Citation
Microsecond- and Submicrosecond-Scale Electric Field Pulses Produced by Cloud and Ground Lightning Discharges

Material Information

Title:
Microsecond- and Submicrosecond-Scale Electric Field Pulses Produced by Cloud and Ground Lightning Discharges
Creator:
NAG, AMITABH
Copyright Date:
2008

Subjects

Subjects / Keywords:
Antennas ( jstor )
Clouds ( jstor )
Electric fields ( jstor )
Electric pulses ( jstor )
Electrical polarity ( jstor )
Geometric mean ( jstor )
Histograms ( jstor )
Lightning ( jstor )
Recordings ( jstor )
Waveforms ( jstor )
City of Gainesville ( local )

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright Amitabh Nag. 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.
Embargo Date:
5/31/2012
Resource Identifier:
660161438 ( OCLC )

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1 MICROSECONDAND SUBMICROSECOND-SCALE ELECTRIC FIELD PULSES PRODUCED BY CLOUD AND GR OUND LIGHTNING DISCHARGES By AMITABH NAG A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007

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2 2007 Amitabh Nag

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3 To my parents, grandparents and Gurudev.

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4 ACKNOWLEDGMENTS I would like to thank Dr. Vladimir Rakov for his guidance at every step of the research that culminated in this thesis. His erudition and atte ntion to detail will always remain benchmarks that I can only hope to achieve. I would also like to thank Dr. Martin Uman and Dr. Douglas Jordan whose comments and questions have help ed me immensely in r eevaluating and shaping my work. I would like to acknowledge the he lp that I received from Rob Olsen III, Jason Jerauld, Brian DeCarlo, Sandip Nallani, Jens Schoene and other members of the University of Florida, Lightning Research Group. Finally, I would like to thank my parents w hose support, patience and sacrifices have enabled me to pursue my own goals in life. Anything that I accomplish is because of them. My mother is and will always be my best teacher.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........7 LIST OF FIGURES................................................................................................................ .........9 ABSTRACT....................................................................................................................... ............19 CHAPTER 1 INTRODUCTION..................................................................................................................21 2 LITERATURE REVIEW.......................................................................................................24 2.1 The Lightning Discharge Process.....................................................................................24 2.1.1 Cloud Discharges....................................................................................................25 2.1.2 Cloud-to-Ground Discharges..................................................................................28 2.2 Preliminary Breakdown....................................................................................................31 2.2.1 Preliminary Breakdown in Cloud Discharges........................................................32 2.2.2 Preliminary Breakdown in Ground Discharges......................................................33 2.3 Lightning Initiation Mechanisms......................................................................................34 2.3.1 Conventional Breakdown.......................................................................................35 2.3.2 Runaway Breakdown..............................................................................................35 2.4 Isolated Narrow Bipolar Pulse..........................................................................................37 3 EXPERIMENTAL SETUP....................................................................................................41 3.1 Overview of the 2006 Experiment....................................................................................41 3.1.1 Theory................................................................................................................... ..41 3.1.2 Experimental Setup at the EMS in 2006................................................................42 3.2 Instrumentation............................................................................................................ .....43 3.2.1 The Antenna...........................................................................................................45 3.2.2 High Input-Impedance Amplifier...........................................................................45 3.2.3 Fiber-Optic Link.....................................................................................................47 3.2.4 LeCroy WavePro 7100 Digitizer............................................................................48 4 DATA PRESENTATION AND ANALYSIS........................................................................49 4.1 Microsecondand Submicrosecond-Scale Pulses............................................................49 4.1.1 Analysis of Pulses in Cloud-to-Ground Discharges...............................................49 4.1.1.1 Data Summary..............................................................................................49 4.1.1.2 Methodology................................................................................................49 4.1.1.3 Analysis........................................................................................................53 4.1.2 Analysis of Pulses in Cloud Discharges.................................................................90

