one meets all too infrequently in life. Martin saw potential in me which I often did not see myself. It has been an honor to be guided and befriended by a man of such character and intellectual abilities. Most of all, Martin taught me how to think as a scientist and instilled in me the desire to discover.

I would like to thank Dr. William Beasley, who has been a friend and teacher for many years and was always there to remind me that it was worth the effort. Bill's scientific guidance and sense of humor were crucial throughout the experiments and analyses which culminated in this thesis.

I would like to express my appreciation to Dr. Vladimir Rakov, my Russian friend, for sharing with me his exceptional insight. His friendship and stimulating conversation were a pleasure. His assistance in motivating me to complete this effort has been invaluable.








I also would like to thank Dr. Richard Orville and Dr. Vince Idone for loaning me their data for analysis. Mr. David Peckham's suggestions were very helpful while I was repairing the optical equipment and his free consulting saved many hours of work. I owe a special debt of gratitude to Dr. James Davidson and the Institute of Food and Agricultural Sciences for loaning me much of the equipment which was used to analyze the data presented in this thesis.


iii
















TABLE OF CONTENTS


Page
ACKNOWLEDGEMENTS.................................. ii

LIST OF FIGURES............................... Vi

LIST OF TABLES................................ xvii

ABSTRACT........................ ........ . xviii

CHAPTER

1. REVIEW ..................................... 1

1.1 Introduction.......................... 1
1.2 Lightning................ ............ 1
1.2.1 Stepped Leader.................. 2
1.2.2 First Return Stroke............ 4
1.2.3 Dart Leader................... 5
1.2.4 Subsequent Return Stroke....... 5 1.2.5 M-component.................... 6
1.3 Photographic Measurements............. 6
1.4 Correlated Measurements of Electric
Fields and Relative Light Intensity. 16

2. UF AND SUNYA EXPERIMENTS AND GENERAL
ANALYSIS TECHNIQUES......................... 22

2.1 Experimental Techniques................. 22
2.1.1 Photographic Techniques........ 22
2.1.2 Electric Field
Measurement Techniques....... 25 2.2 Experiments............................ 29
2.2.1 Tampa Experiment................ 29
2.2.2 Gainesville Experiment......... 35
2.2.3 Data from Orville & Idone...... 41
2.3 Photographic analysis................. 42
2.3.1 Optical System Calibration..... 42 2.3.2 Film Digitization.............. 50
2.3.3 Image Display Techniques....... 53 2.3.4 Computer Analysis Software..... 54
2.3.5 Measurement of Relative Light
Intensity Versus Time........ 55
2.3.6 Measurement of Relative Light
Intensity Versus Height...... 55









2.3.7 Measurement of Leader Speed.... 57
2.4 Electric Field Analysis............... 57
2.4.1 Electric Field System
Calibration.................. 57
2.4.2 Electric Field
Digitization................. 58
2.4.3 Electric Field Analysis
Software...................... 60
2.4.4 Measurement of
Interstroke Time Intervals... 62
2.4.5 Measurement of Return Stroke
Initial Peak Electric Field.. 62
2.4.6 Measurement of Leader
Field Change Duration........ 62
2.4.7 Measurement of Ratio of Leader Field Change to Return Stroke Field Change................. 63

3. DETAILED ANALYSES AND RESULTS............... 70

3.1 Database............................... 70
3.2 First Strokes.......................... 81
3.2.1 Channel Relative Light
Intensity Variations......... 81
3.2.2 Channel Relative Light
Intensity Variations Due to Branches...................... 86
3.3 Subsequent Strokes. .................... 115
3.3.1 Leaders of (Dart leaders)...... 116 3.3.2 Return Strokes................. 143
3.4 M-components........................... 149
3.4.1 M-component Relative Light
Intensity Profiles........... 150
3.4.2 Determination of the Direction
of M-component Propagation... 151

4. DISCUSSION AND CONCLUSIONS.................. 217

REFERENCES ................... ..................... 225

BIOGRAPHICAL SKETCH................................. 230











LIST OF FIGURES


Figure g

1.1 Schematic diagram of a lightning ground
discharge below cloud. (a) Streak schematic
showing, stepped leader, dart leaders, and
return strokes. (b) Still photo schematic of
entire flash. (Adapted from M.A. Uman,
Lightning, McGraw Hill Book Co., 1969)....... 3

1.2 Diagram of improved Boys camera with moving
film and stationary lenses. Adapted from
McEachron (1939)................................ 9

2.1 Photograph of the two research vehicles at
the Tampa, Florida site. The large trailer on
the left housed the electric field recording
equipment while the small truck housed the
streak camera experiment..................... 32

2.2 Beckman and Whitley streak camera. The upper
photo shows the still camera and the twin
lenses of the streak camera. The white circle on the right contained the photodetector. The
lower photo shows the data logging display
and associated electronics which were mounted
on the mobile cart with the camera........... 34

2.3 Diagram of the electric field recording
system used in 1979. Two electric field
antennas were connected to integrators having
slow decay (5 sec) with the other having a fast decay (1 msec). Crossed-loop magnetic
field antennas were connected to fast decay
integrators. A capacitive voltage divider (V)
was used to measure the voltage on a
deenergized power line. Electric field signals
were digitized by Biomation recorders and
displayed on dual-beam oscilloscopes. Electric
fields with various gains and analog time
code signals were recorded on an Ampex FR1900 instrumentation recorder. Thunder, time code and slow decay electric fields were recorded
on Hewlett Packard strip chart recorders..... 37

2.4 Diagram of the electric field and light
recording system used in 1982. The
instrumentation recorder (ITR) was an Ampex
FR3010. The antennas were connected to
integrators with slow (5 sec) and fast (1








msec) decay constants. The photomultiplier
tube (PMT) was followed by two stages of gain
(GI, G2). Time code was IRIG-B analog signal
which was recorded on the ITR and a slow code
digital signal which was recorded on the strip chart. Thunder was recorded on the
strip chart along with slow decay electric
fields........................................ 40

2.5 Film calibration curve produced by Idone and
Orville in 1984 showing relative light
intensity versus film diffuse density and
specular density of the same region as
measured by the microdensitometer............ 49

2.6 Overall electric field change for the 27 July
1979, 2206:51 UT flash which had 16 strokes
with 9 recorded on the streak camera ......... 66

2.7 Overall electric field change for strokes 2
and 3 of the flash on 27 July 1979, 2206:51
UT showing the measurement of the interstroke
interval...................................... 67

2.8 Electric fields preceding stroke 6 of the 27
July 1979, 2206:51 UT flash showing the
measurement of dart leader duration.......... 68

2.9 Electric fields for stroke 6 of the 27 July
1979, 2206:51 UT flash showing the
measurement of initial peak electric field... 69

3.1 Histograms of (a) dart leader field change
duration, (b) initial peak electric field,
(c) previous interstroke interval,
and (d) ratio of leader to return stroke
electric field change versus stroke order for
the 27 July 1979, 2206:51 UT flash. Crosshatched strokes had detectable dart leaders.
Dashed lines indicate ambiguous measurements.
Parallel lines indicate measurements of first
stroke parameters which are off scale........ 75

3.2 Histograms of (a) dart leader field change
duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric
field change versus stroke order for the 27 July 1979, 2246:45 UT flash. Cross-hatched
strokes had detectable dart leaders. The
ratio of leader-return stroke electric field
change was not measurable for strokes 2 and
4. Dashed lines indicate ambiguous
measurements. Parallel lines indicate


vii








measurements of first stroke parameters which
are off scale ................................ 76

3.3 Histograms of (a) dart leader field change
duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric
field change versus stroke order for
the 21 July 1982, 1317:14 EDT flash. Crosshatched strokes had detectable dart leaders.
Leader electric field properties for stroke
6 were not measurable. Dashed lines indicate
ambiguous measurements. Parallel lines
indicate measurements of first stroke
parameters which are off scale............... 78

3.4 Histograms of (a) dart leader field change
duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric
field change versus stroke order for
the 10 August 1982, 1445:35 EDT flash. Crosshatched strokes had detectable dart leaders.
The leader-return stroke electric field ratio
was not measurable for this flash. Dashed
lines indicate ambiguous measurements.
Parallel lines indicate measurements of first
stroke parameters which are off scale........ 79

3.5 Histograms of (a) dart leader field change
duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric
field change versus stroke order for the
10 August 1982, 1446:56 EDT flash. Crosshatched strokes had detectable dart leaders.
Dashed lines indicate ambiguous measurements.
Parallel lines indicate measurements of first
stroke parameters which are off scale........ 80

3.6 Digitized image of first stroke at 1446:56
EDT August 10, 1982. The heights indicated
are referenced in the plots of relative light
intensity versus time. The leader electric
field durations and ratios were not
measurable for this flash..................... 88

3.7 Relative light intensity versus time at for
the stroke at 1446:56 EDT August 10, 1982.... 89

3.8 Relative light intensity versus time for the
lower 500 meters of the stroke at 1446:56 EDT
August 10, 1982............................... 90


viii








3.9 Digitized image of first stroke at 1926:45 UT
July 11, 1978. The heights indicated are
referenced in the plots of relative light
intensity versus time......................... 92

3.10 Relative light intensity versus time for the
stroke at 1926:45 UT July 11, 1978........... 93

3.11 Relative light intensity versus time for the
lower 500 m of the stroke at 1926:45 UT July
11, 1978 ..................................... 94

3.12 Digitized image of first stroke at 2041:00 UT
July 29, 1978. The heights indicated are
referenced in the plots of relative light
intensity versus time........................ 96

3.13 Relative light intensity versus time for the
stroke at 2041:00 UT July 29, 1978........... 97

3.14 Relative light intensity versus time for the
lower 500 m of the stroke at 2041:00 UT July
29, 1978...................................... 98

3.15 Digitized image of first stroke at 2023:46 UT
July 29, 1978. The heights indicated are
referenced in the plots of relative light
intensity versus time......................... 100

3.16 Relative light intensity versus time for the
stroke at 2023:46 UT July 29, 1978........... 101

3.17 Relative light intensity versus time for the
lower 500 m of the stroke at 2023:46 UT July
29, 1978. ..................................... 102

3.18 Digitized image of first stroke at 2032:10 UT
July 29, 1978. The heights indicated are
referenced in the plots of relative light
intensity versus time....................... 104

3.19 Relative light intensity versus time for the
stroke at 2032:10 UT July 29, 1978........... 105

3.20 Digitized image of first stroke at 2032:51 UT
July 29, 1978. The heights indicated are
referenced in the plots of relative light
intensity versus time........................ 107

3.21 Relative light intensity versus time for the
stroke at 2032:51 UT July 29, 1978............ 108

3.22 Digitized image of first stroke at 1749:02 UT
May 27, 1984. The heights indicated are








referenced in the plots of relative light
intensity versus time. ........................ 110

3.23 Relative light intensity versus time for the
stroke at 1749:02 UT May 27, 1984............ 111

3.24 Digitized image of first stroke at 2107:20.81
UT June 9, 1984. The heights indicated are
referenced in the plots of relative light
intensity versus time........................ 113

3.25 Relative light intensity versus time for the
stroke at 2107:20.81 UT June 9, 1984. ........ 114

3.26 Dart leader speed versus stroke order. Solid
lines connect strokes which follow in order
with detectable leaders. Dotted lines connect
strokes of a flash which were separated by
strokes with nondetectable leaders. Schonland
data point is the mean for 55 leaders........ 117

3.27 Apparent leader length versus stroke order.
Apparent lengths of Krehbiel et al. (1979)
were determined from charge source locations for each stroke. Dotted lines bound the data
for 163 strokes reported by Brook et al.
(1962) .................................. .... 118

3.28 Dart leader speed versus interstroke
interval. The dashed line bounds the data of
Brook and Kitagawa ( Winn, 1965) for 100 dart
leaders. The dashed and dotted lines bound
the triggered lightning data of Idone and
Orville (1984).............................. 120

3.29 Dart leader speed versus dart leader electric
field change duration......................... 122

3.30 Dart leader speed versus return stroke
initial peak electric field. The dashed
regression is for 32 triggered lightning
strokes reported by Idone et al. (1984). The
solid regression line is for the UF data
points excluding the leaders for strokes 2
and 3 of 1317:14 UT. Stroke 2 was dart
stepped and stroke 3 was the fastest natural
dart leader ever measured.................... 123

3.31 Dart leader speed versus dart leader relative
light intensity............................ .. 125

3.32 Dart leader relative light intensity versus
return stroke initial peak electric field.
Dart leader relative light intensity








measurements are taken at a height of
approximately 50 m............................ 126

3.33 Dart leader relative light intensity versus
leader electric field change duration.
Optically Nondetected leaders are not shown
for obvious reasons........................... 128

3.34 Dart leader relative light intensity versus
duration of previous interstroke interval.... 129

3.35 Dart leader relative light intensity versus
return stroke relative light intensity....... 131

3.36 Dart leader relative light intensity versus
height for stroke 3 of the 2246:45 UT flash
on July 27, 1979.............................. 134

3.37 Dart leader relative light intensity versus
height for stroke 2 of the 1445:35 EDT flash
on August 10, 1982............................ 135

3.38 Dart leader relative light intensity versus
height for stroke 3 of the 1445:35 EDT flash
on August 10, 1982............................ 136

3.39 Dart leader relative light intensity versus
height for stroke 2 of the 1446:56 EDT flash
on August 10, 1979............................ 137

3.40 Dart leader speed versus return stroke
relative light intensity. Return stroke
relative light intensity is measured at a
height of approximately 50 m................. 138

3.41 Dart leader peak relative light intensity
versus height. Regression lines for the
individual strokes are identified by the
symbols to the right of the plot............. 139

3.42 Dart leader relative light intensity at
plateau after peak versus height.Regression
lines for the individual strokes are
identified by the symbols to the right of the
plot.......................................... 140

3.43 Dart leader rise time versus height. The
system time resolution was approximately
0.5 Asec which produced the effect seen
in the rise time data. ........................ 141

3.44 Dart leader relative light intensity duration
versus height. Regression lines for the








individual strokes are identified by the
symbols to the right of the plot............. 142

3.45 Dart leader electric field change duration
versus return stroke relative light
intensity..................................... 144

3.46 Duration of previous interstroke interval
versus return stroke relative light
intensity..................................... 146

3.47 Return stroke initial peak electric field
versus return stroke relative light
intensity..................................... 147

3.48 Digitized image of stroke 3 of the flash at
2206:51 UT July 27, 1979. The heights
indicated are referenced in the plots of
relative light intensity versus time......... 155

3.49 Relative light intensity versus time for
stroke 3 at 2206:51 UT July 27, 1979......... 156

3.50 Relative light intensity versus time for the
lower 500 m of stroke 3 at 2206:51 UT, July
27, 1979...................................... 157

3.51 Digitized image of stroke 6 of the flash at
2206:51 UT July 27, 1979. The heights
indicated are referenced in the plots of
relative light intensity versus time......... 159

3.52 Relative light intensity versus time for
stroke 6 at 2206:51 UT July 27, 1979......... 160

3.53 Relative light intensity versus time for the
lower 500 m of stroke 6 at 2206:51 UT, July
27, 1979...................................... 161

3.54 Digitized image of stroke 9 of the flash at
2206:51 UT July 27, 1979. The heights
indicated are referenced in the plots of
relative light intensity versus time......... 163

3.55 Relative light intensity versus time for
stroke 9 at 2206:51 UT, July 27, 1979........ 164

3.56 Relative light intensity versus time for the
lower 500 m of stroke 9 at 2206:51 UT, July
27, 1979..................................... 165

3.57 Digitized image of stroke 2 at 2246:45 EDT
July 27, 1979. The heights indicated are


xii








referenced in the plots of relative light
intensity versus time........................ 167

3.58 Relative light intensity versus time for
stroke 2 at 2246:45 UT, July 27, 1979........ 168

3.59 Digitized image of stroke 3 at 2246:45 EDT
July 27, 1979. The heights indicated are
referenced in the plots of relative light
intensity versus time. ........................ 170

3.60 Relative light intensity versus time for
stroke 3 at 2246:45 UT, July 27, 1979........ 171

3.61 Relative light intensity versus time for the
lower 500 m of stroke 3 at 2246:45 UT, July
27, 1979 ..................................... 172

3.62 Digitized image of stroke 2 at 1317:14 EDT
July 21, 1982. The heights indicated are
referenced in the plots of relative light
intensity versus time......................... 174

3.63 Relative light intensity versus time for
stroke 2 at 1317:14 UT, July 21, 1982........ 175

3.64 Digitized image of stroke 3 at 1317:14 EDT
July 21, 1982. The heights indicated are
referenced in the plots of relative light
intensity versus time......................... 177

3.65 Relative light intensity versus time for
stroke 3 at 1317:14 UT, July 21, 1982........ 178

3.66 Digitized image of stroke 2 at 1445:35 EDT
August 10, 1982. The heights indicated are
referenced in the plots of relative light
intensity versus time ........................ 180

3.67 Relative light intensity versus time for
stroke 2 at 1445:35 UT, August 10, 1982...... 181

3.68 Relative light intensity versus time for
lower 500 m of stroke 2 at 1445:35 UT, August
10, 1982...................................... 182

3.69 Digitized image of stroke 3 at 1445:35 EDT
August 10, 1982. The heights indicated are
referenced in the plots of relative light
intensity versus time. ........................ 184

3.70 Relative light intensity versus time for
stroke 3 at 1445:35 UT, August 10, 1982...... 185


xiii








3.71 Relative light intensity versus time for
lower 500 m of stroke 3 at 1445:35 UT, August
10, 1982..................................... 186

3.72 Digitized image of stroke 2 along the first
channel at 1446:56 EDT, August 10, 1982.
The heights indicated are referenced in the
plots of relative light intensity versus
time.......................................... 188

3.73 Relative light intensity versus time for
stroke 2 at 1446:56 UT, August 10, 1982. ..... 189

3.74 Relative light intensity versus time for
lower 500 m of stroke 2 at 1446:56 UT, August
10, 1982..................................... 190

3.75 Digitized image of stroke 2 at 1926:45 UT
July 11, 1978. The heights indicated are
referenced in the plots of relative light
intensity versus time........................ 192

3.76 Relative light intensity versus time for
stroke 2 at 1926:45 UT, July 11, 1978 ........ 193

3.77 Digitized image of stroke 2 at 2023:46 UT
July 29, 1978. The heights indicated are
referenced in the plots of relative light
intensity versus time........................ 195

3.78 Relative light intensity versus time for
stroke 2 at 2023:46 UT, July 29, 1978........ 196

3.79 Return stroke relative light intensity versus
height for stroke 3 at 2206:51 UT, July 27,
1979.......................................... 197

3.80 Return stroke relative light intensity versus
height for stroke 2 at 2246:45 UT, July 27,
1979.......................................... 198

3.81 Return stroke relative light intensity versus
height for stroke 3 at 2246:45 UT, July
27, 1979...................................... 199

3.82 Return stroke relative light intensity versus
height for stroke 2 along the first channel
at 1446:56 EDT, August 10, 1982. ............. 200

3.83 Return stroke relative light intensity versus
height for stroke 2 along the second channel
at 1446:56 EDT, August 10, 1982. .............. 201


xiv








3.84 Return stroke relative light intensity versus
height for stroke 2 at 1445:35 EDT, August
10, 1982...................................... 202

3.85 Return stroke relative light intensity versus
height for stroke 3 at 1445:35 EDT, August
10, 1982.................................... 203

3.86 Return stroke relative light intensity versus
height for stroke 1 at 1926:45 UT, July 11,
1978........................................ 204

3.87 Return stroke relative light intensity versus
height for stroke 2 at 1926:45 UT, July 11,
1978.......................................... 205

3.88 Return stroke peak relative light intensity
versus height. Regression lines for the
individual strokes are identified by the
symbols to the right of the plot............ 206

3.89 Return stroke relative light intensity rise
time versus height............................ 207

3.90 Return stroke relative light intensity
30Asec after peak versus height. Regression
lines for the individual strokes are
identified by the symbols to the right of the
plot.......................................... 208

3.91 Digitized image of stroke at 2041:00 UT
July 29, 1978 showing M-components.
Indicated heights correspond to the
locations of plots of relative light
intensity versus time......................... 210

3.92 Relative light intensity at ground level
showing the return stroke luminosity and the two M-component luminosities for the
stroke at 2041:00 UT July
29, 1978...................................... 211

3.93 Relative light intensity at 600 meters
showing the return stroke
luminosity and the two M-component
luminosities for the stroke at 2041:00 UT
July 29, 1978................................. 212

3.94 Relative light intensity at 1100 meters
showing the return stroke
luminosity and the two M-component
luminosities for the stroke at 2041:00 UT
July 29, 1978................................. 213








3.95 Peak relative light intensity versus height
for the first M-component at 2041:00 UT July
29,1978....................................... 214

3.96 Peak relative light intensity versus height
for the second M-component at 2041:00 UT July
29,1978....................................... 215

3.97 Electric field change following stroke 6 of
the flash 2206:51 UT 27 July, 1979 showing
three typical waveshapes for M-components.... 216


xvi













LIST OF TABLES


Table Page

3.1 Summary of Stroke Parameters Derived from Correlated Optical
and Electric Field Records........ 71

3.2 Summary of Stroke Parameters Derived from Optical Records
Made Available by the State
University of New York at Albany.. 73


xvii















Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy



RELATIVE LIGHT INTENSITY AND ELECTRIC FIELD INTENSITY
OF CLOUD TO GROUND LIGHTNING By

Douglas Max Jordan

May 1990

Chairman: Martin A. Uman
Major Department: Electrical Engineering

Correlated streak camera and single-station electric field records were obtained for: 9 subsequent return strokes in 2 cloud to ground discharges (Tampa, Florida 1979), and 1 first and 14 subsequent strokes in 3 flashes (Gainesville, Florida 1982). Streak camera records for 8 first and 8 subsequent strokes (Kennedy Space Center, Florida 1978) and 3 first and 3 subsequent strokes (Oklahoma 1984) were furnished by the State University of New York at Albany.

For ten first return and 11 subsequent return

strokes, relative light intensity is graphed versus height and time. First stroke light rise times increase immediately after a branch point, then decrease as the


xviii








light front propagates up the channel. First stroke relative light intensity shows an opposite pattern.

Subsequent leader speeds, for 4 flashes, increase

from stroke two to stroke three, then decrease. Positive correlations are shown for leader speed vs. return stroke initial peak electric field, leader speed vs. leader electric field duration, and leader detectability using streak camera techniques vs. return stroke initial peak electric field.

Relative light intensity profiles for five dart

leaders show that the dart leader channel continues to radiate light after the dart leader front has passed and the peak light intensity for dart leaders is constant with height.

From measurements of relative light intensity vs.

height and time for one subsequent stroke followed by two M-components we show that the M-component relative light intensity is constant with height and propagated downward at between 1 and 2 x 108 m/s.

In addition to the new results noted above, for 11

subsequent strokes, peak relative light intensity is shown to be correlated with dart leader speed, correlated with dart leader electric field change duration, and uncorrelated with previous interstroke interval. We confirm previous observations that: dart leader speeds decrease with increasing previous interstroke intervals; there is an increase (more rapid than previously


xix








presented) in apparent channel length with stroke order; subsequent stroke relative light intensity shows a rapid rise to peak which decays with height along the channel; and that a positive correlation exists between initial peak electric field and peak relative light intensity measured near the ground for subsequent return strokes.














CHAPTER 1
REVIEW


1.1 Introduction


In this chapter lightning terminology is introduced and lightning processes and research pertinent to this thesis are discussed. Section 1.2 contains a brief discussion of the lightning process. Sections 1.3 and 1.4 contain a review of relevant previous work in the areas of photographic and electric field observations and measurements.



1.2 Lightning


Lightning is one of the most familiar phenomena in nature. It has been observed, feared, and worshipped for centuries. The lightning process, with its brilliant light intensity, loud thunder, and smell is unique among natural phenomena in that it affects a majority of the human senses.

Although the lightning discharge is considered by

most laymen to be a single event, it is actually a series of distinct processes which occur in less than 1 second, mostly along the same spatial path. A drawing of the








primary lightning processes is shown in Figure 1.1. The discharge begins with electrical breakdown activity in the cloud which is not shown in Figure 1.1. The next phase of the discharge is the stepped leader, followed by the first return stroke. The first return stroke, after an interstroke interval of tens of milliseconds may be followed by a series of dart leader and subsequent return stroke combinations. Often apprecible current, hundreds of amperes, continues to flow for for tens of milliseconds after a return stroke. During this so-called continuing current, discharges not shown in figure 1.1 called Mcomponents, which brighten the channel to ground, may occur. Lightning processes pertinent to the present thesis will be discussed in more detail in the following sections.


1.2.1 Stepped Leader



The stepped leader occurs after the preliminary

breakdown process, which will not be discussed, and is a series of randomly propagating, relatively short-duration, about 1 gsec (Krider et al., 1977; Beasley et al., 1982), discharges which produce "steps" of light. The steps occur about 50 Asec (Kitagawa, 1957) apart high above ground with increasing frequency near ground. The stepped leader normally lowers negative charge, several to tens of coulombs (Brook et al., 1962), from a source in the cloud







2 msec

70psec 60 psec 20 msec mec Jmsec
/sc] IMe


1 msec
-60 psec


Figure 1. 1


(a) (b)

Schematic diagram of a lightning ground discharge below cloud. (a) Streak schematic showing, stepped leader, dart leaders, and return strokes. (b) Still photo schematic of entire flash. (Adapted from M.A. Uman, Lightning, McGraw Hill Book Co., 1969).








and deposits this charge along the paths of the breakdown steps. The stepped leader travels downward with an average speed between 1 and 2 x 105 m/sec (Schonland, 1956; Berger and Vogelsanger, 1966; Thomson et al., 1985). As the stepped leader tips come near the earth, the charge, which has been deposited, creates a large enough electric field to induce positive, upward-moving streamers from many locations beneath the leader. Eventually, one of the upward-moving streamers makes contact with a leader tip, and the first return stroke is initiated.



1.2.2 First Return Stroke



When the positive charges of an upward streamer make contact with a branch of the downward-propagating negative leader, a violent, upward propagating, discharge is initiated near ground and propagates upward during which the charge which was deposited along the leader main channel and branches is lowered to earth. The bulk of the leader charge at each height is lowered within a few tens of microseconds of the return stroke fronts arrival, producing currents at ground which can be as great as hundreds of kiloAmperes, but typically are 35 kA (Berger, 1967b; Berger et al.,1975). The return stroke illuminates the stepped leader path between the cloud and ground as the luminous current wave travels from the ground to the cloud with typical speeds of 1 - 2 x 108 m/sec (Orville,








1982). The return stroke channel normally defines the spatial path of the subsequent events of the flash, although often a single flash may contain return strokes occurring along more than one channel to ground.



1.2.3 Dart Leader



About 20% of the time in Florida (Thomson et al., 1984), the lightning flash ends after the first return stroke. If this is not the case, the first return stroke is followed, after an interval on the order of typically tens of milliseconds (Schonland, 1956; Thomson et al., 1984), by a dart leader. The dart leader is a luminous breakdown which propagates continuously from the cloud to the ground while lowering charge along the channel of the previous return stroke. Typical speeds for the dart leader are 1 - 2 x 107 m/sec ( Orville and Idone, 1982; Idone et al., 1984). The dart leader will occasionally travel almost the entire length of the channel and then suddenly begin to step the remainder of the distance to ground, generally but not always along the previous stroke channel. Such leaders are called dart-stepped leaders.



1.2.4 Subsequent Return Stroke


When the dart leader reaches ground, or is met by an upward leader near ground, the subsequent return stroke








begins. The subsequent return stroke travels from the ground to the cloud along the path of the previous dart leader with speeds on the order of 1 - 2 x 108 m/sec (Orville, 1982) and typically produces peak currents of the order of 15 kA (Berger et al., 1975) at ground.



1.2.5 M-component



If stroke current continues to flow during the

interstroke interval, there may be luminous processes associated with current waves in the channel during this time. These changes in channel light intensity are referred to as M-components (Malan and Schonland, 1947).



1.3 Photographic Measurements


The luminous features of the lightning ground

discharge have been widely studied and have provided considerable insight into the physics of the lightning process. Scientists first studied the light intensity of the lightning flash late in the 19th century. These research efforts consisted primarily of attempts to determine the sequence of events during the lightning discharge to ground. Kayser (1884) was one of the first to observe that the lightning process consisted of multiple strokes down the same spatial path. Hoffert (1889) and Weber (1889) successfully used moving cameras to separate








on film the individual lightning components. Photographs were produced by Walter (1902, 1903, 1910, 1912, 1918) which showed, for the first time, that the lightning discharge was initiated by a branched initial process followed by a return stroke traveling up the same channel. Therefore, by early in the 20th century a coarse view of the lightning discharge had been revealed.

Sir Charles V. Boys (1929) invented a camera system in 1900 which revolutionized the study of lightning. The camera produced relative motion between the film and the lenses of the camera by having the lenses rotate in front of the film. This design allowed the camera to remain fixed yet enabled events which occurred along the same spatial path at different times to be separated. Boys was able to obtain images with his camera (Boys, 1929), but the conclusions of his analysis proved to be incorrect.

The investigation of optical properties of lightning during the period between 1930 and 1960 was dominated by Schonland, Malan, Collens, and coworkers in South Africa. Boys cooperated with the South African team and loaned his camera to be used as a prototype. In their first experiments with Boys' rotating lens camera design (Schonland, Malan and Collens, 1934), the camera lenses, separated by 10.1 cm, were rotated by hand at 1500 rev/min � 10%. At this speed, a distance of 1.0 mm on the film corresponded to approximately 63 microseconds. The authors verified previous findings by Halliday (1933)








which showed light intensity moving both up and down the lightning channel. They observed branches which illuminated in a sequence other than by height along the channel and noticed that the channel light intensity decreased as the return stroke front passed each branch point and finally vanished after it passed the last branch.

The Boys camera was later modified to have still lenses and a rotating film drum (Schonland, Malan, and Collens, 1935), as shown in Figure 1.2. The apertures of the camera lenses were set independently, which allowed sufficient dynamic range to examine processes whose light intensity varied greatly. Using this method, it was determined that the stepped leader paused approximately 100 microseconds between steps. A glow was noticed preceding the stepped leader tip as it neared the ground, and some leader tips glowed quite far above ground. The authors also discovered that the effective stepped leader speeds increased near ground. Orville and Idone (1982) also show that the stepped leader speed increases near the ground.

Malan and Collens (1937) thoroughly investigated

first stroke light variations with particular emphasis on main channel light components initiated by the charge in a branch when a return stroke front passes a branch and releases that charge. These branch components travel down the main channel while the return stroke front travels up


















Rotating Film Drum


B








A

Figure 1. 2


Film


Diagram of improved Boys camera with moving film and stationary lenses. Adapted from McEachron (1939).








the channel above the branch and were often found to be brighter below the branch than the return stroke front which preceded them.

Optical lightning research, in the early years,

concentrated on the subjective evaluation of film records to determine flash properties. As electronic technology improved, it became possible to use calibrated photoelectric detectors to determine quantitative properties of lightning processes. Mackerras (1973) used photomultiplier tubes and a wide-angle camera system to perform a quantitative analysis of the integrated light output of both cloud and ground flashes. Statistics were presented for energy balance, based on assumptions of charge being lowered from an assumed height in the cloud. Comparisons were made with the Bruce and Golde (1941) model of the lightning return stroke.

A parameter of great interest to researchers

developing return stroke models is the speed of the return stroke front as it propagates up the channel. Orville et al. (1978) presented daytime lightning data acquired with a streak camera system including measurements of return stroke speed. Return stroke speeds ranging from 1.2 x 108 to 1.4 x 108 m/sec were observed. The camera system was of the single lens design with rotating film drum. Return stroke speed was computed using a still image from a 35mm camera as a reference image and measuring the displacement of the streaked image from the still image. The








displacement is a function of the height along the channel and the return stroke speed.

Hubert and Mouget (1981) analyzed data from triqqered (artificially initiated by firing small rockets trailing ground wires) lightning at St. Privat d'Allier, France. Their analysis focused on return stroke speed, but some interesting optical waveforms at two heights along the channel are shown as well as current measured in a shunt at the base of the channel. The data were recorded using logarithmic compression, but all of the data presented were in the linear range of the amplifier.

Idone and Orville (1982) presented return stroke speed data from 63 strokes at Kennedy Space Center. Speed as a function of height was measured revealing, in all but one case, a decrease of up to 25% near the top of the visible channel. The cloud base, which determined the visible channel top, for these flashes was approximately 1000 ft.; therefore, only a relatively short section of channel was available for measurement.