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6 4.1.2.1 Data Summary..............................................................................................90 4.1.2.2 Methodology................................................................................................90 4.1.2.3 Analysis........................................................................................................92 4.2 Narrow Bipolar Pulses....................................................................................................130 4.2.1 Data Summary......................................................................................................130 4.2.2 Methodology.........................................................................................................130 4.2.3 Analysis................................................................................................................131 4.3 Leader Duration............................................................................................................ ..137 4.3.1 Data Summary......................................................................................................137 4.3.2 Methodology.........................................................................................................137 4.3.3 Analysis................................................................................................................138 4.4 Ratio of Preliminary Breakdown to Firs t Return Stroke Electric Field Peaks...............139 4.4.1 Data Summary......................................................................................................139 4.4.2 Methodology.........................................................................................................140 4.4.3 Analysis................................................................................................................140 4.5 Ratio of Subsequent to First Re turn Stroke Electric Field Peaks...................................142 4.5.1 Data Summary......................................................................................................142 4.5.2 Methodology.........................................................................................................142 4.5.3 Analysis................................................................................................................146 4.6 Attempted Leaders..........................................................................................................148 4.6.1 Data Summary......................................................................................................149 4.6.2 Methodology.........................................................................................................149 4.6.3 Analysis................................................................................................................154 4.7 Positive Cloud-to-Ground Lightning..............................................................................162 4.7.1 Data Summary......................................................................................................162 4.7.2 Methodology.........................................................................................................162 4.7.3 Analysis................................................................................................................162 5 DISCUSSION..................................................................................................................... ..167 5.1 Microsecondand Submicrosecond-Scale Pulses..........................................................167 5.1.1 Analysis of Pulses in Cloud-to-Ground Discharges.............................................167 5.1.2 Analysis of Pulses in Cloud Discharges...............................................................174 5.2 Narrow Bipolar Pulses....................................................................................................182 5.3 Leader Duration............................................................................................................ ..183 5.4 Ratio of Preliminary Breakdown to Firs t Return-Stroke Elect ric Field Peaks...............183 5.5 Ratio of Subsequent to First Re turn Stroke Electric Field Peaks...................................184 5.6 Attempted Leaders..........................................................................................................187 5.7 Positive Cloud-to-Ground Lightning..............................................................................189 6 RECOMMENDATIONS FOR FUTURE RESEARCH......................................................191 LIST OF REFERENCES.............................................................................................................193 BIOGRAPHICAL SKETCH.......................................................................................................197

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7 LIST OF TABLES Table page 2-1 Characteristics of narrow bipolar electric field pulses and associated HF (3-30 MHz) radiation reported by Sm ith et al. (1999a).........................................................................39 4-1 Characterization of electric field records selected for an alysis of pulses in cloud-toground discharges..............................................................................................................50 4-2 Classification of pulses acco rding to normalized amplitude.............................................53 4-3 Characterization of electr ic field records selected for analysis of pulses in cloud discharges..................................................................................................................... ......93 4-4 Characterization of electric fiel d records of narrow bipolar pulses.................................131 4-5 Summary of electric field characte ristics of each of the eight NBPs..............................131 4-6 Summary of electric field records of cloud-to-ground flashes used to find the duration of the stepped leader..........................................................................................138 4-7 Summary of electric field records used for finding the ratio of subsequent to first return stroke electric field peaks......................................................................................143 4-8 Number of strokes of different order...............................................................................143 4-9 Arithmetic and geometric mean values of the ratio of subsequent to first return stroke field peaks.................................................................................................................... ....146 4-10 Arithmetic and geometric mean values of the ratio of second to first return stroke field peaks.................................................................................................................... ....146 4-11 Arithmetic and geometric mean values of the ratio of first to subs equent return stroke field peaks.................................................................................................................... ....147 4-12 Characterization of elec tric field records acquired during eight thunderstorms in Gainesville, Florida, in 2006............................................................................................149 4-13 Classification of data acc ording to pulse activity (or l ack of such) following the preliminary breakdown (PB) pulse train and presence (or absence) of static ramp........154 4-14 Parameters of microsecond-scale electri c field waveforms produced by the three positive cloud-to-gr ound discharges................................................................................163 5-1 Summary of occurrence of smaller and na rrower pulses observed in the 12 selected cloud-to-ground discharges..............................................................................................171