Orville and Idone (1982) present streak camera

records for 21 dart leaders, 4 dart-stepped leaders, and 3 stepped leaders from Kennedy Space Center, Florida and Socorro, New Mexico in mostly daylight conditions. Dart leader speeds were computed at two heights along the channel with the mean speed in the bottom 800 meters being 11 x 106 m/s while the mean speeds in the upper channel sections was 14 x 106 m/s. Most of the dart leader speeds








decreased near ground while 4 leader speeds increased; however, they commented that the measured difference was not large and was within their experimental error. Dart leader lengths were computed and shown to have a mean value of 27 meters for the Florida data and 42 meters for the New Mexico data. The authors suggested that their use of #92 filters in the Florida experiment could have caused the difference in the means. They found a correlation between the dart leader luminous intensity and the resulting return stroke luminous intensity. They found no correlation between dart leader speed and the luminous intensity of the dart and little correlation between the luminous intensity of the dart and the resulting return stroke speed. Inconclusive results were found for dart leader speed vs. return stroke speed as well as dart leader speed vs. return stroke luminous intensity.

Idone et al. (1984) presented analyses of three

triggered lightnings at Langmuir Laboratory, New Mexico. They computed three-dimensional speeds of the strokes and measured a number of flash parameters, including current at the base of the channel. Return stroke speeds ranged from 6.7 x 107 m/s to 1.7 x 108 m/s. The ratio of threedimensional channel length to two-dimensional channel length ranged from 1.05 to 1.22. Return stroke currents ranged from 4.0 to 32.0 kA with a mean of 6.0 kA.

Mach and Rust (1989) measured return stroke speed

using a 35mm camera housing which was modified to contain









a solid state sensor array. The array had 8 horizontal sections and provided relative light intensity as a function of the corresponding heights and time. Return stroke speed measurements for two channel lengths are presented for 86 natural lightning strokes and 41 triggered strokes. Peak currents for the natural strokes are computed using the Transmission Line Model formula of Uman et al. (1975) and the experimentally derived relationship of Willett et al. (1989). The rise time of their optical waveforms increased with height while the peak relative light intensity decreased with height, supporting the results of Jordan and Uman (1983).

Total power radiated from the return stroke channel is of considerable interest to those engaged in lightning and electromagnetic pulse protection research. Guo and Krider (1982) analyzed data from a calibrated wide-angle photodetector system and fast electric field system. The spectral response of the detector extended from 400 nm to 1000 nm with a peak at 750 nm. The detector lens system covered an angle of approximately 250 above the horizon. Correlated electric field and optical waveforms were shown on a microsecond time scale. The authors assumed that the channel was straight and had uniform light intensity as a function of height. They then calculated the total optical output from a section of channel. They used previous laboratory results, which showed a radiative efficiency of 0.8%, to calculate the average electric power radiated per








meter of channel. Average radiated power was shown to range from 1.3 � 1.2 x 108 W/m for first strokes to

3.1 � 2.3 x 107 W/m for subsequent strokes. Results were also shown for the time derivative of light intensity divided by the peak electric field squared vs. range to the flash. The results of their paper were amended in Guo and Krider (1983a) after receipt of a manuscript in preparation by Jordan and Uman (1983) which showed detailed measurements of light output from the channel as a function of height. These results enabled a more accurate estimate of the peak light output. Peak radiance was shown to be between 6.0 x 105 W/m and 1.0 x 106 W/m in the 400 - 1100 nm range. Guo and Krider (1983a) used the same apparatus to measure the peak electromagnetic power radiated from the return stroke. Peak powers were reported of 2.3 � 1.8 x 109 W for first strokes and

4.8 � 3.6 x 108 W for subsequent strokes.

In order to reduce errors in the calculation of total light output from lightning discharges, Thomason and Krider (1982) conducted a Monte Carlo simulation of the scattering of light by clouds. The light was assumed to be generated inside the cloud, as in an intracloud flash or the portion of the return stroke to ground which is in the cloud. The authors show that essentially all of the original light eventually escapes the cloud but may be time-broadened by tens of microseconds.








Schonland (1938) gave his view of the discharge process which included the observation that the light intensity of subsequent return strokes decreased considerably by the time they had propagated half the distance to the cloud. Cloud bases in South Africa varied from 1000 m to 2900 m. This was the first discussion of the variation in relative light intensity with height along the channel.

Boyle and Orville (1976) used two slits in front of a streak camera to obtain a pair of waveforms which showed light intensity as a function of time at the two slit heights which were 350 m apart. The rise times of the two waveforms were found to be approximately 3 Asec.

Jordan and Uman (1983) were the first to provide a detailed analysis of light profiles as a function of height and time. They used a rotating drum microdensitometer to digitize seven film images, of subsequent strokes, taken with a streak camera in Florida. The images were sampled with a spatial resolution of 25 microns, which yielded a maximum time resolution of 0.5 microseconds. Horizontal elements of the digitized data were plotted to provide a profile of the channel relative light intensity, at a given height, as a function of time. Plots of relative light intensity vs. height and time were analyzed for seven subsequent strokes in two flashes. Results were presented for peak relative light intensity vs. height, rise time vs. height, and relative








light intensity at 30 microseconds after peak. These analyses showed clearly that the peak relative light intensity decreases and rise time increases with height. The authors compared their data to those of Guo and Krider (1983a) in order to provide a rough calibration of the photographic relative light intensities. They integrated the relative light intensity at increasing heights, assuming a return stroke speed of 1.0 x 108 m/sec, to simulate the output of an all-sky detector. Their comparison showed that the total optical power, computed by assuming a uniform channel light output, is underestimated by a factor of between 1.8 to 3.8 with a mean of 2.5.

Ganesh et al. (1984) obtained light profiles by

placing a thin slit in front of a photomultiplier tube. The light intensity data were necessarily at different heights on flashes at different distances but nevertheless were from a relatively short section of channel. The photomultiplier system had a frequency response of approximately 500 KHz and a spectral sensitivity ranging from 200 - 600 nm. A figure was presented of light intensity on a microsecond time scale.


1.4 Correlated Measurements of Electric Fields and Relative Light Intensity

Electromagnetic field measurements have been the primary tool of the lightning theorist. A major








disadvantage of these measurements is that the measured fields are a function of the currents at all heights along the lightning channel. Because of this limitation, it is difficult to determine the current at different heights along the channel from the fields. Correlated electric field and streak photographs have been used in an attempt to gain better understanding of electric field records.

Schonland et al. (1938) reported on the first

correlated measurements of lightning light intensity and electric fields. Electric fields were measured by filming the face of an oscilloscope which was connected to an electric field antenna system. Pulses were found, on the electric field records, which preceded the beginning of the optical signal. This research, for the first time, provided proof that the lightning process begins in the cloud well before the luminous portion can be detected.

Malan and Schonland (1947) presented the results of the best correlated electric field and light intensity measurements to that time. The electric field records revealed many "hook-shaped" waveforms which corresponded to what the authors called "M" components which illuminated the channel to ground. They attributed the "M" components to weak return strokes. It was observed that the steps in the stepped leader process do not produce field changes on the electric field records. They also present evidence of a uniform distribution of charge along the dart leader channel based solely on a uniform dart









leader speed. In a later paper, Malan and Schonland (1951) presented relationships between the height of the charge which is lowered by each subsequent stroke and the stroke number. Krehbiel et al. (1979) have since shown that the charge regions which are discharged by subsequent strokes within the same flash are actually distributed horizontally in the cloud, not vertically. Orville and Idone (1982) showed that dart leader speeds are not constant but most decrease near ground. Schonland (1956) summarized the South African work in a review paper which provided an integrated view of his understanding of the lightning discharge.

Kitagawa and Kobayashi (1958) presented results of correlated optical and electric field signals. The experimental apparatus had relatively coarse time resolution but yielded data which agreed with the findings of Malan and Schonland (1951). The authors found a large number of luminous pulses which occurred before and after the discharge.

Kitagawa et al. (1962) acquired low-speed streak

camera records with correlated electric field records for about 200 cloud-to-ground discharges at Socorro, New Mexico. Thirty six hybrid flashes, containing one or more continuing currents, were analyzed and statistics presented for stroke order of continuing currents, and duration of continuing luminosity. They show hook-shaped electric field waveshapes, or K-changes, associated with








M-components during continuing currents and present data on M-component intervals vs. elapsed time since the return stroke. They suggest that the channel must have several levels of conductivity in order for some strokes to have leader processes while others, such as M-components, can travel down the channel with no detectable leader.

Brook et al. (1962) use photographic records and

correlated electric fields to compute the charge lowered and apparent height of the individual strokes of flashes in New Mexico storms. The apparent height of the charge centers was later shown by Krehbiel et al. (1979) to be a horizontal length as discussed above. Data are presented which show that strokes occurring later in the flash lower less charge.

Schonland, in a presidential address to the British Association, recapped his perception of the entire lightning process (Schonland 1963). In that address, he compared the behavior of the long laboratory spark to lightning and decided that they shared essentially the same breakdown process.

Lundquist and Scuka (1969) analyzed simultaneous measurements of two optical bands and electric fields. They used an all-sky lens system with two optical detectors. The spectral response of one detector was centered in the violet (390 nm) and the other in the red (655 nm). Their results differed from those of Orville (1968) in that often the red band would precede the








violet. Orville found the red wavelengths to lag the violet due to the Ha lines of hydrogen turning on late in the optical process.

Thomson (1980) discussed the results of correlated optical, electric field, and 10 MHz radiation measurements for 282 return strokes in Papua, New Guinea. Wideband optical waveforms, on a millisecond scale, were used primarily to identify specific lightning features. Statistics were presented for flash duration, prestroke duration, continuing current duration, interstroke interval, stepped leader duration, strokes per flash, and channels per flash.

Jordan and Uman (1983) analyzed streak camera data

and simultaneous electric field records. The authors used the plots of peak relative light intensity vs. height, from the streak photographs, to extrapolate a peak relative light intensity at the ground and compared these values with peak electric field (Ep). The best fit was for log peak relative light intensity vs. peak electric field, although statistically significant correlations were found for log peak relative light intensity vs. Ep2 and linear peak relatively light intensity vs. Ep.

Ganesh et al. (1984) presented correlated

light intensity and electric field data on a microsecond time scale. Relative light intensity vs. electric field are plotted on several scales, with a linear relationship appearing to have the best fit. The authors acknowledge








that errors associated with channel position relative to the axis of the slit, atmospheric scattering, and the uncertainty of the distances to the flashes prevented an accurate determination of the relationship between light intensity and electric field.

Additional analysis of the Langmuir Laboratory data by Idone and Orville (1985) examined the correlation between light intensity at the channel base vs. the peak channel current. The analysis was performed in a similar fashion to Jordan and Uman (1983), who compared light intensity to peak electric field. The authors show statistical results for 39 subsequent strokes of 2 triggered flashes. Correlation coefficients were measured for relative light intensity (LR) vs. current (I), log LR vs. log I, log LR vs. I, and log LR vs. 12. All of these had statistical significance at the 1% level, but the highest was LR vs. log I. These findings differ from those of Jordan and Uman (1983), who found the highest correlation between log LR vs. E2.














CHAPTER 2
UF AND SUNYA EXPERIMENTS AND GENERAL ANALYSIS TECHNIQUES


2.1 Experimental Techniques



2.1.1 Photographic Techniques



The lightning process is random in nature, hence to

record it the photographic system must be prepared, over a period of tens of minutes, to respond within a few milliseconds. Lightning ground flashes typically last hundreds of milliseconds and occur separated by several seconds. The camera system must be able to separate the various components of the lightning flash which occur along the same spatial path. The camera configuration used in this research effort varied slightly between years but was based on a Beckman and Whitley model 351 streak camera.

The Beckman and Whitley streak camera is a rotatingdrum fixed-lens camera which is capable of film writing rates of 0.05 mm/microsecond. A 50 mm Nikkor objective lens was used to image the lightning channel through the shutter onto the image plane in front of a collimating lens. The collimating lens transferred the image by way of a 45-degree mirror onto the film drum.








The film used in the streak camera must be loaded and unloaded for each exposure sequence. A length of film (855 mm), which is wound beforehand into a special cassette, is unrolled onto the stationary film drum. The cassette is then disengaged and the drum is rotated with a high-speed motor to the desired operating speed. Typical drum speeds are on the order of 70 revolutions per second. The rotational speed of the film drum is monitored by counting pulses from a magnetic coil pickup. The pickup senses four magnetized screws on the drum face.

The film used in the streak camera was Kodak 5474 shellburst. This film is well suited to lightning photography due to its spectral response which is essentially constant from 300 nm to 670 nm. The extended red sensitivity is desirable for lightning photography due to the high intensity of spectral lines of hydrogen (Ha at 656 nm) emitted by the lightning channel (Orville, 1968). Kodak 5474 has a thick grey base and good antihalation properties. The mechanical properties of the streak camera film are very important since the film must be unrolled onto the drum and retrieved.

A triggering mechanism was necessary for the camera shutters since all of the data were acquired in daylight. A photodiode optical detector connected to a differentiator circuit was used to detect the first stroke of the lightning flash. The parameters of the differentiator were adjusted so that ambient conditions








did not cause a trigger. The trigger circuit also provided an input to the data logging circuitry which printed a record of the events. The time of the event, stroke number, time between strokes, and speed of the camera drum were printed. Film exposure times of approximately

0.5 second were used, enabling the maximum number of subsequent strokes to be recorded without exposing the film to excessive background light.

Once exposed, the film was developed in a darkroom located adjacent to the camera site, using Accufine developer, chilled to 68 OF, with a development time of five minutes and agitation at 30-second intervals. The development process was then stopped and fixed with standard Kodak products. The resulting negatives were air dried.

A still camera was used to document the location of the flash and provide an undistorted view of the channel geometry. The camera was automatically triggered by the streak camera trigger circuit. An exposure time of 0.25 s was used at a lens stop of f8. Kodak Ektachrome film (ASA 64) was used to produce color slides.

A video camera was used to provide a chronological

record of everything that happened in the field of view of the streak camera. The video camera integrated events which occurred within a 16-millisecond interval. Even with such poor time resolution the records were extremely valuable when reconstructing unusual events, such as








multiple channels to ground. System timing and audio were also recorded on the video recorder audio channels.



2.1.2 Electric Field Measurement Techniques



Electric field measurements are an integral part of lightning research, and their omnidirectional nature has made them the primary tool for analysis of the lightning process. The subsystems necessary for an electric field measurement system will be discussed in the following sections.

2.1.2.1 Antennas for measuring vertical electric fields

Electric fields can be detected by many different

types of antennas as discussed by Uman (1987, appendix C). Whip, spherical, and flat plate antennas have all been used to record electric field signals from lightning. All of the electric field records in this thesis were acquired using flat plate antennas.

The vertical electric field terminating on a flat plate conductor induces a surface charge density ( s).



s = 6oEn


If the antenna is connected to an electronic device, the current delivered to the device will be


i = dq/dt, q = sA








= d(EoEnA)/dt



= EoA * dEn/dt



where A is the area of the plate. En is assumed to be uniform across the surface of the plate and the plate is flush with the earth. If the antenna is not mounted flush with the earth the measured electric field is not the true field but is an enhanced value of the field. In order to produce a signal proportional to the electric field, the current from the antenna must be integrated.

The flat plate antennas used in this research were constructed using a ground plane approximately one meter square with a flat circular plate of the appropriate area mounted in a hole in the ground plate. Several antennas with different plate areas were used to provide various gains. The details for the antennas used in the Tampa, Florida experiment are found in Section 2.2.1 and for the Gainesville, Florida experiment are found in Section

2.2.2.

2.1.2.2 Integrators and amplifiers

The signal proportional to the derivative of the electric field from the electric field antenna is connected to an integrator circuit providing an output proportional to the electric field. Integrator circuits may be as simple as a resistor capacitor pair or may









= d(coEnA)/dt



= E0oA - dEn/dt


where A is the area of the plate. En is assumed to be uniform across the surface of the plate and the plate is flush with the earth. If the antenna is not mounted flush with the earth the measured electric field is not the true field but is an enhanced value of the field. In order to produce a signal proportional to the electric field, the current from the antenna must be integrated.

The flat plate antennas used in this research were constructed using a ground plane approximately one meter square with a flat circular plate of the appropriate area mounted in a hole in the ground plate. Several antennas with different plate areas were used to provide various gains. The details for the antennas used in the Tampa, Florida experiment are found in Section 2.2.1 and for the Gainesville, Florida experiment are found in Section

2.2.2.

2.1.2.2 Integrators and amplifiers

The signal proportional to the derivative of the electric field from the electric field antenna is connected to an integrator circuit providing an output proportional to the electric field. Integrator circuits may be as simple as a resistor capacitor pair or may








involve operational amplifier circuits such as those used to record the fields in this thesis.

2.1.2.3 Instrumentation recorders

All signals of interest were recorded on

instrumentation tape recorders with IRIG Group II electronics. In 1979 an Ampex FR1900 was used and in 1982 an Ampex FR3010. The electronics of these recorders are wide-band and provide for both amplitude modulated (direct) and frequency modulated (fm) recording. The direct channel frequency response is down 6db at 400 Hz and 1.5 MHz. The fm channel frequency response extends to dc and is -6db at 500 KHz. FM channels are used to record signals which had low-frequency components such as the slow decay electric field integrators which had decay constants of 5 seconds. An fm channel was also used to simultaneously record time code information. The signals on the instrumentation recorder tapes were later replayed and digitized using the Masscomp digitizing system. This process is discussed in detail in Section 2.4.2.

2.1.2.4 Biomation waveform recorders

The Biomation 805 waveform recorders were used both in data acquisition and data analysis to sample analog data with 8-bit resolution and store up to 2048 samples. The stored data can be displayed at one of several rates through a digital to analog converter. During data acquisition the analog output signals were displayed on dual beam oscilloscopes which were photographed with film








recorders. During data reduction the output was captured on a storage oscilloscope so that a polaroid photograph could be taken of the oscilloscope traces.

The Tampa data and the Gainesville data were

initially analyzed using Biomation waveform recorders. The recorded signals on the instrumentation recorders were played back into the Biomation waveform recorders and time periods of interest were sampled and stored. This process was extremely tedious since the Biomations began recording after a preset delay from the beginning of the event of interest and the delays available on the Biomations were insufficient to analyze a typical flash. Lightning lab personnel overcame this limitation by building special electronics which produced an external trigger to the Biomations after a preset delay from the event trigger. Typically the first stroke of the flash was used as the event trigger and a delay was determined, from a plot of the overall electric field change, which would cause the time period of interest to be sampled and recorded. In order to analyze a typical flash it was not unusual to replay the tape recording of the flash tens of times, sampling a different period of time from each pass with the Biomations. This technique of electric field analysis was laborious and had the disadvantage of deteriorating the quality of the tape recording after many passes. Nevertheless, it was the best available technique at the







30
time and was used in the University of Florida Lightning Laboratory from 1977 until 1984.

2.1.2.5 Oscilloscope film recorders

Dumont oscilloscope film recorders were used in Tampa and Gainesville to record the output signals of the Biomation waveform recorders during data acquisition. The Dumont moving film recorder, containing 500 feet of 35mm panchromatic film, is focused in order to record the signals on the oscilloscope face. A red light-emitting diode attached to the perimeter of the oscilloscope face was used to provide time code information. The film speed was approximately 2 inches per second, which was sufficient to separate the traces from individual strokes which are typically tens of milliseconds apart. The 35mm film records were developed by a commercial film lab.

2.1.2.6 Strip chart recorders

Hewlett Packard 7402a strip chart recorders with a

bandwidth of dc to 100Hz were used to record audio thunder signals, slow decay electric fields, pulses indicating streak camera events, and time code signals. These records are valuable since they are also annotated by the observers during a storm in order to preserve information about instrument settings and visual observations.



2.2 Experiments


2.2.1 Tampa Experiment








During the summer of 1979, the Lightning Research

Laboratory of the University of Florida participated in a research project located near Wimauma, Florida, which is west of Tampa, Florida. The lab was composed of a 14-meter trailer, which housed the electric field measuring system, photographic darkroom, support facilities, and a small truck, located adjacent to the main trailer which contained the camera systems, as shown in Figure 2.1.

A Beckman and Whitley streak camera was used to record streaked photographs of lightning. The camera system consisted of a model 351 streak camera, a 35mm still camera, a television camera with monitor, an audio recorder, and various equipment to provide timing and data logging. The streak camera had a modified face plate which accepted two collimating lenses, thus producing two images on the film, streaked in opposite directions, for each event. The streak camera configuration with the television camera removed is shown in Figure 2.2.

A closed circuit television network was operated in 1979 in order to locate the lightning ground contact points. Five remote cameras were used in addition to the camera in the small truck. The cameras covered an area of approximately 60 km2.

Electric field measurements were made using three

flat plate antennas mounted on top of the 14-meter trailer shown in Figure 2.1 . One of the antennas had an area of

0.5 m2 and the other two had areas of 0.2 m2. North-South

























Figure 2.1


Photograph of the two research vehicles at the Tampa, Florida site. The large trailer on the left housed the electric field recording equipment while the small truck housed the streak camera experiment.











I




it


pi


'.!1


I-


LI


AIL























Figure 2.2


Beckman and Whitley streak camera. The upper photo shows the still camera and the twin lenses of the streak camera. The white circle on the right contained the photodetector. The lower photo shows the data logging display and associated electronics which were mounted on the mobile cart with the camera.











and East-West components of the magnetic fields were measured by loop antennas. Also during this experiment a deenergized power line was instrumented so that the voltage induced on the line by nearby lightning could be measured. An Ampex FR-1900 instrumentation recorder was used to record the electric field signals at a speed of 120 ips. Direct or fm channels were used depending on the frequency range of the particular signal. Figure 2.3 shows the configuration of the electric field recording system in 1979.



2.2.2 Gainesville Experiment


During the summer of 1982 the experiment was located on the thirteenth floor of a dormitory (Beatty Towers) on the University of Florida campus. Two suites were used with one having a window facing south and one facing north. Electric field antennas were mounted on the roof of the dormitory and the electric field recording equipment was fixed in the north suite. The optical systems were portable and could be moved between suites.

Essentially the same streak camera system used in 1979 was used in 1982, with the following differences. Only one lens was used on the streak camera in 1982. The data logging apparatus was modified, for 1982, to use a microprocessor to perform most of the functions previously accomplished with discrete logic.



















Figure 2.3


Diagram of the electric field recording system used in 1979. Two electric field antennas were connected to integrators having slow decay (5 sec) with the other having a fast decay (1 msec). Crossed-loop magnetic field antennas were connected to fast decay integrators. A capacitive voltage divider (V) was used to measure the voltage on a deenergized power line. Electric field signals were digitized by Biomation recorders and displayed on dual-beam oscilloscopes. Electric fields with various gains and analog time code signals were recorded on an Ampex FR1900 instrumentation recorder. Thunder, time code and slow decay electric fields were recorded on Hewlett Packard strip chart recorders.









SE








High speed streak cameras, which rotate more than one revolution during the time between strokes, cause the images from successive strokes to appear out of stroke order on the film. Previous analysis techniques required that electric field records or some form of accurate measurement (� 0.1ms) of the time between strokes be used in order to create a template of where the strokes should be found on the streak camera film. The template was formed by assuming an arbitrary starting position for the first stroke then marking the positions of the successive strokes by knowing the speed of the film and the time between strokes. A time code system was developed, for the 1982 experiment, which was capable of imaging an array of light sources onto the film adjacent to each streak image. The light sources were pulsed for 1 microsecond in order to put the time of the event and the stroke number of the event onto the film adjacent to the actual streaked image. This was a major improvement in the camera system since the sequence of strokes could be more easily indentified.

Electric fields were recorded from 3 flat plate

antennas, one with an area of 0.5 m2 and two with areas of

0.2 m2. The electric field signals were recorded on an Ampex FR3010 instrumentation recorder as discussed in Section 2.1.2.3.

A photomultiplier optical detector, which viewed the lightning through 0.2 mm slit, was operated and the resulting light waveforms were recorded simultaneously


























Figure 2.4


Diagram of the electric field and light recording system used in 1982. The instrumentation recorder (ITR) was an Ampex FR3010. The antennas were connected to integrators with slow (5 sec) and fast (1 msec) decay constants. The photomultiplier tube (PMT) was followed by two stages of gain (GI, G2). Time code was IRIG-B analog signal which was recorded on the ITR and a slow code digital signal which was recorded on the strip chart. Thunder was recorded on the strip chart along with slow decay electric fields.




















777








with the electric field signals on the FR3010. Ganesh et al. (1984) discuss the results of the photomultiplier portion of the experiment.


2.2.3 Data from Orville and Idone


Data have been analyzed in this thesis which were

obtained by Dr. Vince Idone and Dr. Richard Orville of the State University of New York at Albany. The data analyzed were taken during two different years.

In 1978 the SUNYA group made measurements at the Kennedy Space Center in Florida using a Beckman and Whitley streaking camera very similar to the one used by the University of Florida Lightning Laboratory. The Beckman and Whitley camera was operated with 2 collimating lenses and using #92 filters. The filters blocked almost all sunlight while passing the bright hydrogen line (Ha) at 6563 nm emitted by lightning. These filters were used to reduce the fogging of film due to background light since the measurements were made in daylight. The film used was 5474 Shellburst with a gray base.

In 1984 the SUNYA group made measurements in Oklahoma using the same streak camera in the same configuration but without filters. The film used in 1984 was 5474 Shellburst with an estar base which increased the mechanical strength of the film.








2.3 Photographic Analysis


2.3.1 Optical System Calibration


Any optical system used to measure the relative light intensity which caused a change in film density must be calibrated in order to understand and, if necessary, correct the inherent errors of the system. Most important would be system characteristics which cause nonuniform brightness across the film plane when imaging a uniform source. The system used in the measurements for this thesis consisted of two distinct subsystems. The first is the camera apparatus, with its lenses and mirror, which collects the light and focuses the lightning image onto the film. The second is the photographic film which exhibits a nonlinear change in film density in response to incident light intensity.

2.3.1.1 Camera system calibration

Optical systems have errors which can produce

distortion and nonuniform brightness of the focused image at the film plane. Fortunately, because of the sophisticated computer modeling used in the design of lens optics, the distortion present in the lenses used in this study is negligible.

There are system errors which are always present in optical elements. The first of these is due to the fact that the amount of light which passes through a circular








disk varies with the orientation of the disk relative to the source. This is known as the cos4 law since the brightness at the image plane varies as the cos4 of the angle from the lens center to the source location. The errors from this effect can be calibrated using the position of any image point relative to the center of the film frame. For the streak camera only the vertical offset can be determined from the streak camera record alone. If a still photograph was obtained, then the position of the entire lightning channel relative to the lens center can be determined. The data used in this thesis were very nearly in the center of the image frame and subtended angles less than 200. The cos4 of these angles is greater than 0.9 which introduces an error of less than 10 percent to the brightness of the channel at the film plane.

Vignetting is another system error which is a

function of the angle of the source from the lens center. This error is introduced by the physical contruction of the lens and results from the obstruction, by the lens body, of a portion of the bundle of rays which enters the front of the lens before it can exit the rear of the lens. This effect causes a circular bundle of rays to be reduced to a cat's eye pattern at the rear of the lens. The difference in the cross sectional areas of the two patterns results in a reduction in brightness at the film plane. The effect of vignetting is reduced significantly, even eliminated, if the lens aperture stop is small enough








so that the bundle of rays which passes through the aperture can exit the rear of the lens without obstruction. The streak camera objective lens, a 50mm Nikkor, was operated at an aperture setting of f8. At this aperture setting there is no obstruction of the ray bundle until the off center angle excedes 300.

A simple experiment was performed on the streak

camera lens system to determine if vignetting was present. A pinhole source was placed at various positions relative to the center of the lens while remaining in the focal plane at the rear of the collimating lens. The pinhole source produced a circular bundle of rays which exited the front of the objective lens and was focused on a screen. Over the range of off center angles which the lightning data subtended on the film the bundle did not become measureably non-circular.

Additional confirmation of a lack of vignetting is

found in Figure 3.90 which shows relative light intensity 30 psec after the return stroke peak relative light intensity as a function of height to be approximately constant with height. If vignetting were present, it is extremely unlikely that the variations in channel relative light intensity for the strokes presented would decay in such a way as to compensate for the vignetting and produce a uniform value with height.

Additional nonuniformities in image brightness can occur as a function of the angle of incidence of the ray









bundle at the lens surface due to the angle of the source from the lens center. Considering the curvature of the front lens surface, the small aperture stop, and the small off center angles of the data, the difference in angle of incidence for any ray bundle would be minimal. Therefore, the errors due to angle of incidence with the lens surface should be negligible.

2.3.1.2 Film calibration

Photographic film is difficult to use for photometric analysis of optical events since the photographic process does not produce a linear change in film density with respect to incident light intensity. A sensitometric analysis of the film records, as a function of absolute incident light intensity, was not possible since it requires extremely controlled methods of film development as well as optical radiation standards which were not available for this experiment. In this thesis all measurements are in units of relative light intensity.

The measure of film density is in units of diffuse density. Kodak (1973) defines diffuse density as



density = logl0 (Po/Pt)

or

= logl0 (1/T)


where Po is the incident energy, Pt is the transmitted energy, and T is the transmittance of the developed film








image. Measurements of diffuse density assume that the device which collects the light which is transmitted through the film has a collection angle of 1800. If the collection angle is less than 100 the density measured is specular density. Specular density always appears larger than the diffuse density, when measuring photographic film, due to scattering of light as it passes through the film. The light scattered out of the path of the collector is assumed not to be transmitted by the film and is attributed to a higher film density. The microdensitometer used to measure the data for this thesis measured specular density.

In order to determine the exposure, which is the

amount of energy reaching the film surface, that produced a corresponding change in diffuse density it is necessary to produce a plot of film density as a function of exposure. A calibration curve for a batch of film is produced by exposing a section of film to a light source through calibrated densities. A density wedge is a strip of transparent substrate which has had precise amounts of material deposited on its surface in such a way as to produce sections of uniform density which increase in density along the length of the strip. A calibrated density wedge may be purchased from the manufacturer along with precise measurements of the diffuse density in each wedge section. If a uniform light source illuminates the








wedge, the amount of light transmitted through any section of the wedge may be computed by using the following.


Pt = Po / 10D


This expression shows that at D = 0 all of the incident light is transmitted. At D = 1, one tenth of the incident light is transmitted. For a calibration wedge with densities from 0 - 3 D the light transmitted would vary by three orders of magnitude. If the incident energy is not known, the range of relative light intensities provided by the calibration wedge can still provide a series of exposure steps whose relative magnitudes are well understood. The relative magnitudes of exposure steps were used to calibrate the film analyzed in this thesis.

The original density calibration for the data obtained during the Tampa experiment in 1979 was accomplished by exposing strips of the Kodak 5474 Shellburst film through an uncalibrated step wedge to various exposures from a photographic lamp used in the exposure of photographic prints. The film strip with the largest range of densities and the step wedge were then digitized using the Optronics microdensitometer at the Data Analysis Facility at Kennedy Space Center. It was not understood at that time that the measured density values were specular densities, not diffuse densities. A plot was produced of relative exposure versus specular density and








the resulting plot of relative light intensity as a function of specular density was used to produce the results discussed in Jordan and Uman (1983). The original calibration values have been corrected for diffuse density using calibrated step wedges and were found to produce similar results to those using the current calibration values with the exception of an exaggeration of the highest relative light intensity values, such as the peak relative light intensity of the brightest stroke of the 2246:45 UT flash on 79208 as seen in Figure 3.47.

An independent calibration was performed for the 1979 data by using a General Instruments Strobotac strobe to expose a section of film in the streak camera to a 3 psec pulse while monitoring the light with a photodiode detector. The results of this calibration were presented in Jordan and Uman (1983) and showed that the film correctly reproduced the shape of the light signal over the range of relative light intensities produced by the strobe. This provided additional confidence in the original calibration over a range of relative light intensities inclusive of the relative light intensities of all of the data with the exception of stroke "C" in Jordan and Uman (1983).

Figure 2.5 shows diffuse density and specular density as a function of incident relative light intensity for a strip of Kodak 5474 Shellburst film which was exposed, by Dr. Vince Idone at SUNYA, to a 3 gsec pulse of light from















SPECULAR DENSMITY IS) DIFUSE DENSITY (0)


/


EDE FIL F IL
DIFFUSE SPECULA;


.1.

2.0
.L /


I

'i/

/



7/ '


0 2.0 1..0 0.
CALIBRATION WEDGE DENS TY
10 100 10
PRLATIVE LIF" UI:TS


Figure 2.5


Film calibration curve produced by Idone and Orville in 1984 showing relative light intensity versus film diffuse density and specular density of the same region as measured by the microdensitometer.