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8 5-2 Summary of length of record examined be fore and after the trigger pulse and total number of pulses in each of th e 12 selected cloud discharges.........................................178 5-3 Summary of occurrence of smaller a nd narrower pulses in the selected 12 cloud discharges..................................................................................................................... ....179 5-4 Summary of other pulse ac tivities accompanying NBPs.................................................182 5-5 Summary of the subsequent to first str oke electric field (or current) peak ratio reported in different studies.............................................................................................185 5-6 Summary of multiple-stroke flash charac teristics reported in different studies..............186

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9 LIST OF FIGURES Figure page 2-1 Different types of lightning discharges..............................................................................25 2-2 Occurrence statistics of electric field pulses in a cloud discharge.....................................27 2-3 Four types of cloud-to -ground lightning discharge...........................................................28 2-4 Various processes comprising a ne gative cloud-to-ground lightning flash.......................29 2-5 Examples of electric field pulse wavefo rms characteristic of (a ) the active stage in cloud flashes, (b) preliminary breakdown in negative ground flashes..............................33 2-6 Submicrosecond-scale pulses that occurred in a cloud discharge.....................................36 2-7 Narrow bipolar pulses recorded by th e Los Alamos Sferic Array (LASA).......................38 3-1 Electric field antenna system.............................................................................................42 3-2 Experimental setup at the EMS in 2006, individual components of which are described in Section 3.2.....................................................................................................44 3-3 Electronics following the Antenna on the roof of Benton Hall.........................................45 3-4 Schematic of the high input-impedance amplifier used in 2006 EMS..............................46 3-5 Frequency response of the high input-impedance amplifier..............................................46 4-1 Example of an irregular pulse train that was not incl uded in the pulse analysis performed in this study......................................................................................................52 4-2 A 150 ms time-window of the electr ic field record of flash 05/24/06_224.......................54 4-3 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 05/24/06_224......55 4-4 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_224................................................................................................................... ..55 4-5 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_224................................................................................................................... ..56 4-6 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_224................................................................................................................... ..56 4-7 A 150 ms time-window of the electr ic field record of flash 05/24/06_228.......................57

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10 4-8 Occurrence of pulses of diffe rent amplitude in diffe rent parts of flash 05/24/06_228................................................................................................................... ..58 4-9 Occurrence of pulses of diffe rent total duration in different parts of flash 05/24/06_228.............................................................................................................58 4-10 Histogram of distribution of pulses of different amplitude and polarity in flash 05/24/06_228................................................................................................................... ..59 4-11 Histogram of distribution of pulses of di fferent total duration and polarity in flash 05/24/06_228................................................................................................................... ..59 4-12 A 140 ms time-window of the electric field record of flash 05/24/06_1078.....................60 4-13 Occurrence of pulses of different am plitude in different parts of flash 05/24/06_1078.................................................................................................................. .61 4-14 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_1078.................................................................................................................. .61 4-15 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_1078.................................................................................................................. .62 4-16 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_1078.................................................................................................................. .62 4-17 Time-window of about 75 ms of the el ectric field record of flash 05/28/06_1152...........63 4-18 Occurrence of pulses of different am plitude in different parts of flash 05/28/06_1152.................................................................................................................. .64 4-19 Occurrence of pulses of different tota l duration in different parts of flash 05/28/06_1152.................................................................................................................. .64 4-20 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/28/06_1152.................................................................................................................. .65 4-21 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/28/06_1152.................................................................................................................. .65 4-22 A 70 ms time-window of the electri c field record of flash 05/28/06_1360.......................66 4-23 Occurrence of pulses of different am plitude in different parts of flash 05/28/06_1360.................................................................................................................. .67 4-24 Occurrence of pulses of different tota l duration in different parts of flash 05/28/06_1360.................................................................................................................. .67

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11 4-25 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/28/06_1360.................................................................................................................. .68 4-26 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/28/06_1360.................................................................................................................. .68 4-27 A 70 ms time-window of the electri c field record of flash 06/01/06_21...........................69 4-28 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 06/01/06_21........70 4-29 Occurrence of pulses of different tota l duration in different parts of flash 06/01/06_21.................................................................................................................... ...70 4-30 Histogram of occurrence of pulses of different amplitude and polarity in flash 06/01/06_21.................................................................................................................... ...71 4-31 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 06/01/06_21.................................................................................................................... ...71 4-32 Time-window of about 70 ms of the el ectric field record of flash 06/02/06_120.............72 4-33 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 06/02/06_120......73 4-34 Occurrence of pulses of different tota l duration in different parts of flash 06/02/06_120................................................................................................................... ..73 4-35 Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_120................................................................................................................... ..74 4-36 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 06/02/06_120................................................................................................................... ..74 4-37 A 70 ms time-window of the electri c field record of flash 06/02/06_139.........................75 4-38 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 06/02/06_139......76 4-39 Occurrence of pulses of different tota l duration in different parts of flash 06/02/06_139................................................................................................................... ..76 4-40 Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_139................................................................................................................... ..77 4-41 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 06/02/06_139................................................................................................................... ..77 4-42 A 60 ms time-window of the electri c field record of flash 06/02/06_207.........................78 4-43 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 06/02/06_207......79

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12 4-44 Occurrence of pulses of different tota l duration in different parts of flash 06/02/06_207................................................................................................................... ..79 4-45 Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_207................................................................................................................... ..80 4-46 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 06/02/06_207................................................................................................................... ..80 4-47 A 60 ms time-window of the electri c field record of flash 06/02/06_212.........................81 4-48 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 06/02/06_212......82 4-49 Occurrence of pulses of different tota l duration in different parts of flash 06/02/06_212................................................................................................................... ..82 4-50 Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_212................................................................................................................... ..83 4-51 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 06/02/06_212................................................................................................................... ..83 4-52 A 100 ms time-window of the electr ic field record of flash 07/15/06_23.........................84 4-53 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 07/15/06_23........85 4-54 Occurrence of pulses of different tota l duration in different parts of flash 07/15/06_23.................................................................................................................... ...85 4-55 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/15/06_23.................................................................................................................... ...86 4-56 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/15/06_23.................................................................................................................... ...86 4-57 A 90 ms time-window of the electri c field record of flash 07/17/06_54...........................87 4-58 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 07/17/06_54........88 4-59 Occurrence of pulses of different tota l duration in different parts of flash 07/17/06_54.................................................................................................................... ...88 4-60 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_54.................................................................................................................... ...89 4-61 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/17/06_54.................................................................................................................... ...89

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13 4-62 Example of a complex waveform not incl uded in the analysis performed in this study.......................................................................................................................... .........91 4-63 A 140 ms time-window of electric field record of flash 05/24/06_49...............................94 4-64 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 05/24/06_49........95 4-65 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_49.................................................................................................................... ...95 4-66 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_49.................................................................................................................... ...96 4-67 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_49.................................................................................................................... ...96 4-68 Time-window of about 140 ms of elect ric field record of flash 05/24/06_52...................97 4-69 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 05/24/06_52........98 4-70 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_52.................................................................................................................... ...98 4-71 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_52.................................................................................................................... ...99 4-72 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_52.................................................................................................................... ...99 4-73 A 140 ms time-window of electric field record of flash 05/24/06_54.............................100 4-74 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 05/24/06_54......101 4-75 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_54.................................................................................................................... .101 4-76 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_54.................................................................................................................... .102 4-77 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_54.................................................................................................................... .102 4-78 Time-window of about 160 ms of elect ric field record of flash 05/24/06_57.................103 4-79 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 05/24/06_57......104 4-80 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_57.................................................................................................................... .104