.

1








a General Instruments Strobotac xenon strobe after passing through a calibrated wedge. The density of each of the resulting image steps was determined using a diffuse densitometer with a 50 Am spot size. The figure shows that the film has little sensitivity below 10 relative light units but is approximately linear from 10 to 1000 relative light units.

The calibration of the Kodak 5474 Shellburst film by SUNYA is the most thorough calibration made to date for the conditions of exposure and development used by both SUNYA and UF. Since the same film and development conditions were used in Gainesville in 1982 as were used in Tampa in 1979 no additional calibration was performed and the SUNYA conversion factor from density to relative light intensity is used throughout this thesis.


2.3.2 Film Digitization


All of the images for this thesis were analyzed by displaying and manipulating digital images which were digitized using an Optronics Photomation microdensitometer. The Optronics microdensitometer is capable of digitizing a 9 x 9 inch transparency at a spatial resolution of 12.5 Am with 8 bits of density resolution over a selectable scale of either 0 - 2 or 0

3 diffuse density units. The microdensitometer consists of a set of selectable upper apertures, illuminated by a








tungsten light source, which are focused onto the film plane. The image of the aperture is then focused onto a second set of apertures with the light passing through the aperture impinging on a photomultiplier tube. A logarithmic amplifier is used to convert the ratio of the incident light to the transmitted light into units of specular density. The value of incident light is sampled and stored once during each revolution of the drum by sensing the light source through a zero density hole in the drum.

2.3.2.1 Microdensitometer alignment and calibration

It is necessary to align the mechanical carriage of the microdensitometer to insure that valid density values are measured. Mechanical alignment consists of adjusting the position of the light source with respect to the upper aperture and adjusting the relative positions of the upper and lower apertures. The light source position is adjusted such that the image of the light source projected through the upper aperture onto the film is uniform and out of focus. This is accomplished solely by varying the position of the light source. The upper and lower apertures are aligned by iteratively adjusting the rotation and translation of the lower objective lens assembly such that maximum light is received at the photomultiplier tube. The anode voltage of the photomultiplier tube is adjusted after the








mechanical alignment to compensate for the aging of the tube.

In order to produce valid measurements it is also necessary to calibrate the logarithmic amplifier. The logarithmic amplifier converts the ratio of incident light to transmitted light into units of specular density. Calibration is accomplished using several values of neutral density wratten filters. Wratten filters are used since they scatter very little light thereby having a measured specular density very near the actual diffuse density. The logarithmic amplifier is first adjusted to have zero density with nothing in the light path. The amplifier is then adjusted with a 3.0 D filter in the path so that the measured density reads 255 on the 8 bit scale. This procedure was repeated prior to every digitizing session.

2.3.2.2 Microdensitometer interface and software

The microdensitometer is attached to an IBM/AT

class personal computer through a custom interface. The interface has an 8 bit bidirectional bus and control lines implimented with a modified Tecmar 96 line digital I/O board. Custom software was written to control the microdensitometer. The software provides functions to step the position the carriage, digitize a region of film, and place the results into a disk file on the personal computer. The software provides the capability of averaging the image to be digitized a number of times.








This slows the digitizing process but is very effective in reducing the random noise of the photomultiplier and amplifier. All of the data used in this thesis were averaged 8 times.



2.3.3 Image Display Techniques



Digitized data images were manipulated and

displayed using a software system referred to as the Earth Resources Laboratory Applications Software (ELAS).

ELAS was written at the Earth Resources Laboratory of the National Aeronautics and Space Administration at Bay St. Louis, Mississippi. Elas consists of approximately 240 software modules which manipulate and perform calculations on images. The image display modules and polygon manipulation modules were used extensively to analyze the data in this thesis.

As Engineering Manager of the University of Florida Remote Sensing and Imaging Processing Laboratory during the period of 1982-1987, I assisted Mr. Ray Seyfarth in converting the Perkin Elmer version of Elas to the personal computer environment. The software drivers were written to provide ELAS display support for the NUMBER NINE Computer Corporation's 512 x 512 x 32 bit display board. This board provides four 8 bit images arranged as three color images with 8 graphic overlays.








2.3.4 Computer Analysis Software



Software was written for the personal computer, in

addition to the ELAS software, to perform measurements on the digitized data while viewing the data on the monitor. This program was written so that it would run concurrently with ELAS. This configuration allows the program to communicate, using software interrupts, with the root portion of ELAS which controls the positioning of the cursor on the display device. This configuration allowed locations of measurements on the personal computer graphics screen to be reflected in the actual cursor position on the displayed image. The analysis software can display data from one or two-dimensional arrays. The program expects data in standard ELAS data file format and is capable of reading and modifying the ELAS header information.

The software analysis system contains several

sections. The main section allows the user to select a data file to analyze and shows a plot of the first horizontal line of the image on the personal computer screen. The user may display any line in the file or sequence through the lines with a suitable increment between lines and with variable horizontal and vertical magnifications. Horizontal and vertical markers are provided, to assist in measuring differences in position along both axes, and their current values as well as the








values of other pertinent parameters are available on the display screen. A complete plot package is implemented which generates plots with variable parameters in Hewlett Packard Graphics Language format.


2.3.5 Measurement of Relative Light Intensity
Versus Time

Measurements of light intensity as a function of time are accomplished using the personal computer software system. The digitized image of the event of interest is accessed and the line of data corresponding to the desired height is displayed. The time scale for the image is stored in the file header in microseconds per sample. This allows the program to provide a horizontal time scale. Events of interest may be measured directly with the markers or the user may generate a plot of the displayed region of data.


2.3.6 Measurement of Relative Light intensity
Versus Height

Measurement of light intensity as a function of height is more complex than the measurement of light intensity as a function of time since the path along which the measurements are to be made must be delineated. A new module was written for ELAS in order to exploit the line drawing capabilities already implemented. The module is named DENT and works in conjunction with CPPP and POLY.








The function of DENT is to pick out the relative light intensities of an image which fall along a line. The user may choose to specify the exact line path or an approximate path. If an approximate path is specified, the program will examine a range of light intensities near the line and pick the peak in the surrounding data as the new point on the line. This feature enables automated tracking of features such as dart leader or return stroke peak relative light intensity as a function of height. A region may also be specified a certain distance from the line over which the program is to compute the average of the data values. This feature allows an average background density value to be computed for each height. The file produced by DENT is an ascii text file containing one line describing the data file followed by lines containing the vertical line, horizontal element, average of the specified region before or after the polygon, and the value of the data point along the polygon or the peak near the polygon. This format is used for all data files with an extension of .DNT.

A display of relative light intensity as a function

of height is obtained by starting the MDP program with the /D option. This option will display a list of available .DNT files and allow the user to select one. The profile of the relative light intensity will then be displayed as a function of height.








2.3.7 Measurement of Leader Speed


Leader speed is determined by measuring the

displacement of the initial leader light intensity from the leading edge of the return stroke light intensity at a known height along the channel. The ratio of the height in meters to the displacement in time yields the leader speed. This measurement is a reasonable approximation to the actual two-dimensional leader speed if the return stroke speed is assumed to be at least an order of magnitude higher than the leader speed since the measured displacement includes the time for the leader to reach ground and the return stroke to travel back up the channel to the given height. For typical return stroke speeds the error in the dart leader speed due to this measurement technique is approximately 10 percent.



2.4 Electric Field Analysis


The methods by which the electric field records in this thesis were analyzed will be discussed in the following sections. Electric field records are presented from Tampa, Florida in 1979 and from Gainesville, Florida in 1982.


2.4.1 Electric Field System Calibration








Electric field calibration involves the determination of the response of the electric field system to a known vertical electric field applied to the antenna and the measurement of the enhancement factor for the measurement location with respect to ground level. At ground level the flat plate antennas used in these experiments have an enhancement of unity. A detailed discussion of the calibration of the 1979 data is found in Master (1981). Briefly, the enhancement factor was determined by placing an antenna on the ground and simultaneously recording the values obtained on the antennas on the truck and the ground. The enhancement value in 1979 was 2.65. In 1982 the electric field system was calibrated in a manner similar to that used in 1979 with the exception of the determination of the enhancement factor. The enhancement factor for the antenna located on the roof of Beatty Towers was determined by using the average initial peak electric fields of 38 subsequent strokes and setting the average equal to that obtained by Master (1981) from the Tampa data. The enhancement of the 1982 electric field measurements was thus determined to be 5.


2.4.2 Electric Field Digcitization


In 1984 the Lightning Laboratory acquired a Masscomp minicomputer with an analog to digital converter system capable of digitizing several seconds of data at a 500 kHz








rate directly to disk. The effective rate of the digitizing process could be increased to over 5 MHz if the instrumentation tape recorder speed is slowed while digitizing. This system was used exclusively for analysis of the electric field records analyzed in this thesis.

Digitization was accomplished by connecting the

proper channels of the instrumentation recorder to the digitizer and running software on the Masscomp computer which controlled the digitizer. In most cases the sample and hold circuits of the digitizer were used in order to sample several channels simultaneously then multiplex the digital values into the disk file.

The frequency modulated channels of the instrumentation recorder were used almost exclusively so that the playback rate could be slowed down by a factor which would allow several channels of an entire flash to to be sampled and stored to disk. The data were recorded onto the FM channels of another instrumentation recorder before digitizing if the direct channels of the data were of interest. This process reduced the high frequency cutoff of the data from 1.5 MHz, that of a direct channel, to the 500 KHz cutoff of an FM channel. Use of FM channels for digitization was necessary in order to preserve the low frequency portion of the waveshape of the signals when the recorder speed was slowed.

Data were normally sampled at 78.125 kHz if they were played back at 1/64 original speed, equivalent to sampling








the data at 64 x 78125 = 5 MHz in real time. This was an effective sampling rate of 0.2 Asec/sample. Since the response of the recorder was -6 db at 500 kHz, this procedure was equivalent to sampling at 10 times the corner frequency. The sampling rate was above the Nyquist rate for frequencies up to 2.5 MHz. The recorder had negligible frequency response above 2 MHz. Typically, approximately one second of real time data was digitized. The main advantages of the method described are the ability to digitize at high rates for long periods of time and the ease of display and measurement on the Masscomp as discussed below.


2.4.3 Electric Field Analysis Software


The electric field data which were digitized to disk were at first cumbersome to display and plot. The Masscomp computer had general purpose software to plot simultaneous sets of data but it proved too slow for serious analysis purposes. Two new programs were written to facilitate analysis of electric field records.

2.4.3.1 Display software

In order to rapidly display, analyze, and plot

electric field records a software program was developed which exploited hardware available on the Masscomp. The program is very general in that it allows the user to view multiple channels from different data files (up to 6).








Channels of data are presented as time on the horizontal axis versus digitizer units on the vertical axis. Data may be shifted relative to other channels as well as expanded together or separately.

The program uses the array processor of the Masscomp to scale the data onto the screen. This technique bypasses the graphics processor scaling algorithms and allows multiple channels of 5000 points each to be displayed rapidly. The disk I/O rate was a problem for reading large data files (typically 20-40 MB) and necessitated the use of low level file routines in the C language to read large buffers of data.

A plotting package was designed into the system so that a user could produce a hard copy of the screen contents at anytime to either a Gould or Hewlett Packard plotter. The plotting package was linked to a system routine which would continuously plot all available plots asynchronously. Figures 2.6 and 2.7 show examples of electric field plots for the flash at Tampa on July 27, 1979 occurring at 2206:51 UT at a distance of 7.8 km.

2.4.3.2 Averaging software

Many of the data in the electric field records are noisy and must be smoothed in order to find features of interest. Software existed to average a single channel data file but it took over 8 hours to average one second of data. This program was rewritten to use large file buffers and large arrays with efficient looping. The








result was to reduce the execution time from 8 hours to 9 minutes for one second of data (10 MB). The program was modified to accept data with any number of channels and has a user interface consistent with the display program.



2.4.4 Measurement of Interstroke Time Intervals



Interstroke intervals were measured by plotting the

interstroke electric field change on a suitable scale then measuring the time difference between the fast portions of the return stroke electric field change. Figure 2.7 demonstrates the measurement of the interstroke interval between strokes 2 and 3 of the flash on 27 July 1979, 2206:51 UT.


2.4.5 Measurement of Return Stroke Initial Peak Electric

Field



Figure 2.9 demonstrates the technique used to measure the initial peak electric field for stroke 6 of the 27 July 1979, 2206:51 UT. The initial peak electric field must be measured from data with sufficient high frequency response to preserve the peak waveshape. For the data in this thesis the Biomation records taken from the direct recorder channel were used for peak field measurements.


2.4.6 Measurement of Leader Field Change Duration








Figure 2.8 demonstrates the method of determining the duration of the leader field change for stroke 6 of the 27 July 1979, 2206:51 UT flash. Leader field change durations are often difficult to measure due to the noise level of the data and the shape of the beginning of the field change which is the most difficult to identify. The leader field change terminates with the beginning of the return stroke field change which is easily identified.


2.4.7 Measurement of Ratio of Leader Field ChanQe to Return
Stroke Field Change


The ratios of leader electric field change to return stroke electric field change were measured in the manner described by Rakov et al. (1990). As illustrated in Figure 2.8, a line was drawn with the slope of the field change preceding the dart leader. This J field change slope is assumed to continue throughout the leader and return stroke field change (Krehbiel et al., 1979). The dart leader field change terminates at the beginning of the return stroke field change. If the return stroke field change after the initial peak exhibited a "knee" shape, as discussed by Beasley et al. (1982), then the time of occurrence of the "knee" was considered as the end of the return stroke field change. If the return stroke was followed by a continuing current field change then the position of the first M-component "hook" shaped field








change was taken as the end of the return stroke field change.

Thomson (1985) investigated possible errors in

previous techniques for measuring leader-return stroke electric field change ratios used by Schonland et al. (1938) and Beasley et al. (1982). In this thesis we adopt those techniques since the measurements of leader-return stroke ratio presented were made only in order to compare ratios as a function of stroke order. Thomson (1985) stated that two conditions must be met for one to interpret the leader-return stroke ratio in terms of the charge distribution along the leader channel. First, both field changes must be the electrostatic components of the field changes. Second, leader and return stroke field changes must be the result of the same charge which is the charge deposited along the channel by the leader then lowered to ground by the return stroke. He indicated that, if the correct beginning point for the dart leader and ending point for the return stroke are not determined, the measured leader-return stroke ratio may contain significant errors due to the contributions of the field changes of the uniform current assumed in the model of Lin et al. (1980) or from continuing current flowing at the end of the return stroke. He suggested that the field 170 Asec after the start of the return stroke be used as the ending point, again assuming the model of Lin et al. (1980), and that the contribution of the field change due





66



to the uniform current, which is assumed to be flowing in the channel, be subtracted in order to obtain a reasonable return stroke electrostatic field change.







































TIME --


Figure 2.9


Electric fields for stroke 6 of the 27 July 1979, 2206:51 UT flash showing the measurement of initial peak electric field.















_�
Tapa l~gSithStok Retum !5"oUe




rkv " oe v "v� V 1) '1 Lj Rakum :mne Leader Duration.,.
1.1 ms

0.83 Vin






*0.5 ms


TIME -


Figure 2.8


Electric fields preceding stroke 6 of the 27 July 1979, 2206:51 UT flash showing the measurement of dart leader duration.


Tampa, 1979


Sixth Stroke


2206*5 U ILT

























w
0 C)
-j

C) V M
u















Figure 2.7


TIME -


Overall electric field change for strokes 2 and 3 of the flash on 27 July 1979, 2206:51 UT showing the measurement of the interstroke interval.












































Figure 2.6


2206:51 UT


TIME --


Overall electric field change for the 27 July 1979, 2206:51 UT flash which had 16 strokes with 9 recorded on the streak camera.














CHAPTER 3
DETAILED ANALYSES AND RESULTS


3.1 Database


The strokes which are discussed in the following sections are listed in Tables 3.1 and 3.2. The data acquired by the University of Florida Lightning Research Laboratory include two flashes from the Tampa experiment in 1979 and three flashes from the Gainesville experiment in 1982. The data consist of one new channel to ground which is the third stroke of a flash and 22 subsequent strokes with 11 detectable dart leaders, all correlated with electric fields. The data on loan from the State University of New York at Albany contain 11 first strokes and 10 subsequent strokes with 5 detectable dart leaders. No electric fields were recorded in the SUNYA experiments.

Prior to discussions of properties of first and

subsequent strokes it is useful to explore relationships which exist within individual flashes. Dart leader field change duration, previous interstroke interval, initial peak electric field, and the ratio of leader to return stroke electric field change have been measured within five flashes and their values plotted versus stroke order.












Table 3.1


Summary of Stroke Parameters Derived from Correlated Optical and Electric Field Records


Storm Time Distance Stroke Return Stroke Previous Year/Day (UT) km Order Initial Electric Interstroke Field Peak Interval at 100km V/m ms


79208 2206:51 8.2







79208 2246:45 8.8 82202 1317:14 -6
(EDT)





82222 1445:35 ~5.3
(EDT)

82222 1446:56 -6


2**
3
4
5
6
7


223*
58 94 38 43 35 44


4.8 2.3
2.1 4.5 2.6 2.7 3.3

3.1 6.8

4.9 5.1 2.2
2.1 1.7 3.8

5.9 6.5

4.7 4.7 5.5 1.9 3.5 3.1


2***
3
4
5
6
7


* Includes continuing current of 85 ms duration.
** Dart-stepped leader
*** First stroke along a new channel to ground.
















Leader Leader Leader Return Duration Speed Luminosity Stroke ms (relative Luminosity 106 m/s units) (relative units)


1.9


0.98 1.9 1.6
2.1

0.34 0.22

0.95 0.45
1.8 3.3

4.0

0.15 0.36


12.0 14.6




10.7
17.4 5.4 24.4 9.2


7.9 15.7 17.9



16.6


0.5


9
11
5
-1.5
-1.5
6

57
6.4












Table 3.2


Summary of Stroke Parameters Derived from Optical Records Borrowed from the State University of New York at Albany


Storm Time Distance Stroke #92 Leader Year/Day (UT) km Order Filter Speed 106 m/s


78186 - 78192 1926:45 4.6 78210 2009:57 3.5

2023:46 4.6



2025:50 2.0 2032:10 6.0 2032:51 4.0 2041:00 4.9 2041:00 4.9 84148 1749:02 2.0 84161 2107:20 5.0


YES YES YES YES YES YES YES YES YES YES YES YES YES YES YES


3.8


5.9
5.9


5.2
-


2107:20 5.0








Data for the 27 July 1979 2206:51 UT flash are

presented in Figure 3.1. This flash had sixteen strokes of which seven subsequent strokes were imaged by the streak camera. The first stroke had a double termination to ground as seen on the television records. The second stroke initiated a long continuing current (about 85 msec duration). Only stroke six produced a detectable dart leader on the streak film. Stroke six had the shortest dart leader field change duration and the shortest previous interstroke interval of the imaged strokes. It also had the second largest return stroke initial peak electric field. Stroke three, which followed a long interstroke interval (223 msec, including about 85 msec of continuing current), had a slightly larger initial peak electric field than the sixth stroke. Analysis of electric field records from 1979 showed pulses typical for dartstepped leaders prior to the return stroke (Krider et al., 1979).

Data for the 27 July 1979 2246:45 UT flash are shown in Figure 3.2. This flash had four strokes of which two were imaged by the streak camera. Stroke two produced a weak dart leader and had a relatively small return stroke electic field peak. Stroke three had a very bright dart leader which was 1.7 times as fast as as the leader of stroke two and was followed by a return stroke with an initial peak electric field which was twice as large as stroke two.












Tampa 79208
Distance


4 IN I 4
S(a)E M
(a)


* Rm IIH-


9 8 4 8II i II t M
STRE W
(c)


220651 UT
8.2 km


-Z

CP4
CD



C.


STROK M


Figure 3.1


Histograms of (a) dart leader field change duration, (b) initial peak electric field,
(c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 27 July 1979, 2206:51 UT flash. Crosshatched strokes had detectable dart leaders. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale.


�4










I


I












Tampa 79208 224645 UT
Distance 8.8 km


I


I t S 4
STME(a) M
(a)


STKE OMER
(c)


STROKE ORDER
(b)

SThM m


a
U(
S[Ii |


Figure 3.2


Histograms of (a) dart leader field change duration, (b) initial peak electric field,
(c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 27 July 1979, 2246:45 UT flash. Cross-hatched strokes had detectable dart leaders. The ratio of leader-return stroke electric field change was not measurable for strokes 2 and
4. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale.


0.1


I:








Data for the 21 July 1982 1317:14 EDT flash are shown in Figure 3.3. This flash had seven strokes and all but the first stroke were imaged by the streak camera. Four of the seven subsequent strokes produced detectable dart leaders. Stroke three had the fastest dart leader (2.44 x 107m/sec) of all the data which were analyzed. It had the largest return stroke initial peak electric field and the shortest dart leader duration of the flash. The second stroke also had a large initial peak electric field but was preceded by a dart-stepped leader.

Data for the 10 August 1982 1445:35 EDT flash are

shown in Figure 3.4. This flash had three strokes and the two subsequent strokes were imaged by the streak camera. Both of the subsequent strokes produced highly visible dart leaders on the streak film. The measured dart leader durations for both subsequent strokes were unusually short with the duration for stroke three being twice as long as stroke two even though they had practically the same dart leader speeds below the cloud.

Data for the 10 August 1982 1446:56 EDT flash are shown in Figure 3.5. This flash had seven strokes total along two channels to ground. The subsequent stroke for the first channel as well as all strokes for the second channel were imaged by the streak camera. Only the second stroke along the second channel produced a detectable dart leader. The electric field records for this flash were very noisy making it impossible to measure












Gainesville 82222 13�714 UT
Distance 6 km


S2 3 4 5 7

(a)


it



2


1 3 5 4 9- 1 7 STMW WWR
(b)

ST W M
1 2 3 4 5 8 7


STMKE OMER
(c) (d)


Figure 3.3


Histograms of (a) dart leader field change duration, (b) initial peak electric field,
(c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 21 July 1982, 1317:14 EDT flash. Crosshatched strokes had detectable dart leaders. Leader electric field properties for stroke
6 were not measurable. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke


uI
I












Gainesville 82222 i44656 UT
Distance 5 km o,


Leader duration is not ieasurable U

(a)


tflf


STM Om


(EL./AERJ is not m durable
(d)


V. 9* 1 2 3 4 5
STRKE O


Figure 3.5


Histograms of (a) dart leader field change duration, (b) initial peak electric field,
(c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 10 August 1982, 1446:56 EDT flash. Crosshatched strokes had detectable dart leaders. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale.


Is




Full Text

PAGE 1

RELATIVE LIGHT INTENSITY AND ELECTRIC FIELD INTENSITY OF CLOUD TO GROUND LIGHTNING By DOUGLAS MAX JORDAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1990

PAGE 2

ACKNOWLEDGEMENTS I wish to begin by stating that without the support of my wife Beth I would have never completed this effort. Her understanding and compassion have been a comfort and an inspiration. I am grateful to Ross, Clifford, Julia, and Kim for their sacrifices in support of my work. Dr. Martin Uman is the type of person one meets all too infreguently in life. Martin saw potential in me which I often did not see myself. It has been an honor to be guided and befriended by a man of such character and intellectual abilities. Most of all, Martin taught me how to think as a scientist and instilled in me the desire to discover. I would like to thank Dr. William Beasley, who has been a friend and teacher for many years and was always there to remind me that it was worth the effort. Bill's scientific guidance and sense of humor were crucial throughout the experiments and analyses which culminated in this thesis. I would like to express my appreciation to Dr. Vladimir Rakov, my Russian friend, for sharing with me his exceptional insight. His friendship and stimulating conversation were a pleasure. His assistance in motivating me to complete this effort has been invaluable. 11

PAGE 3

I also would like to thank Dr. Richard Orville and Dr. Vince Idone for loaning me their data for analysis. Mr. David Peckham's suggestions were very helpful while I was repairing the optical eguipment and his free consulting saved many hours of work. I owe a special debt of gratitude to Dr. James Davidson and the Institute of Food and Agricultural Sciences for loaning me much of the eguipment which was used to analyze the data presented in this thesis. 111

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TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ii LIST OF FIGURES vi LIST OF TABLES xvii ABSTRACT xviii CHAPTER 1. REVIEW 1 1 . 1 Introduction 1 1 . 2 Lightning 1 1.2.1 Stepped Leader 2 1.2.2 First Return Stroke 4 1.2.3 Dart Leader 5 1.2.4 Subsequent Return Stroke 5 1.2.5 M-component 6 1 . 3 Photographic Measurements 6 1.4 Correlated Measurements of Electric Fields and Relative Light Intensity. 16 2. UF AND SUNYA EXPERIMENTS AND GENERAL ANALYSIS TECHNIQUES 22 2 . 1 Experimental Techniques 22 2.1.1 Photographic Techniques 22 2.1.2 Electric Field Measurement Techniques 25 2 . 2 Experiments 29 2.2.1 Tampa Experiment 29 2.2.2 Gainesville Experiment 35 2.2.3 Data from Orville & Idone 41 2 . 3 Photographic analysis 42 2.3.1 Optical System Calibration 42 2.3.2 Film Digitization 50 2.3.3 Image Display Techniques 53 2.3.4 Computer Analysis Software 54 2.3.5 Measurement of Relative Light Intensity Versus Time 55 2.3.6 Measurement of Relative Light Intensity Versus Height 55 IV

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2.3.7 Measurement of Leader Speed.... 57 2 . 4 Electric Field Analysis 57 2.4.1 Electric Field System Calibration 57 2.4.2 Electric Field Digitization 58 2.4.3 Electric Field Analysis Software 60 2.4.4 Measurement of Interstroke Time Intervals... 62 2.4.5 Measurement of Return Stroke Initial Peak Electric Field. . 62 2.4.6 Measurement of Leader Field Change Duration 62 2.4.7 Measurement of Ratio of Leader Field Change to Return Stroke Field Change 63 3 . DETAILED ANALYSES AND RESULTS 70 3 . 1 Database 70 3.2 First Strokes 81 3.2.1 Channel Relative Light Intensity Variations 81 3.2.2 Channel Relative Light Intensity Variations Due to Branches 86 3 . 3 Subseguent Strokes 115 3.3.1 Leaders of (Dart leaders) 116 3.3.2 Return Strokes 143 3 . 4 M-components 149 3.4.1 M-component Relative Light Intensity Profiles 150 3.4.2 Determination of the Direction of M-component Propagation. . . 151 4 . DISCUSSION AND CONCLUSIONS 217 REFERENCES 225 BIOGRAPHICAL SKETCH 230 v

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LIST OF FIGURES Figure Page 1.1 Schematic diagram of a lightning ground discharge below cloud, (a) Streak schematic showing, stepped leader, dart leaders, and return strokes, (b) Still photo schematic of entire flash. (Adapted from M.A. Uman, Lightning, McGraw Hill Book Co., 1969) 3 1.2 Diagram of improved Boys camera with moving film and stationary lenses. Adapted from McEachron (1939) 2 . 1 Photograph of the two research vehicles at the Tampa, Florida site. The large trailer on the left housed the electric field recording equipment while the small truck housed the streak camera experiment 32 2.2 Beckman and Whitley streak camera. The upper photo shows the still camera and the twin lenses of the streak camera. The white circle on the right contained the photodetector . The lower photo shows the data logging display and associated electronics which were mounted on the mobile cart with the camera 34 2.3 Diagram of the electric field recording system used in 1979. Two electric field antennas were connected to integrators having slow decay (5 sec) with the other having a fast decay (1 msec) . Crossed-loop magnetic field antennas were connected to fast decay integrators. A capacitive voltage divider (V) was used to measure the voltage on a deenergized power line. Electric field signals were digitized by Biomation recorders and displayed on dual-beam oscilloscopes. Electric fields with various gains and analog time code signals were recorded on an Ampex FR1900 instrumentation recorder. Thunder, time code and slow decay electric fields were recorded on Hewlett Packard strip chart recorders 37 2.4 Diagram of the electric field and light recording system used in 1982. The instrumentation recorder (ITR) was an Ampex FR3010. The antennas were connected to integrators with slow (5 sec) and fast (1 vi

PAGE 7

msec) decay constants. The photomultiplier tube (PMT) was followed by two stages of gain (Gl, G2) . Time code was IRIG-B analog signal which was recorded on the ITR and a slow code digital signal which was recorded on the strip chart. Thunder was recorded on the strip chart along with slow decay electric fields 40 2.5 Film calibration curve produced by Idone and Orville in 1984 showing relative light intensity versus film diffuse density and specular density of the same region as measured by the microdensitometer 49 2.6 Overall electric field change for the 27 July 1979, 2206:51 UT flash which had 16 strokes with 9 recorded on the streak camera 66 2.7 Overall electric field change for strokes 2 and 3 of the flash on 27 July 1979, 2206:51 UT showing the measurement of the interstroke interval 67 2.8 Electric fields preceding stroke 6 of the 27 July 1979, 2206:51 UT flash showing the measurement of dart leader duration 68 2.9 Electric fields for stroke 6 of the 27 July 1979, 2206:51 UT flash showing the measurement of initial peak electric field. . . 69 3.1 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 27 July 1979, 2206:51 UT flash. Crosshatched strokes had detectable dart leaders. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale 75 3.2 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 27 July 1979, 2246:45 UT flash. Cross-hatched strokes had detectable dart leaders. The ratio of leader-return stroke electric field change was not measurable for strokes 2 and 4. Dashed lines indicate ambiguous measurements. Parallel lines indicate vn

PAGE 8

measurements of first stroke parameters which are off scale 76 3.3 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 21 July 1982, 1317:14 EDT flash. Crosshatched strokes had detectable dart leaders. Leader electric field properties for stroke 6 were not measurable. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale 78 3.4 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 10 August 1982, 1445:35 EDT flash. Crosshatched strokes had detectable dart leaders. The leader-return stroke electric field ratio was not measurable for this flash. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale 79 3.5 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 10 August 1982, 1446:56 EDT flash. Crosshatched strokes had detectable dart leaders. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale 80 3.6 Digitized image of first stroke at 1446:56 EDT August 10, 1982. The heights indicated are referenced in the plots of relative light intensity versus time. The leader electric field durations and ratios were not measurable for this flash 88 3.7 Relative light intensity versus time at for the stroke at 1446:56 EDT August 10, 1982.... 89 3.8 Relative light intensity versus time for the lower 500 meters of the stroke at 1446:56 EDT August 10 , 1982 90 v in

PAGE 9

3.9 Digitized image of first stroke at 1926:45 UT July 11, 1978. The heights indicated are referenced in the plots of relative light intensity versus time 92 3.10 Relative light intensity versus time for the stroke at 1926:45 UT July 11, 1978 93 3.11 Relative light intensity versus time for the lower 500 m of the stroke at 1926:45 UT July 11, 1978 94 3.12 Digitized image of first stroke at 2041:00 UT July 29, 1978. The heights indicated are referenced in the plots of relative light intensity versus time 96 3.13 Relative light intensity versus time for the stroke at 2041:00 UT July 29, 1978 97 3.14 Relative light intensity versus time for the lower 500 m of the stroke at 2041:00 UT July 29, 1978 98 3.15 Digitized image of first stroke at 2023:46 UT July 29, 1978. The heights indicated are referenced in the plots of relative light intensity versus time 100 3.16 Relative light intensity versus time for the stroke at 2023:46 UT July 29, 1978 101 3.17 Relative light intensity versus time for the lower 500 m of the stroke at 2023:46 UT July 29, 1978 102 3.18 Digitized image of first stroke at 2032:10 UT July 29, 1978. The heights indicated are referenced in the plots of relative light intensity versus time 104 3.19 Relative light intensity versus time for the stroke at 2032:10 UT July 29, 1978 105 3.20 Digitized image of first stroke at 2032:51 UT July 29, 1978. The heights indicated are referenced in the plots of relative light intensity versus time 107 3.21 Relative light intensity versus time for the stroke at 2032:51 UT July 29, 1978 108 3.22 Digitized image of first stroke at 1749:02 UT May 27, 1984. The heights indicated are IX

PAGE 10

referenced in the plots of relative light intensity versus time 110 3.23 Relative light intensity versus time for the stroke at 1749:02 UT May 27, 1984 Ill 3.24 Digitized image of first stroke at 2107:20.81 UT June 9, 1984. The heights indicated are referenced in the plots of relative light intensity versus time 113 3.25 Relative light intensity versus time for the stroke at 2107:20.81 UT June 9, 1984 114 3.26 Dart leader speed versus stroke order. Solid lines connect strokes which follow in order with detectable leaders. Dotted lines connect strokes of a flash which were separated by strokes with nondetectable leaders. Schonland data point is the mean for 55 leaders 117 3.27 Apparent leader length versus stroke order. Apparent lengths of Krehbiel et al. (1979) were determined from charge source locations for each stroke. Dotted lines bound the data for 163 strokes reported by Brook et al. (1962) 118 3.28 Dart leader speed versus interstroke interval. The dashed line bounds the data of Brook and Kitagawa ( Winn, 1965) for 100 dart leaders. The dashed and dotted lines bound the triggered lightning data of Idone and Orville (1984) 120 3.29 Dart leader speed versus dart leader electric field change duration 122 3.30 Dart leader speed versus return stroke initial peak electric field. The dashed regression is for 32 triggered lightning strokes reported by Idone et al. (1984) . The solid regression line is for the UF data points excluding the leaders for strokes 2 and 3 of 1317:14 UT. Stroke 2 was dart stepped and stroke 3 was the fastest natural dart leader ever measured 123 3.31 Dart leader speed versus dart leader relative light intensity 125 3.32 Dart leader relative light intensity versus return stroke initial peak electric field. Dart leader relative light intensity x

PAGE 11

measurements are taken at a height of approximately 50 m 126 3.33 Dart leader relative light intensity versus leader electric field change duration. Optically Nondetected leaders are not shown for obvious reasons 128 3.34 Dart leader relative light intensity versus duration of previous interstroke interval.... 129 3.35 Dart leader relative light intensity versus return stroke relative light intensity 131 3.36 Dart leader relative light intensity versus height for stroke 3 of the 2246:45 UT flash on July 27 , 1979 134 3.37 Dart leader relative light intensity versus height for stroke 2 of the 1445:35 EDT flash on August 10 , 1982 135 3.38 Dart leader relative light intensity versus height for stroke 3 of the 1445:35 EDT flash on August 10, 1982 136 3.39 Dart leader relative light intensity versus height for stroke 2 of the 1446:56 EDT flash on August 10, 1979 137 3.40 Dart leader speed versus return stroke relative light intensity. Return stroke relative light intensity is measured at a height of approximately 50 m 138 3.41 Dart leader peak relative light intensity versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot 139 3.42 Dart leader relative light intensity at plateau after peak versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot 140 3.43 Dart leader rise time versus height. The system time resolution was approximately 0.5 /xsec which produced the effect seen in the rise time data 141 3.44 Dart leader relative light intensity duration versus height. Regression lines for the XI

PAGE 12

individual strokes are identified by the symbols to the right of the plot 142 3.45 Dart leader electric field change duration versus return stroke relative light intensity 144 3.46 Duration of previous interstroke interval versus return stroke relative light intensity 146 3.47 Return stroke initial peak electric field versus return stroke relative light intensity 147 3.48 Digitized image of stroke 3 of the flash at 2206:51 UT July 27, 1979. The heights indicated are referenced in the plots of relative light intensity versus time 155 3.49 Relative light intensity versus time for stroke 3 at 2206:51 UT July 27, 1979 156 3.50 Relative light intensity versus time for the lower 500 m of stroke 3 at 2206:51 UT, July 27, 1979 157 3.51 Digitized image of stroke 6 of the flash at 2206:51 UT July 27, 1979. The heights indicated are referenced in the plots of relative light intensity versus time 159 3.52 Relative light intensity versus time for stroke 6 at 2206:51 UT July 27, 1979 160 3.53 Relative light intensity versus time for the lower 500 m of stroke 6 at 2206:51 UT, July 27, 1979 161 3.54 Digitized image of stroke 9 of the flash at 2206:51 UT July 27, 1979. The heights indicated are referenced in the plots of relative light intensity versus time 163 3.55 Relative light intensity versus time for stroke 9 at 2206:51 UT, July 27, 1979 164 3.56 Relative light intensity versus time for the lower 500 m of stroke 9 at 2206:51 UT, July 27, 1979 165 3.57 Digitized image of stroke 2 at 2246:45 EDT July 27, 1979. The heights indicated are XII

PAGE 13

referenced in the plots of relative light intensity versus time 167 3.58 Relative light intensity versus time for stroke 2 at 2246:45 UT, July 27, 1979 168 3.59 Digitized image of stroke 3 at 2246:45 EDT July 27, 1979. The heights indicated are referenced in the plots of relative light intensity versus time 170 3.60 Relative light intensity versus time for Stroke 3 at 2246:45 UT, July 27, 1979 171 3.61 Relative light intensity versus time for the lower 500 m of stroke 3 at 2246:45 UT, July 27, 1979 172 3.62 Digitized image of stroke 2 at 1317:14 EDT July 21, 1982. The heights indicated are referenced in the plots of relative light intensity versus time 174 3.63 Relative light intensity versus time for stroke 2 at 1317:14 UT, July 21, 1982 175 3.64 Digitized image of stroke 3 at 1317:14 EDT July 21, 1982. The heights indicated are referenced in the plots of relative light intensity versus time 177 3.65 Relative light intensity versus time for stroke 3 at 1317:14 UT, July 21, 1982 178 3.66 Digitized image of stroke 2 at 1445:35 EDT August 10, 1982. The heights indicated are referenced in the plots of relative light intensity versus time 180 3.67 Relative light intensity versus time for stroke 2 at 1445:35 UT, August 10, 1982 181 3.68 Relative light intensity versus time for lower 500 m of stroke 2 at 1445:35 UT, August 10, 1982 182 3.69 Digitized image of stroke 3 at 1445:35 EDT August 10, 1982. The heights indicated are referenced in the plots of relative light intensity versus time 184 3.70 Relative light intensity versus time for stroke 3 at 1445:35 UT, August 10, 1982 185 Xlll

PAGE 14

3.71 Relative light intensity versus time for lower 500 m of stroke 3 at 1445:35 UT, August 10, 1982 186 3.72 Digitized image of stroke 2 along the first channel at 1446:56 EDT, August 10, 1982. The heights indicated are referenced in the plots of relative light intensity versus time 188 3.73 Relative light intensity versus time for stroke 2 at 1446:56 UT, August 10, 1982 189 3.74 Relative light intensity versus time for lower 500 m of stroke 2 at 1446:56 UT, August 10, 1982 190 3.75 Digitized image of stroke 2 at 1926:45 UT July 11, 1978. The heights indicated are referenced in the plots of relative light intensity versus time 192 3.76 Relative light intensity versus time for stroke 2 at 1926:45 UT, July 11, 1978 193 3.77 Digitized image of stroke 2 at 2023:46 UT July 29, 1978. The heights indicated are referenced in the plots of relative light intensity versus time 195 3.78 Relative light intensity versus time for stroke 2 at 2023:46 UT, July 29, 1978 196 3.79 Return stroke relative light intensity versus height for stroke 3 at 2206:51 UT, July 27, 1979 197 3.80 Return stroke relative light intensity versus height for stroke 2 at 2246:45 UT, July 27, 1979 198 3.81 Return stroke relative light intensity versus height for stroke 3 at 2246:45 UT, July 27, 1979 199 3.82 Return stroke relative light intensity versus height for stroke 2 along the first channel at 1446:56 EDT, August 10, 1982 200 3.83 Return stroke relative light intensity versus height for stroke 2 along the second channel at 1446:56 EDT, August 10, 1982 201 xiv

PAGE 15

3.84 Return stroke relative light intensity versus height for stroke 2 at 1445:35 EDT, August 10, 1982 202 3.85 Return stroke relative light intensity versus height for stroke 3 at 1445:35 EDT, August 10, 1982 203 3.86 Return stroke relative light intensity versus height for stroke 1 at 1926:45 UT, July 11, 1978 204 3.87 Return stroke relative light intensity versus height for stroke 2 at 1926:45 UT, July 11, 1978 205 3.88 Return stroke peak relative light intensity versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot 206 3.89 Return stroke relative light intensity rise time versus height 207 3.90 Return stroke relative light intensity 30/xsec after peak versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot 208 3.91 Digitized image of stroke at 2041:00 UT July 29, 1978 showing M-components . Indicated heights correspond to the locations of plots of relative light intensity versus time 210 3.92 Relative light intensity at ground level showing the return stroke luminosity and the two M-component luminosities for the stroke at 2041:00 UT July 29, 1978 211 3.93 Relative light intensity at 600 meters showing the return stroke luminosity and the two M-component luminosities for the stroke at 2041:00 UT July 29, 1978 212 3.94 Relative light intensity at 1100 meters showing the return stroke luminosity and the two M-component luminosities for the stroke at 2041:00 UT July 29, 1978 213 xv

PAGE 16

3.95 Peak relative light intensity versus height for the first M-component at 2041:00 UT July 29,1978 214 3.96 Peak relative light intensity versus height for the second M-component at 2041:00 UT July 29,1978 215 3.97 Electric field change following stroke 6 of the flash 2206:51 UT 27 July, 1979 showing three typical waveshapes for M-components . . . . 216 xvi

PAGE 17

LIST OF TABLES Table Page 3 . 1 Summary of Stroke Parameters Derived from Correlated Optical and Electric Field Records 71 3 . 2 Summary of Stroke Parameters Derived from Optical Records Made Available by the State University of New York at Albany. . 73 x vn

PAGE 18

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy RELATIVE LIGHT INTENSITY AND ELECTRIC FIELD INTENSITY OF CLOUD TO GROUND LIGHTNING By Douglas Max Jordan May 1990 Chairman: Martin A. Uman Major Department: Electrical Engineering Correlated streak camera and single-station electric field records were obtained for: 9 subsequent return strokes in 2 cloud to ground discharges (Tampa, Florida 1979) , and 1 first and 14 subsequent strokes in 3 flashes (Gainesville, Florida 1982) . Streak camera records for 8 first and 8 subsequent strokes (Kennedy Space Center, Florida 1978) and 3 first and 3 subsequent strokes (Oklahoma 1984) were furnished by the State University of New York at Albany. For ten first return and 11 subsequent return strokes, relative light intensity is graphed versus height and time. First stroke light rise times increase immediately after a branch point, then decrease as the xviii

PAGE 19

light front propagates up the channel. First stroke relative light intensity shows an opposite pattern. Subseguent leader speeds, for 4 flashes, increase from stroke two to stroke three, then decrease. Positive correlations are shown for leader speed vs. return stroke initial peak electric field, leader speed vs. leader electric field duration, and leader detectability using streak camera technigues vs. return stroke initial peak electric field. Relative light intensity profiles for five dart leaders show that the dart leader channel continues to radiate light after the dart leader front has passed and the peak light intensity for dart leaders is constant with height. From measurements of relative light intensity vs. height and time for one subseguent stroke followed by two M-components we show that the M-component relative light intensity is constant with height and propagated downward at between 1 and 2 x 10 8 m/s. In addition to the new results noted above, for 11 subseguent strokes, peak relative light intensity is shown to be correlated with dart leader speed, correlated with dart leader electric field change duration, and uncorrelated with previous interstroke interval. We confirm previous observations that: dart leader speeds decrease with increasing previous interstroke intervals; there is an increase (more rapid than previously xix

PAGE 20

CHAPTER 1 REVIEW 1 . 1 Introduction In this chapter lightning terminology is introduced and lightning processes and research pertinent to this thesis are discussed. Section 1.2 contains a brief discussion of the lightning process. Sections 1.3 and 1.4 contain a review of relevant previous work in the areas of photographic and electric field observations and measurements . 1 . 2 Lightning Lightning is one of the most familiar phenomena in nature. It has been observed, feared, and worshipped for centuries. The lightning process, with its brilliant light intensity, loud thunder, and smell is unique among natural phenomena in that it affects a majority of the human senses. Although the lightning discharge is considered by most laymen to be a single event, it is actually a series of distinct processes which occur in less than 1 second, mostly along the same spatial path. A drawing of the

PAGE 21

primary lightning processes is shown in Figure 1.1. The discharge begins with electrical breakdown activity in the cloud which is not shown in Figure 1.1. The next phase of the discharge is the stepped leader, followed by the first return stroke. The first return stroke, after an interstroke interval of tens of milliseconds may be followed by a series of dart leader and subseguent return stroke combinations. Often apprecible current, hundreds of amperes, continues to flow for for tens of milliseconds after a return stroke. During this so-called continuing current, discharges not shown in figure 1.1 called Mcomponents, which brighten the channel to ground, may occur. Lightning processes pertinent to the present thesis will be discussed in more detail in the following sections. 1.2.1 Stepped Leader The stepped leader occurs after the preliminary breakdown process, which will not be discussed, and is a series of randomly propagating, relatively short-duration, about 1 /xsec (Krider et al., 1977; Beasley et al., 1982), discharges which produce "steps" of light. The steps occur about 50 /xsec (Kitagawa, 1957) apart high above ground with increasing freguency near ground. The stepped leader normally lowers negative charge, several to tens of coulombs (Brook et al., 1962), from a source in the cloud

PAGE 22

CM U •H T3 •P C (0 (0 C Q) 3£l» O O M M W Q) tT T3 tr to q) c 0) -H •H U C 4J -P P W M tT~ *0 -H (0 rH ^ * (0 • (U B CM D H O < P • o x X! o o u •H p CO M o ^ o <4-l CQ X5 (0 rH & •h .Q -P CO o a> •h tn * P U tT (0 (0 C 0) O > £! 01 O W T3 tn TJ H a) -h P-H — aw < (0 &>o am CD C-H fc CD J 0) M tn H

PAGE 23

and deposits this charge along the paths of the breakdown steps. The stepped leader travels downward with an average speed between 1 and 2 x 10 5 m/sec (Schonland, 1956; Berger and Vogelsanger, 1966; Thomson et al., 1985). As the stepped leader tips come near the earth, the charge, which has been deposited, creates a large enough electric field to induce positive, upward-moving streamers from many locations beneath the leader. Eventually, one of the upward-moving streamers makes contact with a leader tip, and the first return stroke is initiated. 1.2.2 First Return Stroke When the positive charges of an upward streamer make contact with a branch of the downward-propagating negative leader, a violent, upward propagating, discharge is initiated near ground and propagates upward during which the charge which was deposited along the leader main channel and branches is lowered to earth. The bulk of the leader charge at each height is lowered within a few tens of microseconds of the return stroke fronts arrival, producing currents at ground which can be as great as hundreds of kiloAmperes, but typically are 35 kA (Berger, 1967b; Berger et al.,1975). The return stroke illuminates the stepped leader path between the cloud and ground as the luminous current wave travels from the ground to the cloud with typical speeds of 1 2 x 10 8 m/sec (Orville,

PAGE 24

1982) . The return stroke channel normally defines the spatial path of the subsequent events of the flash, although often a single flash may contain return strokes occurring along more than one channel to ground. 1.2.3 Dart Leader About 20% of the time in Florida (Thomson et al., 1984) , the lightning flash ends after the first return stroke. If this is not the case, the first return stroke is followed, after an interval on the order of typically tens of milliseconds (Schonland, 1956; Thomson et al., 1984) , by a dart leader. The dart leader is a luminous breakdown which propagates continuously from the cloud to the ground while lowering charge along the channel of the previous return stroke. Typical speeds for the dart leader are 1 2 x 10 7 m/sec ( Orville and Idone, 1982; Idone et al., 1984). The dart leader will occasionally travel almost the entire length of the channel and then suddenly begin to step the remainder of the distance to ground, generally but not always along the previous stroke channel. Such leaders are called dart-stepped leaders. 1.2.4 Subsequent Return Stroke When the dart leader reaches ground, or is met by an upward leader near ground, the subsequent return stroke

PAGE 25

begins. The subsequent return stroke travels from the ground to the cloud along the path of the previous dart leader with speeds on the order of 1 2 x 10 8 m/sec (Orville, 1982) and typically produces peak currents of the order of 15 kA (Berger et al., 1975) at ground. 1.2.5 M-component If stroke current continues to flow during the interstroke interval, there may be luminous processes associated with current waves in the channel during this time. These changes in channel light intensity are referred to as M-components (Malan and Schonland, 1947) . 1 . 3 Photographic Measurements The luminous features of the lightning ground discharge have been widely studied and have provided considerable insight into the physics of the lightning process. Scientists first studied the light intensity of the lightning flash late in the 19th century. These research efforts consisted primarily of attempts to determine the sequence of events during the lightning discharge to ground. Kayser (1884) was one of the first to observe that the lightning process consisted of multiple strokes down the same spatial path. Hoffert (1889) and Weber (1889) successfully used moving cameras to separate

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on film the individual lightning components. Photographs were produced by Walter (1902, 1903, 1910, 1912, 1918) which showed, for the first time, that the lightning discharge was initiated by a branched initial process followed by a return stroke traveling up the same channel. Therefore, by early in the 20th century a coarse view of the lightning discharge had been revealed. Sir Charles V. Boys (1929) invented a camera system in 1900 which revolutionized the study of lightning. The camera produced relative motion between the film and the lenses of the camera by having the lenses rotate in front of the film. This design allowed the camera to remain fixed yet enabled events which occurred along the same spatial path at different times to be separated. Boys was able to obtain images with his camera (Boys, 1929), but the conclusions of his analysis proved to be incorrect. The investigation of optical properties of lightning during the period between 1930 and 1960 was dominated by Schonland, Malan, Collens, and coworkers in South Africa. Boys cooperated with the South African team and loaned his camera to be used as a prototype. In their first experiments with Boys 1 rotating lens camera design (Schonland, Malan and Collens, 1934), the camera lenses, separated by 10.1 cm, were rotated by hand at 1500 rev/min ± 10%. At this speed, a distance of 1.0 mm on the film corresponded to approximately 63 microseconds. The authors verified previous findings by Halliday (1933)

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which showed light intensity moving both up and down the lightning channel. They observed branches which illuminated in a sequence other than by height along the channel and noticed that the channel light intensity decreased as the return stroke front passed each branch point and finally vanished after it passed the last branch . The Boys camera was later modified to have still lenses and a rotating film drum (Schonland, Malan, and Collens, 1935), as shown in Figure 1.2. The apertures of the camera lenses were set independently, which allowed sufficient dynamic range to examine processes whose light intensity varied greatly. Using this method, it was determined that the stepped leader paused approximately 100 microseconds between steps. A glow was noticed preceding the stepped leader tip as it neared the ground, and some leader tips glowed quite far above ground. The authors also discovered that the effective stepped leader speeds increased near ground. Orville and Idone (1982) also show that the stepped leader speed increases near the ground. Malan and Collens (1937) thoroughly investigated first stroke light variations with particular emphasis on main channel light components initiated by the charge in a branch when a return stroke front passes a branch and releases that charge. These branch components travel down the main channel while the return stroke front travels up

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Rotating Film Drum Diagram of improved Boys camera with moving film and stationary lenses. Adapted from McEachron (1939).

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10 the channel above the branch and were often found to be brighter below the branch than the return stroke front which preceded them. Optical lightning research, in the early years, concentrated on the subjective evaluation of film records to determine flash properties. As electronic technology improved, it became possible to use calibrated photoelectric detectors to determine quantitative properties of lightning processes. Mackerras (1973) used photomultiplier tubes and a wide-angle camera system to perform a quantitative analysis of the integrated light output of both cloud and ground flashes. Statistics were presented for energy balance, based on assumptions of charge being lowered from an assumed height in the cloud. Comparisons were made with the Bruce and Golde (1941) model of the lightning return stroke. A parameter of great interest to researchers developing return stroke models is the speed of the return stroke front as it propagates up the channel. Orville et al. (1978) presented daytime lightning data acquired with a streak camera system including measurements of return stroke speed. Return stroke speeds ranging from 1.2 x 10 8 to 1.4 x 10 8 m/sec were observed. The camera system was of the single lens design with rotating film drum. Return stroke speed was computed using a still image from a 35mm camera as a reference image and measuring the displacement of the streaked image from the still image. The

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11 displacement is a function of the height along the channel and the return stroke speed. Hubert and Mouget (1981) analyzed data from triggered (artificially initiated by firing small rockets trailing ground wires) lightning at St. Privat d'Allier, France. Their analysis focused on return stroke speed, but some interesting optical waveforms at two heights along the channel are shown as well as current measured in a shunt at the base of the channel. The data were recorded using logarithmic compression, but all of the data presented were in the linear range of the amplifier. Idone and Orville (1982) presented return stroke speed data from 63 strokes at Kennedy Space Center. Speed as a function of height was measured revealing, in all but one case, a decrease of up to 25% near the top of the visible channel. The cloud base, which determined the visible channel top, for these flashes was approximately 1000 ft.; therefore, only a relatively short section of channel was available for measurement. Orville and Idone (1982) present streak camera records for 21 dart leaders, 4 dart-stepped leaders, and 3 stepped leaders from Kennedy Space Center, Florida and Socorro, New Mexico in mostly daylight conditions. Dart leader speeds were computed at two heights along the channel with the mean speed in the bottom 800 meters being 11 x 10 6 m/s while the mean speeds in the upper channel sections was 14 x 10 6 m/s. Most of the dart leader speeds

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12 decreased near ground while 4 leader speeds increased; however, they commented that the measured difference was not large and was within their experimental error. Dart leader lengths were computed and shown to have a mean value of 27 meters for the Florida data and 42 meters for the New Mexico data. The authors suggested that their use of #92 filters in the Florida experiment could have caused the difference in the means. They found a correlation between the dart leader luminous intensity and the resulting return stroke luminous intensity. They found no correlation between dart leader speed and the luminous intensity of the dart and little correlation between the luminous intensity of the dart and the resulting return stroke speed. Inconclusive results were found for dart leader speed vs. return stroke speed as well as dart leader speed vs. return stroke luminous intensity. Idone et al. (1984) presented analyses of three triggered lightnings at Langmuir Laboratory, New Mexico. They computed three-dimensional speeds of the strokes and measured a number of flash parameters, including current at the base of the channel. Return stroke speeds ranged from 6.7 x 10 7 m/s to 1.7 x 10 8 m/s. The ratio of threedimensional channel length to two-dimensional channel length ranged from 1.05 to 1.22. Return stroke currents ranged from 4.0 to 32.0 kA with a mean of 6.0 kA. Mach and Rust (1989) measured return stroke speed using a 35mm camera housing which was modified to contain

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13 a solid state sensor array. The array had 8 horizontal sections and provided relative light intensity as a function of the corresponding heights and time. Return stroke speed measurements for two channel lengths are presented for 86 natural lightning strokes and 41 triggered strokes. Peak currents for the natural strokes are computed using the Transmission Line Model formula of Uman et al. (1975) and the experimentally derived relationship of Willett et al. (1989) . The rise time of their optical waveforms increased with height while the peak relative light intensity decreased with height, supporting the results of Jordan and Uman (1983) . Total power radiated from the return stroke channel is of considerable interest to those engaged in lightning and electromagnetic pulse protection research. Guo and Krider (1982) analyzed data from a calibrated wide-angle photodetector system and fast electric field system. The spectral response of the detector extended from 400 nm to 1000 nm with a peak at 750 nm. The detector lens system covered an angle of approximately 25° above the horizon. Correlated electric field and optical waveforms were shown on a microsecond time scale. The authors assumed that the channel was straight and had uniform light intensity as a function of height. They then calculated the total optical output from a section of channel. They used previous laboratory results, which showed a radiative efficiency of 0.8%, to calculate the average electric power radiated per

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14 meter of channel. Average radiated power was shown to range from 1.3 ± 1.2 x 10 8 W/m for first strokes to 3.1 ± 2.3 x 10 7 W/m for subsequent strokes. Results were also shown for the time derivative of light intensity divided by the peak electric field squared vs. range to the flash. The results of their paper were amended in Guo and Krider (1983a) after receipt of a manuscript in preparation by Jordan and Uman (1983) which showed detailed measurements of light output from the channel as a function of height. These results enabled a more accurate estimate of the peak light output. Peak radiance was shown to be between 6.0 x 10 5 W/m and 1.0 x 10 6 W/m in the 400 1100 nm range. Guo and Krider (1983a) used the same apparatus to measure the peak electromagnetic power radiated from the return stroke. Peak powers were reported of 2.3 ± 1.8 x 10 9 W for first strokes and 4.8 ± 3.6 x 10 8 W for subsequent strokes. In order to reduce errors in the calculation of total light output from lightning discharges, Thomason and Krider (1982) conducted a Monte Carlo simulation of the scattering of light by clouds. The light was assumed to be generated inside the cloud, as in an intracloud flash or the portion of the return stroke to ground which is in the cloud. The authors show that essentially all of the original light eventually escapes the cloud but may be time-broadened by tens of microseconds.

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15 Schonland (1938) gave his view of the discharge process which included the observation that the light intensity of subsequent return strokes decreased considerably by the time they had propagated half the distance to the cloud. Cloud bases in South Africa varied from 1000 m to 2900 m. This was the first discussion of the variation in relative light intensity with height along the channel. Boyle and Orville (1976) used two slits in front of a streak camera to obtain a pair of waveforms which showed light intensity as a function of time at the two slit heights which were 350 m apart. The rise times of the two waveforms were found to be approximately 3 /Ltsec. Jordan and Uman (1983) were the first to provide a detailed analysis of light profiles as a function of height and time. They used a rotating drum microdensitometer to digitize seven film images, of subsequent strokes, taken with a streak camera in Florida. The images were sampled with a spatial resolution of 25 microns, which yielded a maximum time resolution of 0.5 microseconds. Horizontal elements of the digitized data were plotted to provide a profile of the channel relative light intensity, at a given height, as a function of time. Plots of relative light intensity vs. height and time were analyzed for seven subsequent strokes in two flashes. Results were presented for peak relative light intensity vs. height, rise time vs. height, and relative

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16 light intensity at 30 microseconds after peak. These analyses showed clearly that the peak relative light intensity decreases and rise time increases with height. The authors compared their data to those of Guo and Krider (1983a) in order to provide a rough calibration of the photographic relative light intensities. They integrated the relative light intensity at increasing heights, assuming a return stroke speed of 1.0 x 10 8 m/sec, to simulate the output of an all-sky detector. Their comparison showed that the total optical power, computed by assuming a uniform channel light output, is underestimated by a factor of between 1.8 to 3.8 with a mean of 2.5. Ganesh et al. (1984) obtained light profiles by placing a thin slit in front of a photomultiplier tube. The light intensity data were necessarily at different heights on flashes at different distances but nevertheless were from a relatively short section of channel. The photomultiplier system had a frequency response of approximately 500 KHz and a spectral sensitivity ranging from 200 600 nm. A figure was presented of light intensity on a microsecond time scale. 1.4 Correlated Measurements of Electric Fields and Relative Light Intensity Electromagnetic field measurements have been the primary tool of the lightning theorist. A major

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17 disadvantage of these measurements is that the measured fields are a function of the currents at all heights along the lightning channel. Because of this limitation, it is difficult to determine the current at different heights along the channel from the fields. Correlated electric field and streak photographs have been used in an attempt to gain better understanding of electric field records. Schonland et al. (1938) reported on the first correlated measurements of lightning light intensity and electric fields. Electric fields were measured by filming the face of an oscilloscope which was connected to an electric field antenna system. Pulses were found, on the electric field records, which preceded the beginning of the optical signal. This research, for the first time, provided proof that the lightning process begins in the cloud well before the luminous portion can be detected. Malan and Schonland (1947) presented the results of the best correlated electric field and light intensity measurements to that time. The electric field records revealed many "hook-shaped" waveforms which corresponded to what the authors called "M" components which illuminated the channel to ground. They attributed the "M" components to weak return strokes. It was observed that the steps in the stepped leader process do not produce field changes on the electric field records. They also present evidence of a uniform distribution of charge along the dart leader channel based solely on a uniform dart

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18 leader speed. In a later paper, Malan and Schonland (1951) presented relationships between the height of the charge which is lowered by each subsequent stroke and the stroke number. Krehbiel et al. (1979) have since shown that the charge regions which are discharged by subsequent strokes within the same flash are actually distributed horizontally in the cloud, not vertically. Orville and Idone (1982) showed that dart leader speeds are not constant but most decrease near ground. Schonland (1956) summarized the South African work in a review paper which provided an integrated view of his understanding of the lightning discharge. Kitagawa and Kobayashi (1958) presented results of correlated optical and electric field signals. The experimental apparatus had relatively coarse time resolution but yielded data which agreed with the findings of Malan and Schonland (1951) . The authors found a large number of luminous pulses which occurred before and after the discharge. Kitagawa et al. (1962) acquired low-speed streak camera records with correlated electric field records for about 200 cloud-to-ground discharges at Socorro, New Mexico. Thirty six hybrid flashes, containing one or more continuing currents, were analyzed and statistics presented for stroke order of continuing currents, and duration of continuing luminosity. They show hook-shaped electric field waveshapes, or K-changes, associated with

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19 M-components during continuing currents and present data on M-component intervals vs. elapsed time since the return stroke. They suggest that the channel must have several levels of conductivity in order for some strokes to have leader processes while others, such as M-components, can travel down the channel with no detectable leader. Brook et al. (1962) use photographic records and correlated electric fields to compute the charge lowered and apparent height of the individual strokes of flashes in New Mexico storms. The apparent height of the charge centers was later shown by Krehbiel et al. (1979) to be a horizontal length as discussed above. Data are presented which show that strokes occurring later in the flash lower less charge. Schonland, in a presidential address to the British Association, recapped his perception of the entire lightning process (Schonland 1963) . In that address, he compared the behavior of the long laboratory spark to lightning and decided that they shared essentially the same breakdown process. Lundguist and Scuka (1969) analyzed simultaneous measurements of two optical bands and electric fields. They used an all-sky lens system with two optical detectors. The spectral response of one detector was centered in the violet (390 nm) and the other in the red (655 nm) . Their results differed from those of Orville (1968) in that often the red band would precede the

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20 violet. Orville found the red wavelengths to lag the violet due to the Ha lines of hydrogen turning on late in the optical process. Thomson (1980) discussed the results of correlated optical, electric field, and 10 MHz radiation measurements for 282 return strokes in Papua, New Guinea. Wideband optical waveforms, on a millisecond scale, were used primarily to identify specific lightning features. Statistics were presented for flash duration, prestroke duration, continuing current duration, interstroke interval, stepped leader duration, strokes per flash, and channels per flash. Jordan and Uman (1983) analyzed streak camera data and simultaneous electric field records. The authors used the plots of peak relative light intensity vs. height, from the streak photographs, to extrapolate a peak relative light intensity at the ground and compared these values with peak electric field (Ep) . The best fit was for log peak relative light intensity vs. peak electric field, although statistically significant correlations were found for log peak relative light intensity vs. E p 2 and linear peak relatively light intensity vs. E p . Ganesh et al. (1984) presented correlated light intensity and electric field data on a microsecond time scale. Relative light intensity vs. electric field are plotted on several scales, with a linear relationship appearing to have the best fit. The authors acknowledge

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21 that errors associated with channel position relative to the axis of the slit, atmospheric scattering, and the uncertainty of the distances to the flashes prevented an accurate determination of the relationship between light intensity and electric field. Additional analysis of the Langmuir Laboratory data by Idone and Orville (1985) examined the correlation between light intensity at the channel base vs. the peak channel current. The analysis was performed in a similar fashion to Jordan and Uman (1983) , who compared light intensity to peak electric field. The authors show statistical results for 39 subseguent strokes of 2 triggered flashes. Correlation coefficients were measured for relative light intensity (Lr) vs. current (I), log Lr vs. log I, log Lr vs. I, and log Lr vs. I 2 . All of these had statistical significance at the 1% level, but the highest was L R vs. log I. These findings differ from those of Jordan and Uman (1983), who found the highest correlation between log Lr vs. E 2 .