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14 4-81 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_57.................................................................................................................... .105 4-82 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_57.................................................................................................................... .105 4-83 Time-window of about 135 ms of elect ric field record of flash 05/24/06_226...............106 4-84 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 05/24/06_226....107 4-85 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_226...................................................................................................................107 4-86 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_226...................................................................................................................108 4-87 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_226...................................................................................................................108 4-88 Time-window of about 150 ms of elect ric field record of flash 05/24/06_299...............109 4-89 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 05/24/06_299....110 4-90 Occurrence of pulses of different tota l duration in different parts of flash 05/24/06_299...................................................................................................................110 4-91 Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_299...................................................................................................................111 4-92 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 05/24/06_299...................................................................................................................111 4-93 Time-window of about 110 ms of elect ric field record of flash 07/17/06_555...............112 4-94 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 07/17/06_555....113 4-95 Occurrence of pulses of different tota l duration in different parts of flash 07/17/06_555...................................................................................................................113 4-96 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_555...................................................................................................................114 4-97 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/17/06_555...................................................................................................................114 4-98 A 100 ms time-window of electric field record of flash 07/17/06_559...........................115 4-99 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 07/17/06_559....116

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15 4-100 Occurrence of pulses of different tota l duration in different parts of flash 07/17/06_559...................................................................................................................116 4-101 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_559...................................................................................................................117 4-102 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/17/06_559...................................................................................................................117 4-103 A 130 ms time-window of electric field record of flash 07/17/06_565...........................118 4-104 Occurrence of pulses of di fferent amplitude in differe nt parts of flash 07/17/06_565....119 4-105 Occurrence of pulses of different tota l duration in different parts of flash 07/17/06_565...................................................................................................................119 4-106 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_565...................................................................................................................120 4-107 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/17/06_565...................................................................................................................120 4-108 A 100 ms time-window of electric field record of flash 07/21/06_1013.........................121 4-109 Occurrence of pulses of different am plitude in different parts of flash 07/21/06_1013.................................................................................................................122 4-110 Occurrence of pulses of different tota l duration in different parts of flash 07/21/06_1013.................................................................................................................122 4-111 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/21/06_1013.................................................................................................................123 4-112 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/21/06_1013.................................................................................................................123 4-113 A 100 ms time-window of electric field record of flash 07/21/06_1015.........................124 4-114 Occurrence of pulses of different am plitude in different parts of flash 07/21/06_1015.................................................................................................................125 4-115 Occurrence of pulses of different tota l duration in different parts of flash 07/21/06_1015.................................................................................................................125 4-116 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/21/06_1015.................................................................................................................126 4-117 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/21/06_1015.................................................................................................................126

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16 4-118 A 100 ms time-window of electric field record of flash 07/21/06_1018.........................127 4-119 Occurrence of pulses of different am plitude in different parts of flash 07/21/06_1018.................................................................................................................128 4-120 Occurrence of pulses of different tota l duration in different parts of flash 07/21/06_1018.................................................................................................................128 4-121 Histogram of occurrence of pulses of different amplitude and polarity in flash 07/21/06_1018.................................................................................................................129 4-122 Histogram of occurrence of pulses of di fferent total duration and polarity in flash 07/21/06_1018.................................................................................................................129 4-123 Electric field records of eight NBPs analyzed in this study.............................................132 4-124 Electric field record showi ng the stepped-leader duration (TL).......................................138 4-125 Histogram of the stepped-leader duration for individual flashes.....................................139 4-126 Histogram of the ratio of preliminary brea kdown to first return stroke field peaks for individual flashes.............................................................................................................140 4-127 Electric field peaks of the largest PB pulse (APB) and corresponding first returnstroke pulse (ARS) in flash 07/17/2006_77......................................................................141 4-128 Multiple-stroke cloud-to-ground flash.............................................................................143 4-129 Histogram of the ratio of the mean subse quent return stroke field peak to the first return stroke field peak for individual flashes.................................................................147 4-130 Histogram showing the geometric mean of the normalized (to the field peak of the first return stroke in each multiple-stroke flash) field peaks for each stroke order found in the present study (yellow column)....................................................................148 4-131 Electric field record of a nega tive cloud-to-ground lightning flash.................................150 4-132 Electric field record illustrating defi nition of (a) overall preliminary-breakdown pulse train duration (TPB) and (b) pulse duration (TPW) and interpulse interval (TIP) for preliminary-breakdown pulses of an attempted leader..............................................152 4-133 Electric field record of an attempted lead er. a. An attempted leader with no pulse activity following the preliminary brea kdown pulse train. b. Preliminary breakdown pulses of the attempted leader shown in (a).....................................................................155 4-134 Electric field record of an attempte d leader. a. An attempted leader whose preliminary breakdown pulse train is follo wed by static ramp but no other pulse activity. b. Preliminary breakdown pulses of the attempted leader shown in (a)............156

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17 4-135 Electric field record of an attempte d leader. a. An attempted leader whose preliminary breakdown pulse train is fo llowed by non-return-stroke-type pulses (see inset) without static ramp. b. Preliminary breakdown pulses of the attempted leader shown in (a)................................................................................................................... ...157 4-136 Electric field record of an attempte d leader. a. An attempted leader whose preliminary breakdown pulse train is fo llowed by non-return-stroke-type pulses (see inset) and static ramp. b. Preliminary breakdown pulses of the attempted leader shown in (a)................................................................................................................... ...158 4-137 Pulses in a preliminary-breakdown pulse tr ain of an attempted leader. a. A typical “classical” pulse. b. Typical “narrow” pulses..................................................................159 4-138 Histogram of preliminary-breakdown pulse -train duration for attempted leaders. Note that a total of 35 preliminary-breakdow n pulse trains were found in 33 electric field records.................................................................................................................. ...160 4-139 Ranges of variation (vertical bars) and m ean values (diamonds) of pulse duration in individual preliminary breakdown pulse trains................................................................161 4-140 Ranges of variation (vertical bars) and mean values (diam onds) of interpul se interval in individual preliminary breakdown pulse trains...........................................................161 4-141 A 90 ms portion of showing the electric field record containing a positive cloud-toground discharge 07/17/06_567.......................................................................................163 4-142 A 200 ms portion of showing the electric field record contai ning a positive cloud-toground discharge 07/17/06_568.......................................................................................164 4-143 A 140 ms portion of showing the electric field record showin g a positive cloud-toground discharge 07/17/06_583.......................................................................................164 4-144 Slow front (ES1) and fast transition (EF1) of the return stroke waveform of positive cloud-to-ground discharge 07/17/06_567........................................................................165 4-145 Slow front (ES2) and fast transition (EF2) of the return stroke waveform of positive cloud-to-ground discharge 07/17/06_568........................................................................165 4-146 Slow front (ES3) and fast transition (EF3) of the return stroke waveform of positive cloud-to-ground discharge 07/17/06_583........................................................................166 5-1 “Classical” preliminary breakdow n pulse from flash 05/24/06_1078.............................168 5-2 “Narrow” pulses from flashes (a) 05/24/06_1078 and (b), (c) 05/24/06_224.................169 5-3 Histogram of total duration of unipolar and bipolar pulses.............................................172

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18 5-4 Scatter plots of normalized amplitude versus durati on of pulses in flashes (a) 05/24/06_224 (b) 07/15/06_23 (c) 05/28/06_1360 and (d) 06/01/06_21...................172 5-5 “Classical” preliminary breakdow n pulse from flash 05/24/06_49.................................175 5-6 “Narrow” pulses from flashes (a) 07/17/06_555 (b) 05/24/06_57 (c) 05/24/06_226 and (d). 05/24/06_57........................................................................................................176 5-7 Histogram of total duration of unipolar and bipolar pulses.............................................178 5-8 Scatter plots of normalized amplitude versus durati on of pulses in flashes (a) 05/24/06_226 (b) 05/24/06_299 (c) 07/17/06_559 and (d) 07/17/06_565.................180