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CHAPTER 2 UF AND SUNYA EXPERIMENTS AND GENERAL ANALYSIS TECHNIQUES 2 . 1 Experimental Techniques 2.1.1 Photographic Techniques The lightning process is random in nature, hence to record it the photographic system must be prepared, over a period of tens of minutes, to respond within a few milliseconds. Lightning ground flashes typically last hundreds of milliseconds and occur separated by several seconds. The camera system must be able to separate the various components of the lightning flash which occur along the same spatial path. The camera configuration used in this research effort varied slightly between years but was based on a Beckman and Whitley model 351 streak camera . The Beckman and Whitley streak camera is a rotatingdrum fixed-lens camera which is capable of film writing rates of 0.05 mm/microsecond. A 50 mm Nikkor objective lens was used to image the lightning channel through the shutter onto the image plane in front of a collimating lens. The collimating lens transferred the image by way of a 45-degree mirror onto the film drum. 22

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23 The film used in the streak camera must be loaded and unloaded for each exposure sequence. A length of film (855 mm) , which is wound beforehand into a special cassette, is unrolled onto the stationary film drum. The cassette is then disengaged and the drum is rotated with a high-speed motor to the desired operating speed. Typical drum speeds are on the order of 70 revolutions per second. The rotational speed of the film drum is monitored by counting pulses from a magnetic coil pickup. The pickup senses four magnetized screws on the drum face. The film used in the streak camera was Kodak 5474 shellburst. This film is well suited to lightning photography due to its spectral response which is essentially constant from 300 nm to 670 nm. The extended red sensitivity is desirable for lightning photography due to the high intensity of spectral lines of hydrogen (Ha at 656 nm) emitted by the lightning channel (Orville, 1968) . Kodak 5474 has a thick grey base and good antihalation properties. The mechanical properties of the streak camera film are very important since the film must be unrolled onto the drum and retrieved. A triggering mechanism was necessary for the camera shutters since all of the data were acquired in daylight. A photodiode optical detector connected to a differentiator circuit was used to detect the first stroke of the lightning flash. The parameters of the differentiator were adjusted so that ambient conditions

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24 did not cause a trigger. The trigger circuit also provided an input to the data logging circuitry which printed a record of the events. The time of the event, stroke number, time between strokes, and speed of the camera drum were printed. Film exposure times of approximately 0.5 second were used, enabling the maximum number of subseguent strokes to be recorded without exposing the film to excessive background light. Once exposed, the film was developed in a darkroom located adjacent to the camera site, using Accufine developer, chilled to 68 °F, with a development time of five minutes and agitation at 30-second intervals. The development process was then stopped and fixed with standard Kodak products. The resulting negatives were air dried. A still camera was used to document the location of the flash and provide an undistorted view of the channel geometry. The camera was automatically triggered by the streak camera trigger circuit. An exposure time of 0.25 s was used at a lens stop of f8. Kodak Ektachrome film (ASA 64) was used to produce color slides. A video camera was used to provide a chronological record of everything that happened in the field of view of the streak camera. The video camera integrated events which occurred within a 16-millisecond interval. Even with such poor time resolution the records were extremely valuable when reconstructing unusual events, such as

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25 multiple channels to ground. System timing and audio were also recorded on the video recorder audio channels. 2.1.2 Electric Field Measurement Techniques Electric field measurements are an integral part of lightning research, and their omnidirectional nature has made them the primary tool for analysis of the lightning process. The subsystems necessary for an electric field measurement system will be discussed in the following sections. 2.1.2.1 Antennas for measuring vertical electric fields Electric fields can be detected by many different types of antennas as discussed by Uman (1987, appendix C) . Whip, spherical, and flat plate antennas have all been used to record electric field signals from lightning. All of the electric field records in this thesis were acquired using flat plate antennas. The vertical electric field terminating on a flat plate conductor induces a surface charge density ( s ) . s = e o E n If the antenna is connected to an electronic device, the current delivered to the device will be i = dq/dt, q =

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27 = d(e E n A)/dt = e A • dE n /dt where A is the area of the plate. E n is assumed to be uniform across the surface of the plate and the plate is flush with the earth. If the antenna is not mounted flush with the earth the measured electric field is not the true field but is an enhanced value of the field. In order to produce a signal proportional to the electric field, the current from the antenna must be integrated. The flat plate antennas used in this research were constructed using a ground plane approximately one meter sguare with a flat circular plate of the appropriate area mounted in a hole in the ground plate. Several antennas with different plate areas were used to provide various gains. The details for the antennas used in the Tampa, Florida experiment are found in Section 2.2.1 and for the Gainesville, Florida experiment are found in Section 2.1.2.2 Integrators and amplifiers The signal proportional to the derivative of the electric field from the electric field antenna is connected to an integrator circuit providing an output proportional to the electric field. Integrator circuits may be as simple as a resistor capacitor pair or may

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28 involve operational amplifier circuits such as those used to record the fields in this thesis. 2.1.2.3 Instrumentation recorders All signals of interest were recorded on instrumentation tape recorders with IRIG Group II electronics. In 1979 an Ampex FR1900 was used and in 1982 an Ampex FR3010. The electronics of these recorders are wide-band and provide for both amplitude modulated (direct) and frequency modulated (fm) recording. The direct channel frequency response is down 6db at 400 Hz and 1.5 MHz. The fm channel frequency response extends to dc and is -6db at 500 KHz. FM channels are used to record signals which had low-frequency components such as the slow decay electric field integrators which had decay constants of 5 seconds. An fm channel was also used to simultaneously record time code information. The signals on the instrumentation recorder tapes were later replayed and digitized using the Masscomp digitizing system. This process is discussed in detail in Section 2.4.2. 2.1.2.4 Biomation waveform recorders The Biomation 805 waveform recorders were used both in data acquisition and data analysis to sample analog data with 8-bit resolution and store up to 2048 samples. The stored data can be displayed at one of several rates through a digital to analog converter. During data acquisition the analog output signals were displayed on dual beam oscilloscopes which were photographed with film

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29 recorders. During data reduction the output was captured on a storage oscilloscope so that a polaroid photograph could be taken of the oscilloscope traces. The Tampa data and the Gainesville data were initially analyzed using Biomation waveform recorders. The recorded signals on the instrumentation recorders were played back into the Biomation waveform recorders and time periods of interest were sampled and stored. This process was extremely tedious since the Biomations began recording after a preset delay from the beginning of the event of interest and the delays available on the Biomations were insufficient to analyze a typical flash. Lightning lab personnel overcame this limitation by building special electronics which produced an external trigger to the Biomations after a preset delay from the event trigger. Typically the first stroke of the flash was used as the event trigger and a delay was determined, from a plot of the overall electric field change, which would cause the time period of interest to be sampled and recorded. In order to analyze a typical flash it was not unusual to replay the tape recording of the flash tens of times, sampling a different period of time from each pass with the Biomations. This technigue of electric field analysis was laborious and had the disadvantage of deteriorating the guality of the tape recording after many passes. Nevertheless, it was the best available technigue at the

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30 time and was used in the University of Florida Lightning Laboratory from 1977 until 1984. 2.1.2.5 Oscilloscope film recorders Dumont oscilloscope film recorders were used in Tampa and Gainesville to record the output signals of the Biomation waveform recorders during data acguisition. The Dumont moving film recorder, containing 500 feet of 35mm panchromatic film, is focused in order to record the signals on the oscilloscope face. A red light-emitting diode attached to the perimeter of the oscilloscope face was used to provide time code information. The film speed was approximately 2 inches per second, which was sufficient to separate the traces from individual strokes which are typically tens of milliseconds apart. The 35mm film records were developed by a commercial film lab. 2.1.2.6 Strip chart recorders Hewlett Packard 7402a strip chart recorders with a bandwidth of dc to 100Hz were used to record audio thunder signals, slow decay electric fields, pulses indicating streak camera events, and time code signals. These records are valuable since they are also annotated by the observers during a storm in order to preserve information about instrument settings and visual observations. 2 . 2 Experiments 2.2.1 Tampa Experiment

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31 During the summer of 1979, the Lightning Research Laboratory of the University of Florida participated in a research project located near Wimauma, Florida, which is west of Tampa, Florida. The lab was composed of a 14 -meter trailer, which housed the electric field measuring system, photographic darkroom, support facilities, and a small truck, located adjacent to the main trailer which contained the camera systems , as shown in Figure 2.1. A Beckman and Whitley streak camera was used to record streaked photographs of lightning. The camera system consisted of a model 351 streak camera, a 35mm still camera, a television camera with monitor, an audio recorder, and various equipment to provide timing and data logging. The streak camera had a modified face plate which accepted two collimating lenses, thus producing two images on the film, streaked in opposite directions, for each event. The streak camera configuration with the television camera removed is shown in Figure 2.2. A closed circuit television network was operated in 1979 in order to locate the lightning ground contact points. Five remote cameras were used in addition to the camera in the small truck. The cameras covered an area of approximately 60 km 2 . Electric field measurements were made using three flat plate antennas mounted on top of the 14-meter trailer shown in Figure 2.1 . One of the antennas had an area of 0.5 m 2 and the other two had areas of 0.2 m 2 . North-South

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PAGE 51

32

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PAGE 53

34

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36 and East-West components of the magnetic fields were measured by loop antennas. Also during this experiment a deenergized power line was instrumented so that the voltage induced on the line by nearby lightning could be measured. An Ampex FR-1900 instrumentation recorder was used to record the electric field signals at a speed of 120 ips. Direct or fm channels were used depending on the frequency range of the particular signal. Figure 2.3 shows the configuration of the electric field recording system in 1979. 2.2.2 Gainesville Experiment During the summer of 1982 the experiment was located on the thirteenth floor of a dormitory (Beatty Towers) on the University of Florida campus. Two suites were used with one having a window facing south and one facing north. Electric field antennas were mounted on the roof of the dormitory and the electric field recording equipment was fixed in the north suite. The optical systems were portable and could be moved between suites. Essentially the same streak camera system used in 1979 was used in 1982, with the following differences. Only one lens was used on the streak camera in 1982 . The data logging apparatus was modified, for 1982, to use a microprocessor to perform most of the functions previously accomplished with discrete logic.

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5 R (D w ii Hft^rtMaim&tlH-HiHilliBitllD (D (D H# HH 3 H(D 0) 3 H0) H 3 ^< H3 H 3 (t» Oi^Q (DOlftlDBlOftinDi O (D «X> ID 33 IDHftClD rt*Q M m o rt & in It C (Q & 3(0^ DiQiOOOIlK-Hl Vh 0, 03 • n> too oh-oioivQ(DO)0> 3 H a a ha n 13 n-o-rtsoooic in (03(0 H £ N O rt P> 0> wo • o w Hiturorort^ro^^croH) 3 a» rt oi p-^ ^ o, o oi 3 a> & 3 ^ H-(D (D 0) 3 (D ^oiidpoioh-co) 3 in o tJ t-'3h-' 3iQ(D0JOCwa>OHn) roortinC H-t-«cp»n)(D03vDi-' rtsitu p-O-rt il'O ii 0-3 >J It rt ftCrtCH-HdIDKIl(!) ^on an(D 3* 0> n Ho • O 3 (D < lOifDS'rtonrtfCHHO 0> PitT •(DH-313'OiCO w^ n »i R a> v <30 o D> (DfDH-P> , (T>Wrtrt H> •-< a> o o o 3 mo o in 3 o it pOiMOOfi w a> i— • < rt (0 ro h n> (OtiinnoH-nrtouft hH-0>H-(D0>rtO3'(D1i-< p. p. . p. Hft O iQ O O ID ifl PIt •OO O 3 H H0(D tJ 1 ^ O 1-3 3 01 O O H> 3 H> 0» O on>3" I0 3Ha (» g cr rt hi m 3* HC 0> Q) O IBOiH-IllilifllOP-O. 0»(D333O^I-' < rtiQ < ^ (D Hi- (DO 00(D 3 O-iQ in ii g v oi o (DID rt^Q 3" -»d3' h l NOK' 0> C op ooion>< (D X M (D -^ H^ ^ < 3 CD iQ 01 — iQ

PAGE 56

38

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39 High speed streak cameras, which rotate more than one revolution during the time between strokes, cause the images from successive strokes to appear out of stroke order on the film. Previous analysis techniques required that electric field records or some form of accurate measurement (± 0.1ms) of the time between strokes be used in order to create a template of where the strokes should be found on the streak camera film. The template was formed by assuming an arbitrary starting position for the first stroke then marking the positions of the successive strokes by knowing the speed of the film and the time between strokes. A time code system was developed, for the 1982 experiment, which was capable of imaging an array of light sources onto the film adjacent to each streak image. The light sources were pulsed for 1 microsecond in order to put the time of the event and the stroke number of the event onto the film adjacent to the actual streaked image. This was a major improvement in the camera system since the sequence of strokes could be more easily indentified. Electric fields were recorded from 3 flat plate antennas, one with an area of 0.5 m 2 and two with areas of 0.2 m 2 . The electric field signals were recorded on an Ampex FR3 010 instrumentation recorder as discussed in Section 2.1.2.3. A photomultiplier optical detector, which viewed the lightning through 0.2 mm slit, was operated and the resulting light waveforms were recorded simultaneously

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Figure 2.4 Diagram of the electric field and light recording system used in 1982. The instrumentation recorder (ITR) was an Ampex FR3010. The antennas were connected to integrators with slow (5 sec) and fast (1 msec) decay constants. The photomultiplier tube (PMT) was followed by two stages of gain (Gl, G2) . Time code was IRIG-B analog signal which was recorded on the ITR and a slow code digital signal which was recorded on the strip chart. Thunder was recorded on the strip chart along with slow decay electric fields.

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41 A3 7777 I— 7777 7777 A2 %> Slit/PMT TCG 7777 7777 A1 k ms • -4-v -» XI Thunder e>>-»^>^>-»-»ITR 1 FM 2 FM 3 FM 4 FM 5 Direct 6 FM 7 FM NC »SCR »-»Event 1 2 Event

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42 with the electric field signals on the FR3010. Ganesh et al. (1984) discuss the results of the photomultiplier portion of the experiment. 2.2.3 Data from Orville and Idone Data have been analyzed in this thesis which were obtained by Dr. Vince Idone and Dr. Richard Orville of the State University of New York at Albany. The data analyzed were taken during two different years. In 1978 the SUNYA group made measurements at the Kennedy Space Center in Florida using a Beckman and Whitley streaking camera very similar to the one used by the University of Florida Lightning Laboratory. The Beckman and Whitley camera was operated with 2 collimating lenses and using #92 filters. The filters blocked almost all sunlight while passing the bright hydrogen line (Ha) at 6563 nm emitted by lightning. These filters were used to reduce the fogging of film due to background light since the measurements were made in daylight. The film used was 5474 Shellburst with a gray base. In 1984 the SUNYA group made measurements in Oklahoma using the same streak camera in the same configuration but without filters. The film used in 1984 was 5474 Shellburst with an estar base which increased the mechanical strength of the film.

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43 2 . 3 Photographic Analysis 2.3.1 Optical System Calibration Any optical system used to measure the relative light intensity which caused a change in film density must be calibrated in order to understand and, if necessary, correct the inherent errors of the system. Most important would be system characteristics which cause nonuniform brightness across the film plane when imaging a uniform source. The system used in the measurements for this thesis consisted of two distinct subsystems. The first is the camera apparatus, with its lenses and mirror, which collects the light and focuses the lightning image onto the film. The second is the photographic film which exhibits a nonlinear change in film density in response to incident light intensity. 2.3.1.1 Camera system calibration Optical systems have errors which can produce distortion and nonuniform brightness of the focused image at the film plane. Fortunately, because of the sophisticated computer modeling used in the design of lens optics, the distortion present in the lenses used in this study is negligible. There are system errors which are always present in optical elements. The first of these is due to the fact that the amount of light which passes through a circular

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44 disk varies with the orientation of the disk relative to the source. This is known as the cos 4 law since the brightness at the image plane varies as the cos 4 of the angle from the lens center to the source location. The errors from this effect can be calibrated using the position of any image point relative to the center of the film frame. For the streak camera only the vertical offset can be determined from the streak camera record alone. If a still photograph was obtained, then the position of the entire lightning channel relative to the lens center can be determined. The data used in this thesis were very nearly in the center of the image frame and subtended angles less than 20°. The cos 4 of these angles is greater than 0.9 which introduces an error of less than 10 percent to the brightness of the channel at the film plane. Vignetting is another system error which is a function of the angle of the source from the lens center. This error is introduced by the physical contruction of the lens and results from the obstruction, by the lens body, of a portion of the bundle of rays which enters the front of the lens before it can exit the rear of the lens. This effect causes a circular bundle of rays to be reduced to a cat's eye pattern at the rear of the lens. The difference in the cross sectional areas of the two patterns results in a reduction in brightness at the film plane. The effect of vignetting is reduced significantly, even eliminated, if the lens aperture stop is small enough

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45 so that the bundle of rays which passes through the aperture can exit the rear of the lens without obstruction. The streak camera objective lens, a 50mm Nikkor, was operated at an aperture setting of f8. At this aperture setting there is no obstruction of the ray bundle until the off center angle excedes 30°. A simple experiment was performed on the streak camera lens system to determine if vignetting was present. A pinhole source was placed at various positions relative to the center of the lens while remaining in the focal plane at the rear of the collimating lens. The pinhole source produced a circular bundle of rays which exited the front of the objective lens and was focused on a screen. Over the range of off center angles which the lightning data subtended on the film the bundle did not become measureably non-circular. Additional confirmation of a lack of vignetting is found in Figure 3.90 which shows relative light intensity 30 ^sec after the return stroke peak relative light intensity as a function of height to be approximately constant with height. If vignetting were present, it is extremely unlikely that the variations in channel relative light intensity for the strokes presented would decay in such a way as to compensate for the vignetting and produce a uniform value with height. Additional nonuniformities in image brightness can occur as a function of the angle of incidence of the ray

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46 bundle at the lens surface due to the angle of the source from the lens center. Considering the curvature of the front lens surface, the small aperture stop, and the small off center angles of the data, the difference in angle of incidence for any ray bundle would be minimal. Therefore, the errors due to angle of incidence with the lens surface should be negligible. 2.3.1.2 Film calibration Photographic film is difficult to use for photometric analysis of optical events since the photographic process does not produce a linear change in film density with respect to incident light intensity. A sensitometric analysis of the film records, as a function of absolute incident light intensity, was not possible since it requires extremely controlled methods of film development as well as optical radiation standards which were not available for this experiment. In this thesis all measurements are in units of relative light intensity. The measure of film density is in units of diffuse density . Kodak (1973) defines diffuse density as density = log 10 (P /Pt) or = log 10 (1/T) where P is the incident energy, P-^ is the transmitted energy, and T is the transmittance of the developed film

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47 image. Measurements of diffuse density assume that the device which collects the light which is transmitted through the film has a collection angle of 180°. If the collection angle is less than 10° the density measured is specular density . Specular density always appears larger than the diffuse density, when measuring photographic film, due to scattering of light as it passes through the film. The light scattered out of the path of the collector is assumed not to be transmitted by the film and is attributed to a higher film density. The microdensitometer used to measure the data for this thesis measured specular density. In order to determine the exposure . which is the amount of energy reaching the film surface, that produced a corresponding change in diffuse density it is necessary to produce a plot of film density as a function of exposure. A calibration curve for a batch of film is produced by exposing a section of film to a light source through calibrated densities. A density wedge is a strip of transparent substrate which has had precise amounts of material deposited on its surface in such a way as to produce sections of uniform density which increase in density along the length of the strip. A calibrated density wedge may be purchased from the manufacturer along with precise measurements of the diffuse density in each wedge section. If a uniform light source illuminates the

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48 wedge, the amount of light transmitted through any section of the wedge may be computed by using the following. D P t = P / 10 This expression shows that at D = all of the incident light is transmitted. At D = 1, one tenth of the incident light is transmitted. For a calibration wedge with densities from 3 D the light transmitted would vary by three orders of magnitude. If the incident energy is not known, the range of relative light intensities provided by the calibration wedge can still provide a series of exposure steps whose relative magnitudes are well understood. The relative magnitudes of exposure steps were used to calibrate the film analyzed in this thesis. The original density calibration for the data obtained during the Tampa experiment in 1979 was accomplished by exposing strips of the Kodak 5474 Shellburst film through an uncalibrated step wedge to various exposures from a photographic lamp used in the exposure of photographic prints. The film strip with the largest range of densities and the step wedge were then digitized using the Optronics microdensitometer at the Data Analysis Facility at Kennedy Space Center. It was not understood at that time that the measured density values were specular densities, not diffuse densities. A plot was produced of relative exposure versus specular density and

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49 the resulting plot of relative light intensity as a function of specular density was used to produce the results discussed in Jordan and Uman (1983) . The original calibration values have been corrected for diffuse density using calibrated step wedges and were found to produce similar results to those using the current calibration values with the exception of an exaggeration of the highest relative light intensity values, such as the peak relative light intensity of the brightest stroke of the 2246:45 UT flash on 79208 as seen in Figure 3.47. An independent calibration was performed for the 1979 data by using a General Instruments Strobotac strobe to expose a section of film in the streak camera to a 3 jusec pulse while monitoring the light with a photodiode detector. The results of this calibration were presented in Jordan and Uman (1983) and showed that the film correctly reproduced the shape of the light signal over the range of relative light intensities produced by the strobe. This provided additional confidence in the original calibration over a range of relative light intensities inclusive of the relative light intensities of all of the data with the exception of stroke "C" in Jordan and Uman (1983) . Figure 2.5 shows diffuse density and specular density as a function of incident relative light intensity for a strip of Kodak 5474 Shellburst film which was exposed, by Dr. Vince Idone at SUNYA, to a 3 Msec pulse of light from

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50 .o i SPECULAR DENSITY IS) . DIFFUSE DENSITY ID) s >I — I LI L_ ! CZ ID t 2.0 CALIBRATION' KEDSE DENSITY 10 RELATIVE LIST UNITS 1.0 10D 0.0 1000 HESE: FlLr FIL» DIFFUSA SPECULA' 'j es C.li l.V C.12 o.e 1.6C.K C.3t e.« C.li C.5( S.3* C.20 C.« i.! c C.J? 0.K i.cr 0.37 :.r :.90 o.« • rr :.7t 0.M' :.7E i.ti 0.72 :.?c ;.*? 0.61.22 :.3= 0.9i £.*" :.p 1.01 IM 1.0: 1.0° 2.B* '..BE :.is 1.9i 0.73 :.25 t.r. 1.33 o.« 1.36 C.?E 1.46 6.1* 1.5* 0.0( 1.15 Figure 2 . 5 Film calibration curve produced by Idone and Orville in 1984 showing relative light intensity versus film diffuse density and specular density of the same region as measured by the microdensitometer .

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51 a General Instruments Strobotac xenon strobe after passing through a calibrated wedge. The density of each of the resulting image steps was determined using a diffuse densitometer with a 50 /um spot size. The figure shows that the film has little sensitivity below 10 relative light units but is approximately linear from 10 to 1000 relative light units. The calibration of the Kodak 5474 Shellburst film by SUNYA is the most thorough calibration made to date for the conditions of exposure and development used by both SUNYA and UF. Since the same film and development conditions were used in Gainesville in 1982 as were used in Tampa in 1979 no additional calibration was performed and the SUNYA conversion factor from density to relative light intensity is used throughout this thesis. 2.3.2 Film Digitization All of the images for this thesis were analyzed by displaying and manipulating digital images which were digitized using an Optronics Photomation microdensitometer. The Optronics microdensitometer is capable of digitizing a 9 x 9 inch transparency at a spatial resolution of 12.5 /urn with 8 bits of density resolution over a selectable scale of either 2 or 3 diffuse density units. The microdensitometer consists of a set of selectable upper apertures, illuminated by a

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52 tungsten light source, which are focused onto the film plane. The image of the aperture is then focused onto a second set of apertures with the light passing through the aperture impinging on a photomultiplier tube. A logarithmic amplifier is used to convert the ratio of the incident light to the transmitted light into units of specular density. The value of incident light is sampled and stored once during each revolution of the drum by sensing the light source through a zero density hole in the drum. 2.3.2.1 Microdensitometer alignment and calibration It is necessary to align the mechanical carriage of the microdensitometer to insure that valid density values are measured. Mechanical alignment consists of adjusting the position of the light source with respect to the upper aperture and adjusting the relative positions of the upper and lower apertures. The light source position is adjusted such that the image of the light source projected through the upper aperture onto the film is uniform and out of focus. This is accomplished solely by varying the position of the light source. The upper and lower apertures are aligned by iteratively adjusting the rotation and translation of the lower objective lens assembly such that maximum light is received at the photomultiplier tube. The anode voltage of the photomultiplier tube is adjusted after the

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53 mechanical alignment to compensate for the aging of the tube. In order to produce valid measurements it is also necessary to calibrate the logarithmic amplifier. The logarithmic amplifier converts the ratio of incident light to transmitted light into units of specular density. Calibration is accomplished using several values of neutral density wratten filters. Wratten filters are used since they scatter very little light thereby having a measured specular density very near the actual diffuse density. The logarithmic amplifier is first adjusted to have zero density with nothing in the light path. The amplifier is then adjusted with a 3.0 D filter in the path so that the measured density reads 255 on the 8 bit scale. This procedure was repeated prior to every digitizing session. 2.3.2.2 Microdensitometer interface and software The microdensitometer is attached to an IBM/AT class personal computer through a custom interface. The interface has an 8 bit bidirectional bus and control lines implimented with a modified Tecmar 96 line digital I/O board. Custom software was written to control the microdensitometer. The software provides functions to step the position the carriage, digitize a region of film, and place the results into a disk file on the personal computer. The software provides the capability of averaging the image to be digitized a number of times.

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54 This slows the digitizing process but is very effective in reducing the random noise of the photomultiplier and amplifier. All of the data used in this thesis were averaged 8 times. 2.3.3 Image Display Techniques Digitized data images were manipulated and displayed using a software system referred to as the Earth Resources Laboratory Applications Software (ELAS) . ELAS was written at the Earth Resources Laboratory of the National Aeronautics and Space Administration at Bay St. Louis, Mississippi. Elas consists of approximately 240 software modules which manipulate and perform calculations on images. The image display modules and polygon manipulation modules were used extensively to analyze the data in this thesis. As Engineering Manager of the University of Florida Remote Sensing and Imaging Processing Laboratory during the period of 1982-1987, I assisted Mr. Ray Seyfarth in converting the Perkin Elmer version of Elas to the personal computer environment. The software drivers were written to provide ELAS display support for the NUMBER NINE Computer Corporation's 512 x 512 x 32 bit display board. This board provides four 8 bit images arranged as three color images with 8 graphic overlays.

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55 2.3.4 Computer Analysis Software Software was written for the personal computer, in addition to the ELAS software, to perform measurements on the digitized data while viewing the data on the monitor. This program was written so that it would run concurrently with ELAS. This configuration allows the program to communicate, using software interrupts, with the root portion of ELAS which controls the positioning of the cursor on the display device. This configuration allowed locations of measurements on the personal computer graphics screen to be reflected in the actual cursor position on the displayed image. The analysis software can display data from one or two-dimensional arrays. The program expects data in standard ELAS data file format and is capable of reading and modifying the ELAS header information. The software analysis system contains several sections. The main section allows the user to select a data file to analyze and shows a plot of the first horizontal line of the image on the personal computer screen. The user may display any line in the file or seguence through the lines with a suitable increment between lines and with variable horizontal and vertical magnifications. Horizontal and vertical markers are provided, to assist in measuring differences in position along both axes, and their current values as well as the

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56 values of other pertinent parameters are available on the display screen. A complete plot package is implemented which generates plots with variable parameters in Hewlett Packard Graphics Language format. 2.3.5 Measurement of Relative Light Intensity Versus Time Measurements of light intensity as a function of time are accomplished using the personal computer software system. The digitized image of the event of interest is accessed and the line of data corresponding to the desired height is displayed. The time scale for the image is stored in the file header in microseconds per sample. This allows the program to provide a horizontal time scale. Events of interest may be measured directly with the markers or the user may generate a plot of the displayed region of data. 2.3.6 Measurement of Relative Light intensity Versus Height Measurement of light intensity as a function of height is more complex than the measurement of light intensity as a function of time since the path along which the measurements are to be made must be delineated. A new module was written for ELAS in order to exploit the line drawing capabilities already implemented. The module is named DENT and works in conjunction with CPPP and POLY.

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57 The function of DENT is to pick out the relative light intensities of an image which fall along a line. The user may choose to specify the exact line path or an approximate path. If an approximate path is specified, the program will examine a range of light intensities near the line and pick the peak in the surrounding data as the new point on the line. This feature enables automated tracking of features such as dart leader or return stroke peak relative light intensity as a function of height. A region may also be specified a certain distance from the line over which the program is to compute the average of the data values. This feature allows an average background density value to be computed for each height. The file produced by DENT is an ascii text file containing one line describing the data file followed by lines containing the vertical line, horizontal element, average of the specified region before or after the polygon, and the value of the data point along the polygon or the peak near the polygon. This format is used for all data files with an extension of .DNT. A display of relative light intensity as a function of height is obtained by starting the MDP program with the /D option. This option will display a list of available •DNT files and allow the user to select one. The profile of the relative light intensity will then be displayed as a function of height.

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58 2.3.7 Measurement of Leader Speed Leader speed is determined by measuring the displacement of the initial leader light intensity from the leading edge of the return stroke light intensity at a known height along the channel. The ratio of the height in meters to the displacement in time yields the leader speed. This measurement is a reasonable approximation to the actual two-dimensional leader speed if the return stroke speed is assumed to be at least an order of magnitude higher than the leader speed since the measured displacement includes the time for the leader to reach ground and the return stroke to travel back up the channel to the given height. For typical return stroke speeds the error in the dart leader speed due to this measurement technigue is approximately 10 percent. 2.4 Electric Field Analysis The methods by which the electric field records in this thesis were analyzed will be discussed in the following sections. Electric field records are presented from Tampa, Florida in 1979 and from Gainesville, Florida in 1982. 2.4.1 Electric Field System Calibration

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59 Electric field calibration involves the determination of the response of the electric field system to a known vertical electric field applied to the antenna and the measurement of the enhancement factor for the measurement location with respect to ground level. At ground level the flat plate antennas used in these experiments have an enhancement of unity. A detailed discussion of the calibration of the 1979 data is found in Master (1981) . Briefly, the enhancement factor was determined by placing an antenna on the ground and simultaneously recording the values obtained on the antennas on the truck and the ground. The enhancement value in 1979 was 2.65. In 1982 the electric field system was calibrated in a manner similar to that used in 1979 with the exception of the determination of the enhancement factor. The enhancement factor for the antenna located on the roof of Beatty Towers was determined by using the average initial peak electric fields of 38 subsequent strokes and setting the average equal to that obtained by Master (1981) from the Tampa data. The enhancement of the 1982 electric field measurements was thus determined to be 5. 2.4.2 Electric Field Digitization In 1984 the Lightning Laboratory acquired a Masscomp minicomputer with an analog to digital converter system capable of digitizing several seconds of data at a 500 kHz

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60 rate directly to disk. The effective rate of the digitizing process could be increased to over 5 MHz if the instrumentation tape recorder speed is slowed while digitizing. This system was used exclusively for analysis of the electric field records analyzed in this thesis. Digitization was accomplished by connecting the proper channels of the instrumentation recorder to the digitizer and running software on the Masscomp computer which controlled the digitizer. In most cases the sample and hold circuits of the digitizer were used in order to sample several channels simultaneously then multiplex the digital values into the disk file. The freguency modulated channels of the instrumentation recorder were used almost exclusively so that the playback rate could be slowed down by a factor which would allow several channels of an entire flash to to be sampled and stored to disk. The data were recorded onto the FM channels of another instrumentation recorder before digitizing if the direct channels of the data were of interest. This process reduced the high freguency cutoff of the data from 1.5 MHz, that of a direct channel, to the 500 KHz cutoff of an FM channel. Use of FM channels for digitization was necessary in order to preserve the low freguency portion of the waveshape of the signals when the recorder speed was slowed. Data were normally sampled at 78.125 kHz if they were played back at 1/64 original speed, eguivalent to sampling

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61 the data at 64 x 78125 = 5 MHz in real time. This was an effective sampling rate of 0.2 /xsec/sample. Since the response of the recorder was -6 db at 500 kHz, this procedure was equivalent to sampling at 10 times the corner frequency. The sampling rate was above the Nyquist rate for frequencies up to 2.5 MHz . The recorder had negligible frequency response above 2 MHz. Typically, approximately one second of real time data was digitized. The main advantages of the method described are the ability to digitize at high rates for long periods of time and the ease of display and measurement on the Masscomp as discussed below. 2.4.3 Electric Field Analysis Software The electric field data which were digitized to disk were at first cumbersome to display and plot. The Masscomp computer had general purpose software to plot simultaneous sets of data but it proved too slow for serious analysis purposes. Two new programs were written to facilitate analysis of electric field records. 2.4.3.1 Display software In order to rapidly display, analyze, and plot electric field records a software program was developed which exploited hardware available on the Masscomp. The program is very general in that it allows the user to view multiple channels from different data files (up to 6) .

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62 Channels of data are presented as time on the horizontal axis versus digitizer units on the vertical axis. Data may be shifted relative to other channels as well as expanded together or separately. The program uses the array processor of the Masscomp to scale the data onto the screen. This technigue bypasses the graphics processor scaling algorithms and allows multiple channels of 5000 points each to be displayed rapidly. The disk I/O rate was a problem for reading large data files (typically 20-40 MB) and necessitated the use of low level file routines in the C language to read large buffers of data. A plotting package was designed into the system so that a user could produce a hard copy of the screen contents at anytime to either a Gould or Hewlett Packard plotter. The plotting package was linked to a system routine which would continuously plot all available plots asynchronously. Figures 2.6 and 2.7 show examples of electric field plots for the flash at Tampa on July 27, 1979 occurring at 2206:51 UT at a distance of 7.8 km. 2.4.3.2 Averaging software Many of the data in the electric field records are noisy and must be smoothed in order to find features of interest. Software existed to average a single channel data file but it took over 8 hours to average one second of data. This program was rewritten to use large file buffers and large arrays with efficient looping. The

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63 result was to reduce the execution time from 8 hours to 9 minutes for one second of data (10 MB) . The program was modified to accept data with any number of channels and has a user interface consistent with the display program. 2.4.4 Measurement of Interstroke Time Intervals Interstroke intervals were measured by plotting the interstroke electric field change on a suitable scale then measuring the time difference between the fast portions of the return stroke electric field change. Figure 2.7 demonstrates the measurement of the interstroke interval between strokes 2 and 3 of the flash on 27 July 1979, 2206:51 UT. 2.4.5 Measurement of Return Stroke Initial Peak Electric Field Figure 2 . 9 demonstrates the technique used to measure the initial peak electric field for stroke 6 of the 27 July 1979, 2206:51 UT. The initial peak electric field must be measured from data with sufficient high frequency response to preserve the peak waveshape. For the data in this thesis the Biomation records taken from the direct recorder channel were used for peak field measurements. 2.4.6 Measurement of Leader Field Change Duration

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64 Figure 2 . 8 demonstrates the method of determining the duration of the leader field change for stroke 6 of the 27 July 1979, 2206:51 UT flash. Leader field change durations are often difficult to measure due to the noise level of the data and the shape of the beginning of the field change which is the most difficult to identify. The leader field change terminates with the beginning of the return stroke field change which is easily identified. 2.4.7 Measurement of Ratio of Leader Field Change to Return Stroke Field Change The ratios of leader electric field change to return stroke electric field change were measured in the manner described by Rakov et al. (1990). As illustrated in Figure 2.8, a line was drawn with the slope of the field change preceding the dart leader. This J field change slope is assumed to continue throughout the leader and return stroke field change (Krehbiel et al., 1979). The dart leader field change terminates at the beginning of the return stroke field change. If the return stroke field change after the initial peak exhibited a "knee" shape, as discussed by Beasley et al. (1982) , then the time of occurrence of the "knee" was considered as the end of the return stroke field change. If the return stroke was followed by a continuing current field change then the position of the first M-component "hook" shaped field

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65 change was taken as the end of the return stroke field change . Thomson (1985) investigated possible errors in previous techniques for measuring leader-return stroke electric field change ratios used by Schonland et al. (1938) and Beasley et al. (1982). In this thesis we adopt those techniques since the measurements of leader-return stroke ratio presented were made only in order to compare ratios as a function of stroke order. Thomson (1985) stated that two conditions must be met for one to interpret the leader-return stroke ratio in terms of the charge distribution along the leader channel. First, both field changes must be the electrostatic components of the field changes. Second, leader and return stroke field changes must be the result of the same charge which is the charge deposited along the channel by the leader then lowered to ground by the return stroke. He indicated that, if the correct beginning point for the dart leader and ending point for the return stroke are not determined, the measured leader-return stroke ratio may contain significant errors due to the contributions of the field changes of the uniform current assumed in the model of Lin et al. (1980) or from continuing current flowing at the end of the return stroke. He suggested that the field 170 Msec after the start of the return stroke be used as the ending point, again assuming the model of Lin et al. (1980) , and that the contribution of the field change due

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66 to the uniform current, which is assumed to be flowing in the channel, be subtracted in order to obtain a reasonable return stroke electrostatic field change.

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67 Tampsr, 1979 2206:51 UT W^ A*^ Mw '.KmMiI 'i y
PAGE 86

68 Tampa, 1979 Sixth Stroke 2206:51 UT UJ o z. < X o o _J UJ n: o EC O UJ —J UJ \ TIME Figure 2 . 8 Electric fields preceding stroke 6 of the 27 July 1979, 2206:51 UT flash showing the measurement of dart leader duration.

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69 Tampa, 1979 2206:51 UT 6th Strokel 1 J UJ < ^ Wf 71 li Stroke O Q _J UJ EE O DC 1O —5.5 ms*> M, M J /" UJ LU 1 1 f 2 ^r ' r^ / 3 12V/m 1 TIME Figure 2 . 7 Overall electric field change for strokes 2 and 3 of the flash on 27 July 1979, 2206:51 UT showing the measurement of the interstroke interval.

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70 Tampa, 1979 2206:51 UT *
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CHAPTER 3 DETAILED ANALYSES AND RESULTS 3 . 1 Database The strokes which are discussed in the following sections are listed in Tables 3.1 and 3.2. The data acquired by the University of Florida Lightning Research Laboratory include two flashes from the Tampa experiment in 1979 and three flashes from the Gainesville experiment in 1982. The data consist of one new channel to ground which is the third stroke of a flash and 22 subsequent strokes with 11 detectable dart leaders, all correlated with electric fields. The data on loan from the State University of New York at Albany contain 11 first strokes and 10 subsequent strokes with 5 detectable dart leaders. No electric fields were recorded in the SUNYA experiments. Prior to discussions of properties of first and subsequent strokes it is useful to explore relationships which exist within individual flashes. Dart leader field change duration, previous interstroke interval, initial peak electric field, and the ratio of leader to return stroke electric field change have been measured within five flashes and their values plotted versus stroke order . 70

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71 Table 3.1 Summary of Stroke Parameters Derived from Correlated Optical and Electric Field Records Storm Time Distance Stroke Year/ Day (UT) km Order Return Stroke Initial Electric Field Peak at 100km V/m Previous Interstroke Interval ms 79208 2206:51 8.2 3 4 5 6 7 8 9 4.8 2.3 2 4 2 2 3 223' 58 94 38 43 35 44 79208 2246:45 8.8 2 3 3.1 6.8 25 30 82202 1317:14 "6 (EDT) ** 4.9 5.1 2.2 2.1 1.7 3.8 57 51 63 98 23 79 82222 82222 1445:35 (EDT) 1446:56 5.3 2 3 2 3 4 5 6 7 *** 5.9 6.5 4.7 4.7 5.5 1.9 3.5 3.1 44 31 23 53 34 60 41 24 * Includes continuing current of 85 ms duration. ** Dart-stepped leader *** First stroke along a new channel to ground.

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72 Leader Leader Leader Return Duration Speed Luminosity Stroke ms (relative Luminosity 10 6 m/s units) (relative units) 1.9 12.0 41 5 3 19 "0 5 9 24 99 9 11 5 "1.5 "1.5 6 57 6.4 20 20 15 7 7 0.98 14.6 2 1.9 1.6 2.1 — 0.34 10.7 — 0.22 17.4 12 0.95 5.4 0.45 24.4 3 1.8 9.2 3.3 4.0 7.9 0.15 15.7 26 0.36 17.9 18 — 0.5 16.6 4 -

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73 Stroke #92 Leader Order Filter Speed 10 6 m/s Table 3.2 Summary of Stroke Parameters Derived from Optical Records Borrowed from the State University of New York at Albany Storm Time Distance Year/ Day (UT) km 78186 1 YES 78192 1926:45 4.6 1 YES 2 YES 78210 2009:57 3.5 1 YES 2023:46 4.6 1 YES A YES 3 . 8 B YES 2025:50 2.0 1 YES YES YES YES YES 5.9 YES YES 5.2 YES 84148 1749:02 2.0 1 NO 84161 2107:20 5.0 1 NO A NO B NO C NO 2107:20 5.0 1 NO 2032:10 6.0 1 A 2032:51 4.0 1 A 2041:00 4.9 1 2041:00 4.9 A B

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74 Data for the 27 July 1979 2206:51 UT flash are presented in Figure 3.1. This flash had sixteen strokes of which seven subsequent strokes were imaged by the streak camera. The first stroke had a double termination to ground as seen on the television records. The second stroke initiated a long continuing current (about 85 msec duration) . Only stroke six produced a detectable dart leader on the streak film. Stroke six had the shortest dart leader field change duration and the shortest previous interstroke interval of the imaged strokes. It also had the second largest return stroke initial peak electric field. Stroke three, which followed a long interstroke interval (223 msec, including about 85 msec of continuing current) , had a slightly larger initial peak electric field than the sixth stroke. Analysis of electric field records from 1979 showed pulses typical for dartstepped leaders prior to the return stroke (Krider et al., 1979) . Data for the 27 July 1979 2246:45 UT flash are shown in Figure 3.2. This flash had four strokes of which two were imaged by the streak camera. Stroke two produced a weak dart leader and had a relatively small return stroke electic field peak. Stroke three had a very bright dart leader which was 1.7 times as fast as as the leader of stroke two and was followed by a return stroke with an initial peak electric field which was twice as large as stroke two.

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75 Tampa 79208 220651 UT Distance 8.2 kn 14 a* •i n fl 3. .x Si u S3 F ™ 4 6 I 10 12 STROKE ORDER (a) 14 18 Q < < > < > i io a u ii STROKE OROER (b) 24>r M»80fl O.Or 0.4 s a io STROKE ORDER (c) 12 14 18 I' ""-0.0 -1.2-1.8«fl m 8 10 18 14 18 TJ II DO STROKE ORDER (d) Figure 3 . 1 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 27 July 1979, 2206:51 UT flash. Crosshatched strokes had detectable dart leaders. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale.

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76 Tampa 79208 224645 UT Distance 8.8 km g 8.3 t 0.1 1 1 2 3 4 STROKE ORDER (a) STROKE ORDER t — i — i — r STROKE ORDER (O 0.04 0.0S •a • 3 -*.M -O.M -t.Ot H).l 3 M 4 iB * . (d) Figure 3.2 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 27 July 1979, 2246:45 UT flash. Cross-hatched strokes had detectable dart leaders. The ratio of leader-return stroke electric field change was not measurable for strokes 2 and 4. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale.

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77 Data for the 21 July 1982 1317:14 EDT flash are shown in Figure 3.3. This flash had seven strokes and all but the first stroke were imaged by the streak camera. Four of the seven subseguent strokes produced detectable dart leaders. Stroke three had the fastest dart leader (2.44 x 10 7 m/sec) of all the data which were analyzed. It had the largest return stroke initial peak electric field and the shortest dart leader duration of the flash. The second stroke also had a large initial peak electric field but was preceded by a dart-stepped leader. Data for the 10 August 1982 1445:35 EDT flash are shown in Figure 3.4. This flash had three strokes and the two subseguent strokes were imaged by the streak camera. Both of the subseguent strokes produced highly visible dart leaders on the streak film. The measured dart leader durations for both subseguent strokes were unusually short with the duration for stroke three being twice as long as stroke two even though they had practically the same dart leader speeds below the cloud. Data for the 10 August 1982 1446:56 EDT flash are shown in Figure 3.5. This flash had seven strokes total along two channels to ground. The subseguent stroke for the first channel as well as all strokes for the second channel were imaged by the streak camera. Only the second stroke along the second channel produced a detectable dart leader. The electric field records for this flash were very noisy making it impossible to measure

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78 Gainesville 82222 131714 UT Distance 5 km >r 1 2 3 4 S S 7 STROKE ORDER (a) 1 I 3 4 t.. 8 7 STROKE ORDER (b) 100 49 1 2 3 4 3_ 6 7 STROKE ORDER (C) "Si -0.8-1.2-1.8STROKE ORDER 1 2 3 4 S 8 7 (d) Figure 3 . 3 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 21 July 1982, 1317:14 EDT flash. Crosshatched strokes had detectable dart leaders. Leader electric field properties for stroke 6 were not measurable. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke

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79 Gainesville 82222 144656 UT Distance 6 km LBader duration is not MuurablB (a) f v i a 3 4 s STROKE ORDER (b) (AE^/AEfjJ is not leasursbla (d) 1' 2' 1 2 3 4 3 STROKE ORDER (c) Figure 3 . 5 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 10 August 1982, 1446:56 EDT flash. Crosshatched strokes had detectable dart leaders. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale.

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80 Bainesville B2222 144535 UT Distance 5.3 km n Ul , i H S STROKE ORDER (a) 1 8 > STROKE ORDER (b) AE^/aEr > for all strokes (d) i i « STROKE ORDER (c) Figure 3 . 4 Histograms of (a) dart leader field change duration, (b) initial peak electric field, (c) previous interstroke interval, and (d) ratio of leader to return stroke electric field change versus stroke order for the 10 August 1982, 1445:35 EDT flash. Crosshatched strokes had detectable dart leaders. The leader-return stroke electric field ratio was not measurable for this flash. Dashed lines indicate ambiguous measurements. Parallel lines indicate measurements of first stroke parameters which are off scale.

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81 dart leader durations for strokes other than the stroke with the visible dart leader. 3.2 First Strokes Streak camera records are available for eleven first strokes and one new channel to ground which is the third stroke of a flash. The eleven first strokes are from SUNYA and the new channel to ground is fron the 1982 Gainesville experiment. Only six of the twelve records were suitable for detailed analysis. 3.2.1 Channel Relative Light Intensity Variations Variations in relative light intensity for first strokes must be carefully made near branch locations along the channel. It is at these branch points that the channel light intensity varies the most abruptly. The first stroke at 1446:56 EDT August 10, 1982 occurred at a distance of about 6 km and was the second channel to ground in a seven stroke flash. This stroke was the third stroke in the flash and was followed by four subsequent strokes down the same channel. Electric field records are available for all of the strokes in this flash. A photograph of the digitized image of this stroke is shown in Figure 3.6. Figure 3.7 and Figure 3.8 contain the relative light intensity profiles versus height for

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82 this stroke. A detailed analysis of the variations in light intensity for this stroke was not made due to the image quality. The first stroke at 1926:45 UT on July 11, 1978 is shown in Figure 3.9. This flash was located at a distance of 4.6 km and was imaged through a #92 filter. Twin lens streak camera images were obtained for this stroke and a subsequent stroke. Relative light intensity profiles at increasing heights for the first stroke are shown in Figures 3.10 and 3.11. Two prominent branches are visible on the streak camera images. The rise time of the return stroke increased from 2 . 3 jusec below the lower branch to 13 jusec above the branch. The return stroke rise time was 7 /xsec below the upper branch and 18 /usee above the branch. These changes in rise time occur within ten meters along the channel. The return stroke relative light intensity decreased from 45 rlu to 21 rlu while passing the lower branch point and decreased from 71 rlu to 27 rlu while passing the upper branch point. The first stroke at 2041:00 UT on July 29, 1978 is shown in Figure 3.12. Relative light intensity profiles as a function of height for this stroke are shown in Figures 3.13 and 3.14. Only one stroke was photographed in this flash. The image was taken with a #92 filter and is of particular interest since it has a double termination to ground. No electric field records are available for this stroke; therefore it is impossible to separate the spatial

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83 and temporal components of the flash near the ground. It can be seen in Figure 3.13 that at meters the peak relative intensities of the terminations appear to be approximately the same. This is somewhat misleading since the relative light intensity of the left channel must be subtracted from that of the right to obtain a valid comparison. The left termination exhibits a marked decrease in intensity from 60 rlu to 22 rlu at 98 meters for no apparent reason. It is possible that a branch is present but obscured by the return stroke streaking. At 257 meters the right termination joins the left. There is a decrease in intensity at this point from 67 rlu to 32 rlu with an increase in rise time from 2.3 /usee to 6.5 Msec. At 402 meters the rise time is 9.4 /xsec (47 rlu), decreasing to 5.6 /xsec (50 rlu) at 410 meters then increasing to 6.4 /xsec (41 rlu) at 417 meters with these changes occuring on either side of a branch which is barely visible. Above the small branch at 650 meters the rise time decreases to 3 /usee with the light intensity dropping to 15 rlu. Rise time measurements were not made above 700 meters due to the diffuse channel light intensity. The first stroke at 2023:46 UT on July 29, 1978 is shown in Figure 3.15. The relative light intensity profiles at increasing heights for this stroke are shown in Figures 3.16 and 3.17. A #92 filter was also used for this stroke. Two subseguent strokes were also obtained for

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84 this flash. The rise time of the return stroke relative light intensity at 46 meters is 2.3 /usee and increases to 4.7 /xsec at 151 meters then decreases again to 2 . 5 /xsec at 197 meters. At an altitude of 271 meters the rise time again increases to 6.4 /xsec but within 10 meters at 280 meters has decreased to 3 . 5 jusec. This effect is also seen at altitudes between 350 meters and 565 meters where the rise time decreases from 11 jusec to 3 . 5 /xsec. The first stroke at 2032:10 UT on July 29, 1978 is shown in Figure 3.18. The relative light intensity profiles at increasing heights for this stroke are shown in Figure 3.19. The first stroke at 2032:51 UT on July 29, 1978 is shown in Figure 3.20. This flash had one subseguent stroke and was imaged with a #92 filter. The relative light intensity profiles at increasing heights for the first stroke are shown in Figure 3.21. This flash was partially obscured but significant branching was visible. The first stroke at 1749:02 UT on May 27, 1984 is shown in Figure 3.22. This image was obtained in Oklahoma and no filter was used. The relative light intensity profiles for this stroke are shown in Figures 3.23. Only one stroke was recorded for this flash. The distance to this stroke was approximately 2 km. Branch components are visible for this stroke along the lower portion of the channel. It is not obvious which of the branches caused

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85 the branch component since some of the branches on the right side of the channel are obscured by the streaked light. The rise time of the return stroke relative light intensity is 2 . 1 /usee just below the branch point at 100 m then increases to 4 . 2 /xsec above the branch point at 104 m. At 160 m the rise time has decreased to 2.1 /xsec but increases to 8 /xsec above the branch point at 223 m. The branch component peak relative light intensity at 20 m is 8 rlu with a return stroke peak relative light intensity of 54 rlu and at 120 m is 12 rlu with a return stroke peak relative light intensity of 49 rlu. The first stroke at 2107:20.81 UT on June 9, 1984 is shown in Figure 3.24. The relative light intensity profiles for this stroke are presented in Figure 3.25. There was only one stroke recorded for this flash. These data are not filtered and a calibration strip was developed at the same time as the data. The data presented above suggest that the physics of the initial light front which propagates up the channel is a highly non-linear process since the rise time of a linear process, which is assumed to be losing energy, can not decrease once it has increased. Even though the initial front loses a significant amount of energy at a branch point, as evidenced by the increase in rise time and decrease in relative light intensity just above the branch, it rebuilds the sharp front after traveling further up the channel. This effect can be seen in the

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86 first stroke shown in Figure 3.9 at altitudes between 330 m and 350 m and between 500 and 550 m, the first stroke shown in Figure 3.12 at altitudes between 400 m and 420 m, the first stroke shown in Figure 3.15 at altitudes between 46 m and 197 m, between 271 m and 280 m, and between 350 m and 365 m, and the first stroke shown in Figure 3.22 between 100 m and 160 m. 3.2.2 Channel Relative Light Intensity Variations Due to Branches The first stroke at 1749:02 UT on May 27, 1984 shown in Figure 3.22 exhibits relatively large changes in channel relative light intensity when the return stroke passes a point where a branch connects to the channel. Examples are seen at altitudes of 100 m and 219 m. The increase in channel relative light intensity which travels down the return stroke channel below the branch point as the return stroke is traveling up the channel above the branch point is called a branch component . The peak relative light intensity of the branch component along the lower channel of this stroke was 6 rlu at the bottom of the channel while the return stroke relative light intensity was 60 rlu. At 120 m the branch component peak relative light intensity was 12 rlu while the return stroke peak was 49 rlu. At 140 m the branch component peak relative light intensity is 7 rlu and the return stroke

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3 H> F(1) M D ID H3 ^( a hP (D rt (D .-3iQ en H (D HC a 3 ^ > r ^ tn (D C H(U & H MilQ N cr C rt (D d (D m i>< I~S cn a CD 0) (D rt rt < 3 HMl H(D H 1 3 O ^ CD O 0) 1 3 cn a ^ cn d CD rt cn HH 3 0> 3 i£> O H3 rt (XI H, n Pi H rt to 3 3" • H) H, h( (D (D H|_i ft| • i-3 h 0) rt ^ 3* cn cn pHMO) rt 3" 3" o • cn (D rt 3* W cn CD rt £ H HM (D CD O iQ O ^ 0) h, 3* CD a rt fl> (D i-i CO 3 l-< (D 0J O (Hrt rt (t fti 3 H* ft&H (D HH4^ < n ^ rt (0 fu
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88 is! f t LU O S£ ».» f t H o o ©« O CO >. N nQ >0 ID

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89 1080 m 2: UJ X IxJ > a: Figure 3 . 7 T 25 50 75 MICROSECONDS 100 Relative light intensity versus time at for the stroke at 14 4 6. '56 EDT August 10, 1982.

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90 in UJ X UJ > UJ a: 480 m 420 m 360 m 300 m 240 m 180 m 120 m 60 m 25 50 75 MICROSECONDS 100 Figure 3 . 8 Relative light intensity versus time for the lower 500 meters of the stroke at 1446:56 EDT August 10, 1982.

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H2 (0 U3 (D H 0) ft < ID ph a 3 WK HCTi H•• ft .&• HUl N (t> ft a rt (D 3 n ft < rt) i-( en d •I a> (D Hi (D i-i rt) o ft) 3 a H^ 3 H O H Hi a oo Hrt tr 01 (D fo tr ID rfd hHMvQ 3 O S3* (D ft rt • 01 01 o Ml Hi H•1 tfl rt 01 t-3 rt o 01 rt

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92

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93 906 m to z: UJ X o UJ > UJ a: t r 25 50 75 MICROSECONDS 100 Figure 3.10 Relative light intensity versus time for the stroke at 1926:45 UT July 11, 1978.

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94 515 m 400 m z LxJ 354 m Z 1X o 331 m —1 LU > UJ a: 297 m 193 m 90 m m Figure 3.11 ""I 1 25 50 75 MICROSECONDS 100 Relative light intensity versus time for the lower 500 m of the stroke at 1926:45 UT July 11, 1978.

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Jq 2 h CD CO to H h <-t a 3 IB c Hft H> HlQ (D CD •< H3 H rt cn CD to HH3 i£ N rt o ^ CD »< CD a a i< UD HCD H-J 3 h 3 00 0) cn • U2 C ft CD cn 3" H (D 3* rt CD H> p-'O 3 M 3" M» ID O CD H• ft H^ cn 03 en 3* rt O rt M) tfl cn rt ^ Ht-c CD 3 o m a x » HCD rt H0) 0) < rt rt (D CD a M H O P0) J^ iQ >-i H 3" CD • • rt O O

PAGE 115

96

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97 924 m in LJ UJ > UJ a: 25 50 75 MICROSECONDS 100 Figure 3 . 13 Relative light intensity versus time for the stroke at 2041:00 UT July 29, 1978.

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98 o UJ > a: 453 m 417 m 306 m 257 m 230 m 206 m 172 m 147 m 98 m m MICROSECONDS Figure 3 . 14 Relative light intensity versus time for the lower 500 m of the stroke at 2041:00 UT July 29, 1978.

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ID H Ul H•1 M HHD UD N rt o ^ a> *< ID a a h < VD Hn> H^J 3 ^ 3 CO CD Ul • *£> c rt (T> w U H a> 3* O rt ID l-h H-^O 3 HCTM) ID o fl> H• ft H^ en <£ Ul 3 rt o rt M) 01 Ul rt h h>~i (D 3 O m a ^ 0) H(D rt p01 0) < rt rt (D (0 d to H O PCD M vQ ^ CJ U (I • • rt 4^

PAGE 119

100

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101 -z. UJ > 966 m 915 m 856 m 764 m 672 m 626 m 580 m 566 m 524 m 511 m 419 m 396 m 350 m 281 m 271 m 258 m 198 m 152 m 46 m m n 1 r~ 25 50 75 MICROSECONDS 100 Figure 3.16 Relative light intensity versus time for the stroke at 2023:46 UT July 29, 1978.

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102 V) z UJ UJ > UJ a: 511 m 419 m 396 m 350 m 281 m 271 m 258 m 198 m 152 m 46 m m n 1 r~ 25 50 75 MICROSECONDS 100 Figure 3.17 Relative light intensity versus time for the lower 500 m of the stroke at 2023:46 UT July 29, 1978.

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1 ID H CO H3 ft fD en prt < ID en C en 4 <^ a CD C PHi MiQ fl> ^< Hh rt to p«3 N ' CD a rt P3 CD -J 3 00 DJ 0) M o rt tr CD 3CD P3* O rt Hi w CO I1 rt PCD P3rt o H> H) Ptn rt rt 3 O Pa> o (U pj rt rt CD Q, M O 0J LO l-l N> CD •• H O

PAGE 123

104

PAGE 124

105 840 m in Ui > a: 100 MICROSECONDS Figure 3.19 Relative light intensity versus time for the stroke at 2032:10 UT July 29, 1978.

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^ H03 C 1 (D CO to o H^ q a 3 (D c Hrt m> miQ (D (D ^< H3 ^ rt to (D tO pH3 U3 N rt ^ n> < n> a a h < KD H(D H<\ 3 h 3 00 PJ W 03 e rt (D en tr H (D cr o rt (D i-h H-W 3 HUM) n> O (D H• rt h^ CO vQ CO 3" rt o rt n» tn tfl rt ^ Hh (D 3 O h a *ft> H(D rt O H0> 0) < rt rt (D ID Pi to M o HQ> U) 03 V M 3* (D * • rt (J1

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107 j£ r.j O O (0 W« IX .. '"•J '_j hO •& >>" IT* <<1 t-» £S f f f f £ £ t t 000 04 co
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10 S in o UJ > uj 600 m 510 m 460 m 440 m 430 m 320 m 280 m 240 m 200 m 160 m 120 m 80 m 40 m m r~ 25 50 75 MICROSECONDS 100 Figure 3.21 Relative light intensity versus time for the stroke at 2032:51 UT July 29, 1978.

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*0 ri (D u M W H 11 3 a 3 (D Hrt M)k< (£ ID ID H3 i-f t-o rt cn (D -J HK 3 *» N ft O (D ^< 0) H a vo < oo H(B H^ 3 ^ 3 • ii) [/) ^Q C rt i01 3 3* a> (D O rt M) p-tJ 3 3 M (0 H) 0) HP• rtiQ ^ (0 3 to rt lio en H) W Hrt ^ 3 ^ 0> O, o M H* Bi o (D rt (u Hrt cu < (D rt (D a h M 01 -J H^ ^ 03 (D i£ 3 • • rt o to

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110

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Ill 405 m V) z UJ LjJ > UJ a: 25 50 75 MICROSECONDS 100 Figure 3.23 Relative light intensity versus time for the stroke at 1749:02 UT May 27, 1984.

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2 ID to H•1 a D 3 ID k| Hft H> iQ (D (D C-l H 3 ^ C rt 01 (D 3 HH3 (D tsl rt (D »< (D UD p a "» < HID HH 3 H. 3 VO PI oi COkQ C rt *>• ID oi 3 • ID O rt 1-3 M H-'O 33 H ID Ml m O Hrt 3" •-< en n> 01 Hrt O iQ H> 3* 0! rt rt M. 01 i-i O H pW PJ 3 (D rt a HH0) < O rt 0) 0) rt to H (D M Ha o vQ vl 3" 0» • • rt h to o • CO H

PAGE 132

113

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114 1223 m i 1 r 25 50 75 MICROSECONDS 100 Figure 3.25 Relative light intensity versus time for the stroke at 2107:20.81 UT June 9, 1984.

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115 peak is 29 rlu. At 160 m, where the branch component first becomes distinct from the return stroke, its peak relative light intensity is 2 3 rlu while the return stroke peak is 10 rlu. One obvious feature of this branch component is the slow, tens of miroseconds, rise time which is very similar in shape to the rise times associated with Mcomponents; which are discussed in Section 3.4. Malan and Collens (1937) observed that branch components appear to travel down the return stroke channel and concluded that they are caused by a sudden increase in availabile charge as the return stroke passes the branch point. 3 . 3 Subsequent Strokes A total of twenty two streak camera images of subsequent strokes were recorded with correlated electric fields during 1979 and 1982. Ten of the twenty two subsequent UF strokes had dart leaders which were detectable on the streak camera records but only 6 were of sufficient quality to allow the measurement of their relative light intensity variations. An additional ten subsequent strokes, five strokes having detectable leaders, were available from the SUNYA data. In the following sections the dart leader results are presented followed by the results for subsequent return strokes and M-components .

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116 3.3.1 Leaders Of (Dart Leaders) As discussed in Section 2.2.2, the availability of electric field records enabled the determination of stroke order for all of the UF data but there is no stroke order information for the SUNYA data. In the following sections the results are presented for a number of leader parameters as a function of stroke order. 3.3.1.1 Leader speed versus stroke order A plot of dart leader speed, as determined from streak camera records, as a function of stroke order is shown in Figure 3.26. The minimum dart leader speed was 5.4 x 10 6 m/sec and the maximum was 2.4 x 10 7 m/sec. It can be seen that in general the dart leader speed increased from stroke two to stroke three then decreased with later strokes. This result in not conclusive considering the limited number of flashes available for analysis. Schonland et al. (1935) compared dart leader speed with the total number of strokes in the flash with no obvious correlation but never discussed leader speed as a function of stroke order. Brook et al. (1962) measured dart leader speed and knew the stroke order of the strokes but did not discuss this relationship. 3.3.1.2 Apparent channel length versus stroke order. Dart leader apparent channel length versus stroke order is shown in Figure 3.27. The apparent channel length is determined in the manner first described by Malan and

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117 25 20 X In a UJ UJ a. en cr UJ o UJ 15 10 5 Plot Stori Flash Sytbol Year Day (UD D 79208 220651 X 79208 224645 X 82202 131714 82222 144535 A 82222 144656 H Data of Schonland (1956) ^D -X B 1 _L 1. 3 4 5 STROKE ORDER Figure 3.2 6 Dart leader speed versus stroke order. Solid lines connect strokes which follow in order with detectable leaders. Dotted lines connect strokes of a flash which were separated by strokes with nondetectable leaders. Schonland data point is the mean for 55 leaders.

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118 20 IB 16 E jx U X htD UJ 12 _l cr UJ a 10 < UJ Plot Ston Flash Syibol Year Day (UT) 79208 220651 K 79208 224645 X 82202 131714 o 82222 144535 A 82222 144656 B CO 2 B a. < A 2 0, E Filth IE Of Krebiel et al . 11979) r r _J _j 3 4 5 STROKE ORDER B Figure 3.27 Apparent leader length versus stroke order. Apparent lengths of Krehbiel et al. (1979) were determined from charge source locations for each stroke. Dotted lines bound the data for 163 strokes reported by Brook et al. (1962) .

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119 Schonland (1951). The dart leader speed is determined, as discussed in Section 2.3.7, and, assuming this speed is constant into the cloud, is multiplied by the duration of the leader electric field change, see Section 2.4.6. Brook et al. (1962) made a number of speed measurements in which the speed appeared to change with height. This figure shows a clear tendency for leaders later in the flash to have longer apparent lengths. This is in agreement with data presented by Krehbiel et al. (1979). 3.3.1.3 Leader speed versus previous interstroke interval Dart leader speed as a function of interstroke interval is shown in Figure 3.28. The dashed lines on the figure indicate the limits of the data for 103 dart leaders acguired by Brook and Kitagawa (Winn, 1965) . It is noted that the speed measured for stroke 3 of the flash on 21 July 1982, 1317:14 EDT (2.4 x 10 7 m/sec) falls far outside the previously measured data. This is the highest dart leader speed reported in the literature for a dart leader measured with optical technigues. Richard (Richard et al., 1986) reported dart leader speeds as high as 5.0 x 10 7 m/sec using locations of vhf sources along the leader path. Also shown on the graph is the range of dart leader speeds and previous interstroke intervals found by Idone et al. (1984) for 32 dart leaders from triggered lightning. Triggered lightning dart leaders appear to be twice as fast as those for natural lightning. 3.3.1.4 Leader speed versus leader electric field change duration

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120 45 40 25 S 15 JX UJ H UJ 10 Plot Stora Flaih Syatool Year Day WT) D 78206 220651 IK 73208 224645 X 82202 131714 82222 144535 A 82222 144656 1 / o Data of Idone et al. (1964) I Data of Brook and I /Kltaoawa Winn. 1965) 1 "1 _L 20 40 60 80 PREVIOUS INTERSTROKE INTERVAL. RS 100 Figure 3.28 Dart leader speed versus interstroke interval. The dashed line bounds the data of Brook and Kitagawa ( Winn, 1965) for 100 dart leaders. The dashed and dotted lines bound the triggered lightning data of Idone and Orville (1984).