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19 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science MICROSECONDAND SUBMICROSECOND-SCALE ELECTRIC FIELD PULSES PRODUCED BY CLOUD AND GR OUND LIGHTNING DISCHARGES By Amitabh Nag May 2007 Chair: Vladimir A. Rakov Cochair: Martin A. Uman Major: Electrical and Computer Engineering This study examines microsecondand subm icrosecond-scale pulses (with emphasis on initial breakdown pulses) in electric field r ecords of cloud and cloud-to-ground discharges acquired in summer 2006, in Gainesville, Florida. A total of 12 cloud and 12 ground flashes were an alyzed in detail, with the electric field records being 96 or 200 ms long. Occurrence of pulse s in different parts of the flash and pulse waveshape characteristics were examined. Th e majority of pulses in both cloud and ground discharges analyzed in this study were relative ly small in amplitude and duration, the durations being an order of magnitude smaller than thos e of “classical” preliminary breakdown pulses. Eight so-called narrow bipolar puls es (NBPs) were also analyzed. Five of these appeared to be temporally isolated (within at least 40 ms) from any other discharg e activity (that is, not associated with full-fledged lightning flashes). The other three NBPs were accompanied by some other pulse activity. The geometric mean stepped-leader dura tion in negative cloud-to-ground flashes was estimated to be about 23 ms. The geometric mean of the ratio of the initial peak of the largest preliminary breakdown pulse to that of the corre sponding first return stroke pulse was equal to

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20 0.45. Also, the relative magnitudes of the electric field peaks of first and subsequent return strokes in negative cloud-to-ground discharges were examined. The geometric mean ratio of the subsequent-to-first return stroke peak field was found to be 0.59. Electric field pulse trains that are characteristic of pre liminary breakdown in negative cloud-to-ground discharges, but are not followed by return-stroke waveforms were identified and analyzed. These events are refe rred to in this thesis as at tempted cloud-to-ground leaders, although some of them were followed by full-fle dged cloud discharges. Some of the attempted cloud-to-ground leaders, which should belong to the cloud discharge category, can be misclassified by lightning locating systems, such as the U.S. NLDN, as negative cloud-to-ground discharges. The analysis also yielded three positive cloudto-ground discharges. Various characteristics of the slow front and fast transition of each positive return stroke were examined.

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21 CHAPTER 1 INTRODUCTION Lightning was present on Earth long before human life evolved and it may even have played a crucial role in the e volution of life on our planet. Each year some 25 million cloud-toground lightning discharges occur in the United States alone. Lightning strikes involve the formation of channels carrying tens of kiloam peres of electric current with channel peak temperatures of the order of 30,000 K. Thus, lightning strikes have far-reaching and often disastrous consequences affecting essential se rvices such as aviation, power transmission and distribution, communicati on, as well as the day-to-day human life. Lightning is the second most effective weathe r-related killer in the United States. According to the US National Oceanographi c and Atmospheric Administration (NOAA) publication Storm Data, the annual average numbe r of lightning-related deaths in the United States between 1965-95 is 85. Also about 300 indivi duals are injured by lightning each year in the United States. The recent Sago coal-mine explosi on in West Virginia l eading to the death of 12 miners in January 2006 and many California wildfires causing immense damage to property and wildlife are believed to have been caused by lightning. Lightning and electrical storms have been a major concern for the aviation industry. Lightning damage to aircraft varies from minor pitting of the aluminum skin to complete destruction of the aircra ft. Studies suggest that a typical comm ercial plane is struck by lightning once a year on average (Rakov and Uman, 2003, C h. 10). There were two well documented cases where lightning was initiated by large rockets la unched from Earth, the Saturn V vehicle of NASA’s Apollo 12 and US Air Force’s Atlas-Centau r 67. The latter suffered damage that led to the loss of the vehicle and its payload. On 25 August 2006, a direct li ghtning strike to the lightning rod on top of the Space Station Launc h Pad at Kennedy Space Center, Florida caused