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121 Dart leader speed as a function of the leader electric field change duration is shown in Figure 3.29. The duration of the leader electric field change duration has a range of 0.15 msec to 4.0 msec with a mean of 0.99 msec. From the figure it can be inferred that leaders with higher speeds have shorter electric field change durations. 3.3.1.5 Leader speed versus initial peak electric field Dart leader speed as a function of the electric field peak of the following return stroke, normalized to 100 km, is shown in Figure 3.30. Also shown on the figure is a regression line for dart leader speed as a function of peak return stroke current from Idone et al. (1984) for 32 triggered strokes in New Mexico. The peak current values from Idone et al. have been plotted in terms of peak electric field instead of current using the formula presented by Willett et al. (1989) relating the two for triggered lightning in Florida. Ip = -3.9 X 10" 2 D E' p 2.7 X 10 3 where I p is peak current, D is the distance, and E' p is the modified or truncated peak electric field. Excluding the leader identified as dart-stepped, a linear fit of the data had an intercept of 3.45 and slope of 2.14 and a correlation coefficient of 0.55 at a 0.05 significance level, indicating a positive correlation of dart leader

PAGE 141

122 25 20 X V) Q LU UJ CL en (X LU a 15 10 0.5 Plot Ston Flash SyBbol Year Day tUT) D 79208 ??0B5i * 79208 224645 X 82202 131714 o B2222 144535 A 82??? 144656 _L 1.5 2 2.5 3 LEADER DURATION, ms 3.5 4.5 Figure 3.29 Dart leader speed versus dart leader electric field change duration.

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123 25 20 X In 15 cn CD < 10 V-U.4+4.96Ep V-3.45+2.14%) I I I / Plot Start Flash Syrtol Year Day (UT) D 79208 220651 * 79208 224645 X 82202 131714 82222 144535 A B???3 144656 J_ 4 6 B INITIAL ELECTRIC FIELD PEAK NORMALIZED TO 100 km. V/m 10 12 Figure 3.30 The Dart leader speed versus return stroke initial peak electric field. The dashed regression is for 32 triggered lightning strokes reported by Idone et al. (1984) . solid regression line is for the UF data points excluding the leaders for strokes 2 and 3 of 1317:14 UT. Stroke 2 was dart stepped and stroke 3 was the fastest natural dart leader ever measured.

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124 speed and return stroke initial peak electric field. A linear fit of the data of Idone et al. had an intercept of 11.4 and slope of 4.96 with a correlation coefficient of 0.84. If the results of Willett et al. (1989) are valid for New Mexico triggered lightning, the dependence between dart leader speed and peak return stroke electric field is significantly different for triggered and natural lightning. 3.3.1.6 Leader speed versus leader relative light intensity Dart leader speed as a function of dart leader relative light intensity is shown in Figure 3.31 . All dart leader speeds are average speeds over the maximum channel section possible. Dart leader relative light intensity was measured near ground (height of approximately 50 m) as was done by Orville and Idone (1982) . No obvious relationship can be seen for these parameters. This result is in contradiction to the findings of Schonland et al. (1935) who report an increase in dart leader speed with an increasing dart leader light intensity, but confirm the results of Orville and Idone (1982) who found no relationship between these parameters for natural lightning. 3.3.1.7 Leader relative light intensity versus initial peak electric field Dart leader peak relative light intensity as a function of initial peak electric field normalized to 100 km is shown in Figure 3.32. The initial peak electric fields of all subsequent strokes in the flashes recorded

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125 25 20 •w-l X en *>» e 15 o LU LU D. a: UJ a UJ 10 Plot Ston Syrtol Year Day D 7920B * 79206 X 82202 82222 A 82222 Flash _ML_ 220651 224645 131714 144535 144656 10 15 20 LEADER LUMINOSITY 25 30 Figure 3.31 Dart leader speed versus dart leader relative light intensity.

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126 7 a 3 e * D. a uj a LU d 9 3 ifr f] Plot Stop* Flash Syibol Year Day (UD D 79208 220651 X 79208 224645 X 82202 131714 82222 144535 A 1 1 82222 144656 1 10 15 20 LEADER LUMINOSITY 25 30 Figure 3.32 Dart leader relative light intensity versus return stroke initial peak electric field. Dart leader relative light intensity measurements are taken at a height of approximately 50 m.

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127 are presented. Dart leaders which were not detectable are plotted with a relative light intensity of zero. The figure shows that for initial peak electric fields below 4 . V/m the dart leader was not detectable with the techniques used in our analyses. The data set is too small for a conclusive result but the trend seems to be increasing dart leader relative light intensity with increasing initial peak electric field. 3.3.1.8 Leader relative light intensity versus leader duration Leader relative light intensity versus leader field change duration is shown in Figure 3.33. It can be seen that leaders with shorter durations are brighter. This result is interesting in view of the relationship presented for leader speed versus leader light intensity; which showed no obvious relationship. If leaders with higher speeds tend to have shorter leader durations then one would not expect shorter durations to correlate with an increase in light intensity. 3.3.1.9 Leader relative light intensity versus previous interstroke interval Leader relative light intensity as a function of previous interstroke interval is shown in Figure 3.34. No obvious relationship can be found for these parameters . 3.3.1.10 Leader relative light intensity versus return stroke relative light intensity A plot of leader relative light intensity as a function of return stroke relative light intensity is

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128 to 3.5 3 2.5 § 1.5 UJ 1 0.5 Plot Stom Flash Syabol Year Day (UT) D 73208 220E51 X 79206 224645 X B2202 131714 o B2222 144535 A RPPPP 144E5E 10 15 20 LEADER LUMINOSITY 25 30 Figure 3.33 Dart leader relative light intensity versus leader electric field change duration. Optically Nondetected leaders are not shown for obvious reasons.

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129 100 CO cr w i£ o cr i— en cr [) BO ~ BO Plot Ston FlBSh Syabol Year Day (UT) D 79208 220651 X 7S20B 224645 X 62202 131714 o B2222 144535 A B8882 144656 IL 40 en ID O • 20 Q_ X J. I 10 15 20 LEADER LUMINOSITY 25 30 Figure 3.34 Dart leader relative light intensity versus duration of previous interstroke interval.

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130 presented in Figure 3.35. With only six leaders suitable for analysis, the scatter is such that only a trend is recognizable. In general the brighter return strokes have brighter leaders. This result confirms previous findings by Schonland et al. (1935) and of Orville and Idone (1982) . 3.3.1.11 Leader relative light intensity versus height Dart leader peak relative light intensity as a function of height along the channel for four dart leaders is shown in Figures 3.36-3.39. The leader for stroke 3 of the 2246:45 UT flash on July 27, 1979, Figure 3.36, exhibits a decrease in light intensity with increase in height of approximately 50% over a channel height of 1600 m. The leader for stroke 2 of the 1445:35 EDT flash on August 10, 1982, Figure 3.37, has a decrease of approximately 30% with increasing height over a channel height of 1350 m. The leader for stroke 3 of the 1445:35 EDT flash on August 10, 1982, Figure 3.38, has a decrease of less than 20% over a channel height of 1350 m. The leader for stroke 2 of the 1446:36 EDT flash on August 10, 1982, Figure 3.39, does not decrease but, instead, increases slightly above 700 m. Figure 3.41 shows the peak relative light intensities for all three leaders on the same plot. 3.3.1.11 Leader relative light intensity versus height and time Relative light intensity versus height and time for all of the dart leaders are shown with their respective

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131 BO 70 60 in o 5 50 3t £ 40 o to z cr UJ cr 30 20 £ 10 Plot Stom Flash Sy stool Year Day (UT) D 79206 220651 * 79208 224645 X 82202 131714 82222 144535 A 82222 144656 10 15 20 LEADER LUMINOSITY 25 Figure 3.35 Dart leader relative light intensity versus return stroke relative light intensity.

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132 return strokes in the figures following section 3.4.2. As can be seen in Figure 3.68 and Figure 3.71, the dart leader light intensity does not return to zero after the dart has passed a point along the channel. Instead, the dart leader light intensity returns to a non-zero level and maintains this level until the return stroke front passes. Measurements of the dart leader residual relative light intensity value were made at the point which the trailing edge of the dart leader peak relative light intensity changed slope to become approximately flat. Figure 3.42 is a plot of this residual dart leader light intensity as a function of height along the channel. Regression lines for the data points appear to be approximately constant with height for one leader while two leaders decrease approximately 50% with height over 1000 meters. It should be noted that the correlation coefficients for the regression lines above are 0,49, 0.17, and 0.24 indicating very little statistical significance. This residual light intensity lasts much too long to be any type of decay of the dart peak light intensity and suggests that a current flows down the channel after the dart has passed. This is reasonable in that the charge which is deposited along the channel by the dart leader must be coming from the cloud and delivered by a current along the channel. The rise time of the dart leader relative light intensity as a function of height for three leaders is

PAGE 152

133 shown in Figure 3.43 to be 1.0 ± 0.5 /xsec for all measured heights with the exception of 3 data points. The streak camera speeds used to acquire these data limited the time resolution to approximately 0.5 /Lisec, explaining the quantized nature of the data points. It is possible and likely that the dart leaders had light rise times less 0.5 /xsec. Dart leader width as a function of height is shown in Figure 3.44 to be relatively constant along the channel with a value of 4.0 ± 2.0 /xsec. Dart leader width was measured by taking the leading edge of the dart light intensity at the point it increased above background and the trailing edge of the dart light intensity at the point at which it intersected the relatively flat value which continues until the return stroke light begins. If dart widths are reproduced by multiplying the dart lengths by the dart speeds presented in Table 4. of Orville and Idone (1982) , it is shown that their mean width along the lower part of the channel is 2.5 /Ltsec and along the upper part of the channel is 2.3 /usee. This is consistent with the findings of this study showing the width to be constant with height. The mean widths measured by Orville and Idone are less than those found in this study probably due to a difference in measurement technique. Orville and Idone (1982) found the dart length as a function of dart leader speed to be best fit by a second order polynomial. If the

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134 50 T LU 40DART LEADER STROKE 3 22:46:45 UT 79208 TAMPA 3020LU UL 10.. 200 400 600 BOO 1000 HEIGHT meters 1200 1400 1600 Figure 3.3 6 Dart leader relative light intensity versus height for stroke 3 of the 2246:45 UT flash on July 27, 1979.

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135 100 T CO UJ CD LU LU 80-DART LEADER STROKE 2 14:45:35 EDT 82222 GAINESVILLE arc 400 600 BOO HEIGHT meters 1000 1200 1400 Figure 3.37 Dart leader relative light intensity versus height for stroke 2 of the 1445:35 EDT flash on August 10, 1982.

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136 100 T LU X CD LU LU QZ BO-BO40.. ». . RETURN STROKE STROKE 3 14:45:35 EDT 82222 GAINESVILLE 400 600 BOO HEIGHT meters 1200 1400 Figure 3.38 Dart leader relative light intensity versus height for stroke 3 of the 1445:35 EDT flash on August 10, 1982.

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137 50 T en UJ 40.. 30-CD LU DZ » DART LEADER STROKE 2 SECOND CHANNEL 14:46:56 EDT 82222 GAINESVILLE 400 500 GOO HEIGHT meters 900 1000 Figure 3.39 Dart leader relative light intensity versus height for stroke 2 of the 1446:56 EDT flash on August 10, 1979.

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138 BO 70 BO in o z 50 5" ID -J LU 40 o a: (~ to z 30 cr Z3 R Ul cr Plot StorB Flash Syibol Year Day IUT) a 79208 220651 HI 79208 224645 X 82202 131714 o 82222 144535 A 82222 144656 20 10 D 10 15 LEADER SPEED, (m/sjxio 6 20 25 Figure 3.40 Dart leader speed versus return stroke relative light intensity. Return stroke relative light intensity is measured at a height of approximately 50 m.

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139 40 >t -» * z £ 111 t 3 DC . £ Ul £ O) DC > 0)
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140 15 ! a. t 55 z uu fz 5 I UJ ra Q « 2 £ 2 oH o O 82220 1445:35 EDT Stroke 2 a 82220 1445:35 EDT Stroke 4 E] A 79208 2246:45 UT Stroke 3 -Q ab a ^ o O n 2D D^0 a o 1A_J Ja^qq q_\e a . Q A o* DD O i i — A i A a o 300 600 900 1200 1500 CHANNEL HEIGHT (meters) Figure 3.42 Dart leader relative light intensity at plateau after peak versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot.

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141 x -iu P < hi _i w DC a> Q W < Z cc < 2o 82220 1445:35 EDT Stroke 2 a 82220 1445:35 EDT Stroke 4 A 79208 2246:45 UT Stroke 3 pofto £aa ad dad no a 4d a DDDDDO D CODDOOODDOODD 300 600 900 1200 1500 CHANNEL HEIGHT ( meters ) Figure 3.4 3 Dart leader rise time versus height. The system time resolution was approximately 0.5 ^isec which produced the effect seen in the rise time data.

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142 fc x o cc >© UJ CO tr < a 10 8o 82220 1445:35 EDT Stroke 2 a 82220 1445:35 EDT Stroke 4 A 79208 2246:45 U T Stroke 3 2OD OO O O OO D a a — i — 300 600 — I — 900 — I 1200 1500 CHANNEL HEIGHT ( meters ) Figure 3.4 4 Dart leader relative light intensity durati versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot. on

PAGE 162

143 dart width is assumed constant with height then the best fit should be a linear function of dart leader speed. 3.3.2 Return Strokes A total of 32 subsequent strokes were available for analysis. Electric field records were available for 5 flashes containing 22 strokes. 3.3.2.1 Return stroke peak relative light intensity versus leader speed Figure 3.40 shows subsequent return stroke peak relative light intensity as a function of leader speed. The figure shows that faster leaders produce brighter return strokes. This contradicts the findings of Schonland et al. (1935) who found that slower leaders produce brighter return stokes. Orville and Idone (1982) found their data to be inconclusive concerning these parameters. 3.3.2.2 Return stroke peak relative light intensity versus leader field change duration Figure 3.45 shows subsequent return stroke peak relative light intensity as a function of leader electric field change duration. Field change duration is a function of both the speed of the dart leader and the total channel length. The figure shows that leaders with long field change durations are followed by return strokes with lower relative light intensity. This result is consistent with that of Brook et al. (1962) who found smaller charge lowered by strokes with longer apparent leader lengths.

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144 BO 70 >60 »— < en o t-H X 50 LU O cx t— en 40 30 cx UJ cr 20 10 A X X x Plot Ston Flash Svabol Year Day CUT) D 79208 220651 X 79208 224645 X 82202 131714 82222 144535 A 82222 144656 _L 0.5 Figure 3.4 5 1 1.5 2 2.5 3 3.5 LEADER DURATION, ms Dart leader electric field change duration versus return stroke relative light intensity.

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145 3.3.2.3 Return stroke peak relative light intensity versus previous interstroke interval Subsequent return stroke peak relative light intensity as a function of the duration of the previous interstroke interval is shown in Figure 3.46. No obvious relationship can be found between these two parameters. This contradicts the results of Schonland et al. (1935) who found that their brighter return strokes followed longer interstroke intervals. 3.3.2.4 Return stroke peak relative light intensity versus return stroke initial peak electric field Subsequent return stroke peak relative light intensity as a function of initial peak electric field is shown in Figure 3.47. It can be seen from the figure that there is definitely some type of correlation between these two parameters with larger peak fields associated with brighter return strokes. Idone and Orville (1985) show regression lines for peak return stroke relative light intensity as a function of peak return stroke current for 39 subsequent strokes in two triggered lightning flashes. They obtain a better fit for the log of the relative light intensity versus the log of the peak current compared to the log relative light intensity versus linear initial peak electric field found by Jordan and Uman (1983). It should be noted that the range of peak currents (2-20 kA) in their data is somewhat lower than that for natural lightning (10-35 kA) obtained by using our electric fields and the relationship between peak current and electric field peak presented by Willett et

PAGE 165

146 100 80 >i— »— i en I £ 60 UJ IX ^cr. 5 e— LU cr 40 20 X Plot Stori Flash Syibol Year Day CUT) D 79208 220S51 X 79208 224645 X 82202 131714 O 82222 144535 A n???p 144E56 NOTE: Stroke 3 of Flash 220651 mbe not used because of Its long previous interstroke interval D * X JX. _j 20 40 60 80 PREVIOUS INTERSTROKE INTERVAL, ms 100 Figure 3.46 Duration of previous interstroke interval versus return stroke relative light intensity.

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147 100 BO >in -60 Plot Sjgbol D * X o A UJ cn 40 LU 20 Figure 3.47 Stom Flash Year Day CUT] 79208 220651 73208 224645 82202 131714 82222 144535 82222 144656 D H* Ac B xa d r D ' r 2 ^3 a INITIAL ELECTRIC FIELD PEAK NORMALIZED TO 100 km. V/m Return stroke electric field peak versus return stroke relative light intensity.

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148 al. (1989) . It must be reiterated that the relationship derived by Willett et al. is for triggered lightning and is possibly inappropriate for natural lightning, but it is the only available relationship between peak current and peak electric field. 3.3.2.5 Return stroke relative light intensity versus height and time Photographs of the digitized images and relative light intensity profiles at various heights for all of the subseguent strokes analyzed in this thesis are presented in Figures 3.48 3.78. The profiles are consistent with results presented by Jordan and Uman (1983) showing the subseguent stroke relative light intensity to consist of a rapid rise to peak followed by a relative constant value. The peak relative light intensity is seen to decay as a function of height. Detailed plots of return stroke peak relative light intensity are shown in Figures 3.79 3.87. These plots show the marked decrease in peak relative light intensity with height for all strokes. Figures 3.86 and 3.87 show peak light intensity as a function of height for a first stroke and a subsequent stroke 78192 1926:45 UT. The large decreases in light intensity at the branch points of the first stroke can be seen at 350 m and 550 m. The relative light intensity of the subsequent stroke shows relatively little change at the locations of the branch points of the previous stroke.

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149 Figure 3.89 shows the 20% to 80% rise time of the return stroke relative light intensity as a function of height. These are presented in order to compare more recent data with those of Jordan and Uman (1983) . The rise times along the lower portion of the channel are 1.5 ± .5 Msec and increase to 4.0 ± 2.5 jusec in the upper portion of the channel. The rise times measured in this thesis have the same range as those reported by Jordan and Uman (1983) but their mean for the lower portion of the channel was approximately 2.5 /xsec. This discrepancy is most likely due to the difference in measurement techniques. Their original work was measured by hand on paper plots as compared to the software display and measurement techniques used in this current work. Figure 3.90 shows the relative light intensity value during the relatively constant period 30 /xsec after the return stroke light intensity leading edge. As shown by Jordan and Uman (1983) this plot shows the 30 /usee value to be approximately constant with height. 3 . 4 M-components The flash at 2041:00 UT on July 29, 1978 which was obtained by Idone and Orville contained two subsequent strokes; one of which was followed by two very well defined M-components. A digitized image of this flash is shown in Figure 3.91. The optical records for these M-

PAGE 169

150 components have been analyzed in detail and are presented in the following sections. There were no electric field records available for this flash. 3.4.1 M-component Relative Light Intensity Profiles Relative light intensity profiles at three heights along the channel of both the return stroke and Mcomponents are presented in Figure 3.92-3.94. It is evident from this figure that M-component relative light intensity has a different character than the return stroke. The M-component relative light intensity does not have a sharp rise to peak nor does it possess the characteristic decaying peak of the typical return stroke. The M-component relative light intensity is essentially constant with height along the channel, as shown in Figure 3.95. Figure 3.96 shows the relative light intensity as a function of height for the second M-component. The valley relative light intensity between the M-components , as seen in the plots at different heights, indicates that the Mcomponents occurred during a continuing current which was keeping the channel luminous. This is consistent with observations by Malan and Schonland (1947) and Kitagawa et al. (1962) who found that M-components were only observed during continuing currents. During the analysis of the electric field records of 1979 and 1982 it was observed that there were many hook-

PAGE 170

151 shaped waveforms, typical of M-components, during continuing currents which were not detectable on the streak camera records. Figure 3.98 shows the electric field change following stroke 6 of the 2206:51 UT flash on July 27, 1979 in Tampa, Florida. Three field changes, typical for M-components, are evident following the return stroke field change. The second of these field changes is comparable to the field change of the preceding dart leader which was easily detectable on the streak camera records. None of the M-component field changes were detectable on the streak camera records. 3.4.2 Determination of the Direction of M-component Propagation Malan and Collens (1937) measured the direction of propagation for nine M-components, finding that seven traveled downward while two traveled upward. They measured the speeds for four of the M-components which had a range of 2.0 4.7 x 10 7 m/sec. It was observed while measuring the M-component waveshapes that the time between the return stroke leading edge and the M-component leading edge was greater near the bottom of the channel than at the top of the channel by approximately 25 /isec. At 1 km height the displacement was approximately 2 /xsec. This relative displacement can be used, in a manner similar to that described in Section 2.3.7 for dart leaders, to measure the speed of Mcomponent progagation. The displacement of the return

PAGE 171

152 stroke channel from the ideal (V r =oo) channel is h/V r and the displacement of the M-component channel from the ideal channel (V m =«>) is h/V m where h is the channel height and V r and Vjn are the return stroke speed and M-component speed respectively. The the difference in the absolute displacements above is the relative displacement between the two and is h/V r h/V m which is the measured displacement of approximately 2 /zsec at the height h. The M-component speed as a function of return stroke speed can then be calculated as below. V m = 1/V r s where S is 20 ^sec / 1000 m. The eguation above has a discontinuity and sign reversal at a return stroke speed of 5.0 x 10 7 m/sec, therefore; the M-component must be traveling downward if the return stroke speed is greater than 5.5 x 10 7 m/sec and upward if the return stroke speed is lower. Idone and Orville (1982) presented statistics for 17 first stroke speeds and 43 subseguent stroke speeds with a mean subseguent stroke speed of 1.2 x 10 8 m/sec. The distribution of return stroke speeds shows only 6 of 70 strokes, including first

PAGE 172

153 strokes which typically have lower speeds, having speeds less than 6.0 x 10 7 m/sec. Assuming the return stroke speed was greater than 5.5 x 10 7 m/sec, the M-component downward speed is calculated to be 1.0 x 10 8 m/sec for a return stroke speed of 1.0 x 10 8 m/sec and 6.7 x 10 7 m/sec for a return stroke speed of 2.0 x 10 8 m/sec.

PAGE 173

*1 Hs fl fl) • OD n HCu D fl) 3 rt HM 0aa V Hto Hft to rt H0) o H< rt o\ N fl) 1 a s 3" fl) h a ft vfl •1 Q fl) H(D c 3 Hi H O rt fl) *< Hi (D H 3 (D M CO CO 3 -J rt Ha "» H rt fl) o »< CL H * V0 fl) < H-J (D 3 kO CJ H • co rt o c 3" H H) en (D 3* ID rt rfo 3 HH 3 fl) 3 (D IS rt HHi • CO iQ H 3" ft) o rt to M> co 3

PAGE 174

155 i 1 i I 4 i " ™*m « t ? ® S CM -T ? ji -O CO •.«. O <& E £
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156 1520 m to z: LU X O > § a: 7 — i r 25 50 75 MICROSECONDS Figure 3.49 Relative light intensity versus time for stroke 3 at 2206:51 UT July 27, 1979.

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157 in 2: UJ F z hX o UJ > a: 480 m 400 m 320 m 240 m 160 m 80 m ^^A^ ^W•V • ^»V*w^» j \j **\^f*y* » m» < * iyi I »^V 25 50 75 MICROSECONDS 100 Figure 3.50 Relative light intensity versus time for the lower 500 m of stroke 3 at 2206:51 UT, July 27, 1979.

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x: w W • 0) 0) B a) x: r•H X! Q.4J -P n • M VD CTi c Q) t> -H > i o a) 4J ^ o •H -p t^ c 01 W (N <1) C u 0) <«-l >i u tn 4-1 (0 E-« a) jc e D U tF •r4 IX} H H -H TJ in T3 d) .. Q) 0) N <& 4-> > •H o (0 H 4J OJ U 4J •H (N H to 0* T3 H •H 4-> C QJ Q nJ H *-l ID m tn •H

PAGE 178

159

PAGE 179

160 z: UJ x o > a: 1120 m 1040 m A/YAA_/yW MA-, I 960 m 880 m 800 m 720 m ffJ ^n/Ar-vAJvyiLyAwvA—^ T 50 75 MICROSECONDS 100 Figure 3.52 Relative light intensity versus time for stroke 6 at 2206:51 UT July 27, 1979.

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161 480 m £ 4UU m (/) z UJ F z 320 m hX o ZJ UJ 240 m > 3 UJ o: 160 m T 25 50 75 MICROSECONDS 100 Figure 3.53 Relative light intensity versus time for the lower 500 m of stroke 6 at 2206:51 UT, July 27, 1979.

PAGE 181

X! m 4-1 W -P (0 si r-\ Cr> w • 4-1 •H P 0) 0) o e 0) X -H S3 ftp -P Q) A (1) U) 4-1 H X d O +J (/) • M o\ en c (1) rH > i 0) P M *» O »H -P rc W W eg a) c h QJ 4-1 >l (D O rc 3 a) •H Q) 1M tX> P a B a) s E u tP H Id •H H H Tl IT) T3 a) • • 0) CD N \D P > -H O rt •H P CM O P •H (N -H id CP TJ r-\ -rH P C a; a id •rH M -* ID m a) M & -H Pm

PAGE 182

163 r5 .CM Q <1 rvj . eg <\J o. . f©QO s: *fflB'" o cq ob evi
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164 V) Ul X o LU > 1360 m Liu^^ 1280 myy^^^^ 1200 m g^^^^ 800 m 1040 m ^^/y^^ 960 m yt idiJ 4»^^ 880 ^jKlVvyl^^ 720 m 640 m p 560 m LU a: 480 m 400 m 320 m 240 m 160 m 100 m m r^f^rff^ r fV ' i r 25 50 75 MICROSECONDS 100 Figure 3.55 Relative light intensity versus time for stroke 9 at 2206:51 UT, July 27, 1979.

PAGE 184

165 480_m tsjpU*^^ V) Ul 320 m ix CD > DC 400 rn f^y^^j^ .^yi^y^yy^^u^^ ^A*v*AV>*/v>/fy/v ^[^^^^/^^ 25 50 75 MICROSECONDS 100 Figure 3.56 Relative light intensity versus time for the lower 500 m of stroke 9 at 2206:51 UT, July 27, 1979.

PAGE 185

a CD en •-J 3 rt CD 01 Hrt < 01 c oi rt HCD (D £ H^ rt CD to H 3 -J O CD a h N CD H 3 rt CD v£> H-1 3 CD t-3 CD CD O rt h01 t£ o rt H, 01 CD h1 OJ rt < CD 3 CD ft O i-h 01 rt H o CD to rt to M £> On • • (Jl w a h3

PAGE 186

167 I I I 1 i I I L I a a i wm I *— «-j in UJ — i -f
PAGE 187

168 X o Z3 LU > UJ a: 880 m 800 m 720 m 640 m in z 560 m 480 m 400 m 320 m 240 m 160 m r^YV^wwVvA^V^ T r 25 50 75 MICROSECONDS 100 Figure 3.58 Relative light intensity versus time for stroke 2 at 2246:45 UT, July 27, 1979.

PAGE 188

H3 (D en Hi-< ^ O 3 (D C Hrt H> M^a (D n> < H3 ^ rt n (D NJ HH3 -J N rt ^ (D *< (D Oa h < KD HID H^1 3 h 3 VO 0) (/) • £> C rt ID cn tr H O) 3* o rt fl> Ml H-'O 3 h-* cr ui (D o (D rt • rt hhi cn as O 3 * rt ro H> 01 LaJ ^ >-•• ID 3 0J Hd(t PI Hrt M H0) to < rt ^ (D (D Q\ a • • M ^ H0J Ul vQ M 3" (D W rt a

PAGE 189

170

PAGE 190

171 1440 m UJ X o LJ > UJ a: i r 25 50 75 MICROSECONDS 100 Figure 3.60 Relative light intensity versus time for stroke 3 at 2246M5 UT, July 27, 1979.

PAGE 191

172 X o IJ LU > LU a: T 25 50 75 MICROSECONDS ~l 100 Figure 3.61 Relative light intensity versus time for the lower 500 m of stroke 3 at 2246:45 UT, July 27, 1979.

PAGE 192

fr Q , p W a) .c M it * id •H H rH • • -d r0) (1) H p > r> H T TJ a) >. Q) O P N H c -H H rsj 0) 01 P M C •H >i a 0) 0>rH <4-l P •H fl 0) c Q b U •H eg n 0) -H

PAGE 193

174

PAGE 194

175 720 m . kAAAnr — vyw r -tf^\_/\r*MA/V\A 660 m ./n_f\_/w/vw_/v/v — A600 m £ 540 m in X UJ > a: 480 m 420 m 360 m 300 m _/fc/*/VYV "w> — \rv r a_o/\_/u— r-v _T^ ^ JV_A_ TV V -\a/wt-v — nr -rwn_r -a__/ irv — v-AJLAATWJ-A_/r-un T-A/Vruvn-Mr -rn_fk_ -y/ n-rw "v ur— v — UWTA, 25 50 75 MICROSECONDS 100 Figure 3.63 Relative light intensity versus time for stroke 2 at 1317:14 UT, July 21, 1982.

PAGE 195

EH Q P W 0) X M tn ^r (S •H H rH • • TS r^ Q) 0> H P > n (0 •H H U -P •H ro P t (0 C 0) •H ^ n W <4-l Q) -P O * X3 tP w U •H P • P 0) O 0) w j: g a-H H T 73 QJ >, 0) i O P N H c H •H r\j Q) W P p, c •H >i 0> Q) tpH (4-| +J -H 3 0) C Q h> M H -^ KD n 0) rH Pm

PAGE 196

177 © © us «• v> >..• fH ru IU o »-• o 3! rr m* Cil i-. h — • ao o * t *^*^r I , ' i! Iiij.fi : ! ikMiuis:" * t

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178 UJ UJ > UJ DC 720 m 660 m 600 m 540 m 480 m 420 m 360 m 300 m 240 m 180 m 120 m 60 m -» » — \r • y vi — r /vwvnrf-VA_MA_A_ -v — r r^ -nr-UL. -» — r>u»v-\A/w 7VWI -wrv — y-\_/w "V VAT-LTTV* -TV m -ryj-\_rwnn_A /\_ _^-LA_n»_r' "Vy\_AAAA A__A_ -wry *» »» w— um_rwr— ' I ,/ " VATUV^KL "\M. * fcAy l_AA_ TW — fV_ MUUM. T A^ "V\_ T T nrv — vwu 25 50 75 MICROSECONDS 100 Figure 3.65 Relative light intensity versus time for stroke 3 at 1317:14 UT, July 21, 1982.

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Eh a) Q ^ -p W QJ rH • • P in (0 0) <« u > ^j* -H •H H T c KJ P •H rH id 0) V) ^ (N p xi o X •H o 0) W m si -p • -p o (1) in 0) i-C a-H -0 TJ c 0) >1 Q) rH O -P N c •H -H -p 0) tf] •P w in c •H a) a) tr tyu -P •H 3 QJ c Q < u -H VO VO m 0) •H b

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180 * ^ '•'i i J; G G Q C'-J <2 © © iu W *tf in <\J UJ o -«r ru 2! d m <\i •-^ l— tr Ol nr

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181 tn -z. LiJ X > I UJ a: 1440 m 1380 m 1320 m 1260 m 1200 m 1140 m 1080 m 1020 m 960 m 900 m 840 m 780 m 720 m 660 m 600 m 540 m 480 m 420 m 360 m 300 m 240 m 180 m "120 m 60 m m 25 50 75 MICROSECONDS 100 Figure 3,67 Relative light intensity versus time for stroke 2 at 1445:35 UT, August it), i§§2.

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182 V) UJ X o UJ > UJ 01 25 50 75 MICROSECONDS 100 Figure 3.68 Relative light intensity versus time for lower 500 m of stroke 2 at 1445:35 UT, August 10, 1982.

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H| R ID u o 3 fl> C Hrt t-hiQ 03 (D (D d H3 ^ W ft tfl (D ft HH-3 N ft H (D *< (D O (X a< H(D HM g *13 wpi (fl <»iQ C rtwiD 01 3* • (D O ft 1-3 H) P-'O 3" 3 hid in (DO ft • rt tr hj W (D H-* vQ (D i-h cr ft u ^ en a> o) HH-ri 0) 3 rt&h HH* < O * (B (u O ft •• H (D o ha u iQ 3* 0) f rt i^ C (D i-

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184 r t £ £ t t . © <5> £ £ £ © J o © a o in © '.£} «-4 -T © ' £ £ © © © 02 © © © « UJ M * OJ z cc M OJ — • H-* OJ
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185 1440 m in z: o > § LU T I r 25 50 75 MICROSECONDS 100 Figure 3.70 Relative light intensity versus time for stroke 3 at 1445:35 UT, August 10, 1982.

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186 in UJ x o > § a: Figure 3.71 25 50 75 MICROSECONDS 100 Relative light intensity versus time for lower 500 m of stroke 3 at 1445:35 UT August 10, 1982.

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p (1) 0] x: M p •H • w *. U &>o c >H c rH OJ -P o M •H rH -P 0) Ul (0 U) Q) vo T •rH OW C -P (0 ^ •H « e H rH -H U) OJ -P -P r-l TS to x: d) tJMH N rH -H o -H (1) OJ P c jc w • -H c -P Q) tJ> (0 0) o e •H X! X3 rH -H D O H a-p (M 0) U & -H

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188

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189 X o Id > UJ a: 1080 m 420 m 360 m 300 m 240 m 25 50 75 MICROSECONDS 100 Figure 3.73 Relative light intensity versus time for Stroke 2 at 1446:56 UT, August 10, 1982.

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190 z: x o UJ > § UJ a: 480 m 420 m 360 m 300 m 240 m 180 m 120 m 60 m T 25 50 75 MICROSECONDS Figure 3.74 Relative light intensity versus time for lower 500 m of stroke 2 at 1446:56 UT, August 10, 1982.

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^ HvQ C ^ ID CJ -J ai H h li a 3 ID c Hrt h> miQ ID tt> ^< h3 ^ rt V) ID (-> HH' 3 H N rt n " (D ^ a a H < U) pCD H-J 3 1 3 03 a> m • £) d rt (D m tr H (D 3 o rt (D Hi H-*d 3 HffOl ID CD rt • ct HH. W il tr * rt a> n 01 N) n Hro 3 QJ m a n 0) Hrt o M HD) VD < rt to CD (D CT* a • • M *> H0) Ul v£) ^ 3"
PAGE 211

192

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906 m 768 m 193 653 m £ 538 m z LxJ hX UJ > UJ T 25 50 75 MICROSECONDS 100 Figure 3.76 Relative light intensity versus time for stroke 2 at 1926:45 UT, July 11, 1978.

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H p D 0) X u IT 1 VD Id -H *tf rH • • TS n Q> 0) o (0 •H r\] U P •H 10 P T flj C 0) •H (h (N (0 <4-l (U p o * Xi O tr> w ^ H p • -P 0) Q) W X 6 a-H H e r* •H Q) •H o\ > rH T3 TS (1) >i 0) •fc U P N Ch crH •H CN Q) Ifl -P M c •H >1 a) Q) (JlrH
PAGE 214

'jL Q O
PAGE 215

196 in z: UJ X Ld > UJ 966 m 915 m 856 m 764 m 672 m 626 m 580 m 566 m 524 m 511 m 419 m 396 m 350 m 281 m 271 m 258 m 198 m 152 m 46 m m 1 r 25 50 75 MICROSECONDS 100 Figure 3.78 Relative light intensity versus time for stroke 2 at 2023:46 UT, July 29, 1978.

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197 § a a 9 a A1ISN31NI 1H9I1 3AI1V13U -p •H BO C r> •H r-l Q) M o p (0 > H P (0 H 0) i > 3 n

PAGE 217

198 O (/)(M > •H P (0 tr>cr> -rH H Q) 01 CM 01 >i U H 0) 3 > h o co n QJ >H &

PAGE 218

199 LJ O {/) ro m ZUJ-t a: ^ =>o tj£= i 4J in in •• c ai J c -H P id -p xi -H H 0) O U P 0) (1) > P id 0) u an o p c p o « o P 0^ a) rw CJ P in >i U H 0) ^ > h> H CO •H A1ISN31NI 1H9I1 3AI1V13U

PAGE 219

200 111 z < X. O COO in M • H 1 M Q UJ -H -P W L-p U) 0) id VO -t-J rH M IT) n) a) • • F Mi-ivo ^ 0) -P "< 1 o tn rH LL> M •H +J i— i P a) (0 UJ w x ~r rH C UJ 0) M 3 c 3 n c p M (0 0) Q) £! « > CM CO n Q) H B •H Pn A1ISN31NI 1H9I1 3AI1V13H

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201 -d c • 0) CM >i to < p CTl •H o h w x: c p Q) o -p tJ>H c c •H P rH 01 P (0 3 £! tp tPOJ 3 -H < H x p a UJ •H +J W c P w a) (0 VD 4-> rH P in m o •• e P. * i— (DP-* i!jCH zn o rp II) u •H p 1— 1 p a> u m co ro Q) P & •H Pn $88 A1ISN31NI 1H9I1 3AI1V13U

PAGE 221

202 O OH Q F Ui m z urn po!£ a: to ^15 N CM 00 CD g P -H 01 c 0) P C •H -P CP -H Q W ID • • in P m a) o p c/i P Ul o C. (0 tf a) -H ^ +j a) o • m ^ 0) 0) 3 3 K co > < g a) •H 3 8? A1ISN31NI 1H9I1 3AI1V13U

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203 LU o hF uj to to m 2 UJfO K * in > LO UJ CM CM CM CM CO O as? A1ISN31NI 1H9I1 3AI1V13U H Q W >i P in •H ID c 0) -p c -H n • • in P (0 U si tr>n 0) > H P m 0) o M -P M C M -P 0) K d) o u p U) M O • X5 H tP o H P n < in co m
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204 in c. 4 * Qi CD » ^ a s A1ISN31NI 1H9I1 3AI1V13U +j in •H * W •• C VD 0) CM c en X! (1) > •H •P (0 H Q) ID o kl -p tfl U a> -P oo * 43 l» o Cr>cri M-H H -p a) H C 01 H P W >i -P U -H b CO g, •H U-,

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205 H D >i +j in -H tJ. w •• C vo 0) CM P CTi C H •H P -P (0 .C tJ>CM -H rH (I) * 0> > u •H +J (0 +J W C(0 rH U UJ d) o -*--> M i +JMH 0) 0) 3 « > h> co d) tP •rH 9 8 ta A1ISN31NI 111911 3AI1V13U

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206 100 t — * 25 m < 2 2. °"z 3 LU — ** * l_ -= El" c/) V tu < _ tc -J UJ n A + 1200 1500 CHANNEL HEIGHT ( meters ) Figure 3.88 Return stroke peak relative light intensity versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot.

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207 t X w UJ > S UJ ot *" *E g be! M z 5 DC cc / o 82220 1445:35 EDT STROKE 2 6n 82220 1445:35 EDT STROKE 4 O + 79208 2246:45 UT STROKE 2 A 79208 2246:45 UT STROKE 3 5 A A 4 D A + 3O A + . + O OOO A 2j o >0& O OA Df DO -HDO ODDOOODD + DD + pnnn Aooaaof so a + + 1k + A + + + A A o, i i , i i , i , i i 300 600 900 1200 1500 CHANNEL HEIGHT ( meters ) Figure 3.89 Return stroke relative light intensity rise time versus height.

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208 O < -J a. ^ £a: » ^: ai ULI V> * =1. <1> O o = W>£ ?l cc ? 50-140O 82220 1445:35 EDT STROKE 2 82220 1445:35 EDT STROKE 4 + 79208 2246:45 UT STROKE 2 A 79208 2246:45 UT STROKE 3 A 1200 1500 CHANNEL HEIGHT ( meters ) Figure 3.90 Return stroke relative light intensity 30/isec after peak versus height. Regression lines for the individual strokes are identified by the symbols to the right of the plot.

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C ID UD HH h c-i a 3 3 C Hrt Q, MiQ (D 0> h-k; H3 ft o rt 01 H(U to HHrt id n ft 3 (D ^i (D •«< 01 a H a < 3 V£ HID M, (t -J 3 n p 0) 01 tJ iQ 03 d m 3 oi 0) oi rt 3 ft 01 O rt Ui K H) Hn H3 o 3 01 fl> M) h rt • H n h (D 3 o H 3 £Hrt r H 1 iQ 3 oi • 3 (D • O rt O a

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210

PAGE 230

211 A1ISN3Q HVin33dS

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212 § CO o c\) 3 £ m nt 041:00 UT 0) a) (n o * c o o P vo ^ a (o -p s P w o a) (0 o * c 1 o >i»n : -P 3 p •H -p 01 ui a) > CD C U P CD +J 0) tl) -P ~z. C 43 XJ O •H +J + CJ o LU P tPTJ a> en rH >i-H H X! -P P 1— l a) io •H -H * s: > in mm -H W (N •p M c c (0 a) •-I-H >, rH P sen a) a) 3 d 3 a i-i rH ho n en n a) M 3 Cn Pm A1ISN3Q HVinOBdS

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213 O i u •P 3 H p (0 0) C Vi P •H t/1 -P M rd 0) •H -P 0)
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214 s u o P T •H O C Q) •P c -p Si 0) > •H -P id u n 0) -p (0 «J c
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215 a 5 ul a -P 3 £ h tr> § •H E-< a) b •«< jC o w o 3 •• 8 10 H ft >-l "tf 0) O > CM >i-P I in nsit nt a QJ a) a) ., J -P c fi OJ d o J= •h a, -P o hligh M-c § CD l-H Tl UJ a> c DC > •H 9 _J -P Q) UJ (0 If)
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216 Tampa, 1979 2206:51 UT 6th Stroke jW^"*" IU i 71 i Stroke o o _J LU o cc o ^-5.5 ms*Ml ' M A /* LU -I LU 1 1 / 2 J 1 l~ M, / 3 12V/m 1 TIME Figure 3.97 Electric field change following stroke 6 of the flash 2206:51 UT 27 July, 1979 showing three typical waveshapes for M-components.

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CHAPTER 4 DISCUSSION AND CONCLUSIONS Streak camera data have been presented for 44 return strokes; 11 of them first strokes. Electric field records were available for 1 new channel to ground, which was the third stroke of a flash, and for 22 subsequent strokes. First stroke relative light intensity rise times have been shown to increase sharply with corresponding decreases in light intensity immediately above branch points. The rise time then decreases and the light intensity increases as the light front propagates further up the channel. This evidence suggests that the optical front of the first stroke is certainly not a linear process with an initial energy input which is dissipated along the channel. Rather it is more like a shock wave whose sharp front is rebuilt after the disturbing effects of the junction. Data have been shown which suggest that the current in the dart leader channel continues to flow after the dart leader tip has passed until the return stroke propagates up the channel. It is no longer appropriate to think of the dart leader as a simple length of light which travels unattached from the cloud to the ground. The dart leader relative light intensity appears to behave 217

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218 similarly to that of the return stroke with a very sharp rise to peak followed by a plateau which continues until the beginning of the return stroke light intensity. Dart leader properties have been presented which show dart leader speeds in this study to have a mean value of 13.7 x 10 6 m/sec, which is more than twice as fast as that reported by Schonland et al. (1935) who reported a mean of 5.5 x 10 6 m/sec. It is however consistent with speeds reported by Orville and Idone (1982) . All of the dart leaders which were detectable in this study had peak return stroke electric field values greater than 4 v/m normalized to 100 km, as is evident from Figure 3.32. If it is assumed that all subseguent strokes have dart leaders then the distribution of peak return stroke electric fields shown by Master, et al. (1984) and Rakov and Uman (1990) would suggest that approximately 50% of all dart leaders are not detectable in daylight streak photographs. In this study, 11 of the return strokes had detectable dart leaders or 48% of the leaders were not detectable. Schonland et al. (1935), whose data were obtained at night, report that 94 leaders were found in 158 strokes or 41% of the strokes had no detectable leader. If slower dart leaders have the lowest light intensity, as indicated in Figure 3.31, then it seems probable that the mean dart leader speeds reported in the literature of 11 x 10 6 m/sec for 17 leaders by McEachron (1939), 9.7 x 10 6 m/sec for 103 leaders by Brook and

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219 Kitagawa (Winn, 1965), 9.0 x 10 6 m/sec for 80 leaders by Berger (1967b) , and 11 x 10 6 m/sec for 21 leaders by Orville and Idone (1982) , with the exception of Schonland et al. (1935), who reported 5.5 x 10 6 m/sec for 55 leaders, are skewed toward higher dart leader speeds. Figure 3.28 indicates the range of dart leader speeds and previous interstroke intervals found by Idone et al. (1984) for 32 dart leaders from triggered lightning is twice as fast as those for natural lightning. Dart leader speed as a function of the electric field peak of the following return stroke has been shown in Figure 3.30. The figure presents a regression line for dart leader speed as a function of initial peak electric field using data from Idone et al. (1984) for 32 triggered strokes in New Mexico. Electric field values were plotted in terms of current using the formula presented by Willett et al. (1989) relating the two for triggered lightning in Florida. A linear fit of the data had an intercept of 3.45 and slope of 2.14 and a correlation coefficient of 0.55 at a 0.05 significance level, indicating a positive correlation of dart leader speed and return stroke initial peak electric field. A linear fit of the data of Idone et al. had an intercept of 11.4 and slope of 4.96 with a correlation coefficient of 0.84. If the results of Willett et al. (1989) are valid for New Mexico triggered lightning, the dependence between dart leader speed and

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220 peak return stroke electric field is significantly different for triggered and natural lightning. No obvious relationship is seen in Figure 3.31 for dart leader speed vs dart leader relative light intensity. This result is in contradiction to the findings of Schonland et al. (1935) who report an increase in dart leader speed with an increasing dart leader light intensity, but confirms the results of Orville and Idone (1982) who found no relationship between these parameters for natural lightning. It can be seen that leaders with shorter durations are brighter in Figure 3.33. This result is interesting in view of the relationship presented for leader speed versus leader light intensity; which showed no obvious relationship. If leaders with higher speeds tend to have shorter leader durations then one would not expect shorter durations to correlate with an increase in light intensity. In general the brighter return strokes have brighter leaders, as shown in Figure 3.35. This result confirms previous findings by Schonland et al. (1935) and of Orville and Idone (1982) . The figure shows that faster leaders produce brighter return strokes which contradicts the findings of Schonland et al.(1935) who found that slower leaders produce brighter return stokes. Orville and Idone (1982) found their data to be inconclusive concerning these parameters. Figure 3.40 indicates that

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221 leaders with long field change durations are followed by return strokes with lower relative light intensity. This result is consistent with that of Brook et al. (1962) who found smaller charge lowered by strokes with longer apparent leader lengths. In agreement with data presented by Krehbiel et al. (1979) , dart leader apparent channel lengths vs stroke order, shown in Figure 3.27, exhibit a clear tendency for leaders later in the flash to have longer apparent lengths . Relationships have been shown for return stroke relative light intensity as a function of dart leader speed, dart leader light intensity, and leader electric field duration. As discussed in Section 3.3.1.8, the leader light intensity seems to be correlated with the leader electric field duration but not with leader speed. These results are consistent with those of Brook et al. (1962) if dart leader light intensity is assumed to be a function of the charge lowered from the cloud along the channel. They found that strokes with longer dart leader electric field durations lower less charge. The dart leader electric field duration seems to be the most important parameter in the determination of dart leader light intensity. It also suggests why brighter dart leaders produce brighter return strokes; since the return stroke is assumed to neutralize the charge lowered by the dart leader.

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222 Subsequent return stroke peak relative light intensity as a function of initial peak electric field is shown in Figure 3.47. It can be seen from the figure that there is definitely some type of correlation between these two parameters with larger peak fields associated with brighter return strokes. Idone and Orville (1985) show regression lines for peak return stroke relative light intensity as a function of peak return stroke current for 39 subsequent strokes in two triggered lightning flashes. They obtain a better fit for the log of the relative light intensity versus the log of the peak current compared to the log relative light intensity versus linear initial peak electric field found by Jordan and Uman (1983) . It should be noted that the range of peak currents (2-20 kA) in their data is somewhat lower than that for natural lightning (10-35 kA) obtained by using our electric fields and the relationship between peak current and electric field peak presented by Willett et al. (1989) . It must be reiterated that this is for triggered lightning and is possibly inappropriate for natural lightning, but it is the only available relationship between peak current and peak electric field. The rise times along the lower portion of the subsequent return stroke channel are 1.5 ± .5 /usee and increase to 4.0 ± 2.5 /isec in the upper portion of the channel, as shown in Figure 3.89. The rise times measured in this thesis have the same range as those reported by

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223 Jordan and Uman (1983) but their mean for the lower portion of the channel was approximately 2.5 /usee. This discrepancy is most likely due to the difference in measurement techniques. Their original work was measured by hand on paper plots as compared to the software display and measurement techniques used in this current work. Figure 3.90 shows the relative light intensity value during the relatively constant period 30 |xsec after the return stroke light intensity leading edge to be approximately constant with height. This is in agreement with results presented by Jordan and Uman (1983). Results have been presented for dart leader peak relative light intensity and relative light intensity rise time as a function of height. M-component, and branch component light intensity profiles have been shown as function of height and time. The data set is too small for conclusive results but the data indicate that the breakdown process for dart leaders is significantly different from return strokes, M-components , and branch components. Dart leader peak relative light intensity and relative light intensity rise time are relatively constant with height, as shown in figures 3.41 and 3.43, while for return strokes the relative light intensity peak decays with height and the relative light intensity rise time increases with height, as seen in figures 3.88 and 3.89. M-components and branch components which occur in the already luminous channel seem to be similar in that both

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224 have smooth relative light intensity waveshapes with relative light intensity rise times of tens of microseconds as shown in Figure 3.23 for the branch component and figures 3.92, 3.93, and 3.94 for the Mcomponent . Three field changes, typical for M-components , are evident following the return stroke field change shown in Figure 3.98. The second of these field changes is comparable to the field change of the preceding dart leader which was easily detectable on the streak camera records. None of the M-component field changes were detectable on the streak camera records.

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REFERENCES Beasley, W.H., M.A. Uman, and P.L. Rustan: Electric Fields Preceding Cloud to Ground Lightning Flashes, J. Geophys. Res., 87:4883-4902 (1982). Berger, K. : Novel Observations on Lightning Discharges: Results of Research on Mount San Salvatore, Journal Franklin Inst., 283:478-525, (1967b). Berger, K. and E. Vogelsanger: New Results of Lightning Observations, In "Planetary Electrodynamics" (S.C. Coroniti and J. Hughes, eds.), pp. 489-510, Gordon & Breach, New York, (1969). Berger, K. , R.B. Anderson, and H. Kroninger: Parameters of Lightning Flashes, Electra, 80:23-37, (1975). Boyle, J.S. and R.E. Orville: Return Stroke Velocity Measurements in Multistroke Lightning Flashes, J. Geophys. Res., 81:4461-4466 (1976). Boys, C.V.: Progressive Lightning, Nature, 124: 54-55 (1929) . Brook, M. , N. Kitagawa, and E.J. Workman: Quantitative Study of Strokes and Continuing Currents in Lightning Discharges to Ground, J. Geophys. Res., 67:649-659 (1962). Bruce, C.E.R., and R.H. Golde: The Lightning Discharge, J. Inst. Elec. Eng. (London), 88, 487-520 (1941). Ganesh, C. , M.A. Uman, W.H. Beasley, and D.M. Jordan: Correlated Optical and Electric Field Signals Produced by Lightning Return Strokes, J. Geophys. Res., 89:4905-4909 (1984) . Guo, C. and E.P. Krider: The Optical and Radiation Field Signatures Produced by Lightning Return Strokes, J. Geophys. Res., 87:8913-8922 (1982). Guo, C. and E.P. Krider: The Optical Power Radiated by Lightning Return Strokes, J. Geophys. Res., 88:8621-8622 (1983a) . Guo, C. and E.P. Krider: Anomolous Light Output From Lightning Dart Leaders, J. Geophys. Res., 90:13,07313,075 (1983b). 225

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226 Halliday, E.C.: On the Propagation of a Lightning Discharge Through the Atmosphere, Phil. Mag., 15:409-420 (1933) . Hoffert, H.H.: Intermittent Lightning Flashes, Phil. Mag., 28, 106-109 (1889). Hubert, P. and B. Mouget: Return Stroke Velocity Measurements in Two Triggered Lightning Flashes, J Geophys. Res., 86:5253-5261 (1981). Idone, V.P. and R.E. Orville: Lightning Return Stroke Velocities in the Thunderstorm Research International Program (TRIP). J. Geophys. Res., 87:4903-4915 (1982). Idone, V.P., R.E. Orville, P. Hubert, L. Barret, and A. Eybert-Berard: Correlated Observations of Three Triggered Lightning Flashes, J. Geophys. Res,. 89:1385-1394 (1984). Idone, V.P. and R.E. Orville: Correlated Peak Relative Light Intensity and Peak Current in Triggered Lightning Subsequent Return Strokes, J. Geophys. Res., M90: 6159-6154 (1985). Jordan, D.M. and M.A. Uman: Variation in Light Intensity With Height and Time from Subsequent Lightning Return Strokes, J. Geophys. Res., 88:6555-6562 (1983). Kayser, H. : Uber Blitzphotographien, Ber. Konigliche Akad. Berlin, 611-615 (1884). Kitagawa, N. , M. Brook, and E.J. Workman: Continuing Currents in Cloud-to-Ground Lightning Discharges, J. Geophys. Res., 67:637-647 (1962). Kitagawa, N. and M. Kobayashi: Field Changes and Variations of Luminosity due to Lightning Flashes, Rec. Advan. in Atmos. Elec. pp. 485-501 (L.G. Smith, Ed.) Pergamon Press, New York (1958) . Kodak Plates and Films for Scientific Photography, Eastman Kodak Company, Rochester, New York (1973) . Krehbiel, P.R., M. Brook, and R. McCrory: An Analysis of the Charge Structure of Lightning Discharges to the Ground. J. Geophys. Res., 84:2432-2456 (1979). Krider, E.P., Weidman, CD., and Levine, D.M. : The Temporal Structure of the HF and VHF Radiation Produced by Intracloud Lightning Discharges, J. Geophys. Res., 84:5760-5762, (1979). Lin, Y.T., M.A. Uman, and R.B. Standler: Lightning Return Stroke Models, J. Geophys. Res., 85:1571-1583, (1980)

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227 Lundquist, S. and V. Scuka: Some Time Correlated Measurements of Optical and Electromagnetic Radiation from Lightning Flashes. Arkiv For Geofysik Band 5 nr 39, 585-593 (1969) . Mach, D.M. and W.D. Rust: Photoelectric Return-Stroke Velocity and Peak Current Estimates in Natural and Triggered Lightning, J. Geophys. Res., 94:13,237-13,247, (1989). Mackerras, D. : Photoelectric Observations of the Light Emitted by Lightning Flashes. J. of Atmos. and Terrestrial Phy., Pergamon Press, New York, Vol. 35 : 521-535 (1973). Malan, D.J., and H. Collens: Progressive Lightning-III, Proc. Roy. Soc. (London), A162:175-203 (1937). Malan, D.J., and B.F.J. Schonland: Progressive Lightning-VII, Proc. Roy. Soc. (London), A191:485-503 (1947). Malan, D.J. and B.F.J. Schonland: The Distribution of Electricity in Thunderclouds, Proc. Roy. Soc, A209:158-177 (1951) . Master, M.J., M.A. Uman, Y.T. Lin, and R.B. Standler: Calculations of Lightning Return Stroke Electric and Magnetic Fields Above Ground, J. Geophys. Res., 86:12,12712,132 (1981). Master, M.J., M.A. Uman, W.H. Beasley, and M. Darveniza: Lightning Induced Voltages on Power Lines: Experiment, IEEE Trans. PAS, PAS-103:2519-2529 (1984). McEachron, K.B.: Lightning to the Empire State Building, J. Franklin Inst. , 227:149-217, (1939). Meyer-Arendt, J.R.: Radiometry and Photometry: Units and Conversions, Applied Optics, 7: 2081 (1968). Orville, R.E., A High-Speed Time-Resolved Spectroscopic Study of the Lightning Return Stroke: Part I-III. A Qualitative Analysis, J. Atmospheric Sci., 25: 827-856 (1968) . Orville, R.E., G.G. Lala, and V.P. Idone: Daylight TimeResolved Photographs of Lightning, Science, 201:59-61 (1978). Orville, R.E. and V.P. Idone: Lightning Leader Characteristics in the Thunderstorm Research International Program (TRIP), J. Geophys. Res., 87:11,177-11,192 (1982). Plooster, M.N.: Numerical Model of the Return Stroke of the Lightning Discharge. The Physics of Fluids, Vol. 14, Num. 10, 2124-2248 (1971).

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228 Rakov, V.A., Uman, M.A., Jordan, D.M., Priore III, C.A. : Ratio of Leader to Return-Stroke Electric Field Change for First and Subsequent Lightning Return Strokes, J. Geophys. Res., Accepted for Publication Spring 1990. Richard, P., A. Delannoy, G. Labaune, and P. Laroche: Results of Spatial and Temporal Characterization of the VHF-UHF Radiation of Lightning, J. Geophys. Res., 91:12481260, (1986). Schonland, B.F.J. : Progressive lightning IVThe Discharge Mechanism, Proc. Roy. Soc. (London) A164:132-150 (1938). Schonland, B.F.J. : The Pilot Streamer in Lightning and the Long Spark, Proc. Roy. Soc. (London) A220:25-38 (1953). Schonland, B.F.J. : The Lightning Discharge, Handbuch Der Physik, 22:576-628, Springer-Verlag, Berlin (1956). Schonland, B.F.J. : Lightning and the Long Electric Spark, Advan. Science, 306-313 (1963). Schonland, B.F.J, and J. Craib: The Electric Fields of South African Thunderstorms, Proc. Roy. Soc. (London), A114:229-243 (1927) . Schonland B.F.J. , D.B. Hodges and H. Collens: Progessive Lightning V. A comparison of photographic and electrical studies of the discharge process., Proc. Roy. Soc. (London), A166:56-75 (1938). Schonland, B.F.J, and D.J. Malan: Progressive Lightning VI, Proc. Roy. Soc. (London) A168:455-469 (1938). Schonland, B.F.J. , D.J. Malan, and H. Collens: Progressive Lightning, Proc. Roy. Soc. (London), A143:654-674 (1934). Schonland, B.F.J. , D.J. Malan, and H. Collens: Progresssive Lightning II, Proc. Roy. Soc. (London), A152:595-625 (1935) . Thomason, L.W. and Krider, W.P.: The Effects of Clouds on the Light Produced by Lightning, J. of the Atmos. Sc. , 39:20512065 (1982). Thomson, E.M. : Characteristics of Port Moresby Ground Flashes, J. Geophys. Res., 85:1027-1036 (1980). Thomson, E.M. : Characteristics of Lightning in Papau New Guinea, Ph.D. Dissertation, Uman, M.A. : Lightning, McGraw-Hill, New York, (1969).

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229 Uman, M.A. : The Lightning Discharge, Academic Press (Orlando) , International Geophysics Series, Volume 39 (1987) . Uman, M.A. , D.K. McLain, and E.P. Krider: The Electromagnetic Radiation from a Finite Antenna, Am. J. Phys., 43, 33-38, (1975). Walter, B. : Ein photographischer Apparat ur genaueren Analyse des Blitxes, Physik. S., 3, 168-172 (1902). Walter, B. : Uber die Entstehungsweise des Blitzes, Ann. Physid, 10, 393-407 (1903). Walter, B. : Uber Doppelaufnahmen von Blitzen — , Jahrbuch Hamb. Wiss. Anst. , 27(beihefte 5), 81-118 (1910). Walter, B. : Stereoskopische litzaufnahmen, Physik. Z., 13, 1082-1084 (1912). Walter, B. : Uber die Ermittelung der zeitlichen Aufeinanderfolge zusammengehoriger Blitze sawie uber ein bemerkenswerted Beispiel dieser Art von Entladungen, Physik. Z., 19, 273-279 (1918). Weber, L. : Uber Blitzphotographien, Ber. Konigliche Akad. Berlin, 781-784 (1889). Willett, J.C., Bailey, J.C., Idone, V.P., EybertBerard, A., and Barret, L. : Submicrosecond Intercomparison of Radiation Fields and Currents in Triggered Lightning Return Strokes based on the "Transmission Line Model", J. Geophys. Res. (1989). Winn, W.P.: A Laboratory Analog to the Dart Leader and Return Stroke of Lighting, J. Geophys. Res., 70:3265-3270 (1965) .

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BIOGRAPHICAL SKETCH Douglas M. Jordan was born August 13, 1949, in Panama City, Florida. He attended public schools in Panama City and graduated from Rutherford High School in June, 1967. In August, 1967 he entered Gulf Coast Community College and received the Associate of Arts degree in June, 1969. In September, 1969 he entered the University of Florida as a Junior in the College of Engineering and was enrolled until he entered the U.S. Navy in April, 1971. He served with the U.S. Navy until March, 1977. In March, 1977 he reentered the University of Florida and received the degree of Bachelor of Science in December, 1979. He entered the Graduate School of the University of Florida in January, 1980 and received the degree of Master of Science in December, 1981. He entered the Doctoral Program of the University of Florida in January, 1982. In January, 1983 he became the Engineering Manager of the University of Florida Remote Sensing and Image Processing Laboratory under the Institute of Food and Agricultural Sciences of the University of Florida. In June, 1987 he became the Scientific and Tecnical Director of Florida Agricultural 230

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231 Services and Technology, a not for profit corporation established for technology transfer with the Institute of Food and Agricultural Sciences of the University of Florida. In August, 1988 he reentered the Doctoral Program of the Graduate School of the University of Florida.

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy RELATIVE LIGHT INTENSITY AND ELECTRIC FIELD INTENSITY OF CLOUD TO GROUND LIGHTNING By Douglas Max Jordan May 1990 Chairman: Martin A. Uman Major Department: Electrical Engineering Correlated streak camera and single-station electric field records were obtained for: 9 subsequent return strokes in 2 cloud to ground discharges (Tampa, Florida 1979) , and 1 first and 14 subsequent strokes in 3 flashes (Gainesville, Florida 1982). Streak camera records for 8 first and 8 subsequent strokes (Kennedy Space Center, Florida 1978) and 3 first and 3 subsequent strokes (Oklahoma 1984) were furnished by the State University of New York at Albany. For ten first return and 11 subsequent return strokes, relative light intensity is graphed versus height and time. First stroke light rise times increase immediately after a branch point, then decrease as the

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light front propagates up the channel. First stroke relative light intensity shows an opposite pattern. Subseguent leader speeds, for 4 flashes, increase from stroke two to stroke three, then decrease. Positive correlations are shown for leader speed vs. return stroke initial peak electric field, leader speed vs. leader electric field duration, and leader detectability using streak camera technigues vs. return stroke initial peak electric field. Relative light intensity profiles for five dart leaders show that the dart leader channel continues to radiate light after the dart leader front has passed and the peak light intensity for dart leaders is constant with height. From measurements of relative light intensity vs. height and time for one subseguent stroke followed by two M-components we show that the M-component relative light intensity is constant with height and propagated downward at between 1 and 2 x 10 8 m/s. In addition to the new results noted above, for 11 subseguent strokes, peak relative light intensity is shown to be correlated with dart leader speed, correlated with dart leader electric field change duration, and uncorrelated with previous interstroke interval. We confirm previous observations that: dart leader speeds decrease with increasing previous interstroke intervals; there is an increase (more rapid than previously

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presented) in apparent channel length with stroke order; subsequent stroke relative light intensity shows a rapid rise to peak which decays with height along the channel; and that a positive correlation exists between initial peak electric field and peak relative light intensity measured near the ground for subsequent return strokes.

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the deqree of Doctor of Philosophy. Martin A. Uman, Chairman Professor of Electrical Enqineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. fV\ lA*UK^ Ewen M. Thomson Associate Professor of Electrical Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Alexander Domi; Assistant Professor of Electrical Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. IJjct^ K &*U**U Dennis P. Carroll Professor of Electrical Engineering

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the deqree of Doctor of Philosophy. Humbert oHHBfipl n s Assistant" Professor of Astronomy This dissertation was submitted to the Graduate Faculty of the College of Engineering and to the Graduate School and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. JL<__ Winfred M. Phillips May 1990 m 1-UtJtuuj (J ' A-HaaC, Winfred M. Phillips Dean, College of Engineering Madelyn M. Lockhart Dean, Graduate School