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22 NASA to delay the launch of the space shuttle At lantis over concerns about possible damage caused by the strike. According to the Electric Power Research In stitute (EPRI), lightning is a major cause (4050%) of electric service interruptions, resulting in $50 million per year in damage and restoration expenses. Lightning is involved in 5% of all US residential-property-da mage insurance claims, including those from tens of thousands of home fi res, with total claims of over one billion dollars annually. Over the years, a large number of resear ch projects have be en conducted to better understand and model the lightning discharge, which in turn has led to th e evolution of lightning protection standards and techniqu es that are followed in the industry. However, quite a few aspects of lightning are stil l insufficiently understood. A better and more detailed characterization of the lightning electromagnetic field will definitely provide new insights into the phenomenon of lightning initia tion and other processes. This thesis presents data acquired using an experimental setup designed to measure the electric field environment of lightni ng. This experiment is herein re ferred to as the Electric Field Measurement Station (EMS). Data acquired in th e summer of 2006 are analyzed in this thesis. The main goals of this experiment are listed below. Analyze microsecondand submicrosecond-scal e pulses in the light ning electric field. Report the occurrence of and analyze pulse trai ns characteristic of preliminary breakdown in cloud-to-ground lightning that are not followed by return stroke pulses. Examine the stepped-leader duration in Florida. Report the occurrence of and characterize Narrow Bipolar Pulses (NBPs). Find the ratio of the initial electric field p eak of the largest preliminary breakdown pulse and that of the corresponding first return stroke pulse in a flash.

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23 Find the ratio of the electric field peaks of the subsequent and fi rst return-strokes in multiple-stroke cloud-to-ground flashes. Examine positive cloud-to-ground lightning. While some of the above-listed features have been examined in previous works, it is the first time that a detailed study about the ex istence of submicrosec ond-scale pulses in the lightning electric field is being conducted. Chapter 2 gives a review of the existing literat ure concerning lightning th at is relevant to this thesis. Chapter 3 gives a detailed descriptio n of the instrumentation at the EMS. Chapter 4 presents a summary and detailed analyses of th e data collected in summ er 2006. Each section of Chapter 5 discusses the results obtained from the analysis of the da ta described in the corresponding section of Chapter 4. Finally, Chapte r 6 suggests avenues for future research in this area that can lead to bette r understanding of the various feat ures of lightning described in this thesis.

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24 CHAPTER 2 LITERATURE REVIEW This chapter presents a review of the exis ting literature concer ning lightning that is relevant to this thesis. Section 2.1 presents a br ief introduction to the physics of natural lightning, both cloud and cloud-to-ground types. Section 2.2 discusses preliminary breakdown in cloud and ground discharges. Section 2.3 presents a brief revi ew of the mechanism of lightning initiation in thunderclouds. Finally, Section 2.4 discusses the so-called isolat ed narrow bipolar pulses. 2.1 The Lightning Discharge Process Lightning is a transient, high-current electrical discharge that transfers charge between the atmosphere and the Earth or between different pa rts of the atmosphere. The primary sources of lightning are the separated char ge centers located in clouds termed cumulonimbus, commonly referred to as thunderclouds (Uman, 1987). The charge structure of a cumulonimbus can be approximated as a vertical tripole consisting of three charge centers, main positive at the top, main negative in the middle, and an additional smaller positive at the bottom. The tw o upper charges, located respectively at heights of about 12 and 7 km for Florida, are usually specified to be equa l in magnitude (typically some tens of coulombs) and therefore form a dipole. The magnitude of the lower positive charge (probably about 10 C or less), locat ed approximately at a height of 2 km, is significantly smaller than that of the dipole charges (Rakov and Uman, 2003). Around 75% of lightning discharges occur within the cloud a nd include intracloud, intercloud and cloud-to-air discha rges. Cloud discharges are often referred to as ICs. Lightning discharges involving charge transfer to the gr ound constitute of around 25% of all lightning discharges and are called cloud-to -ground discharges, often referred to as CGs. Figure 2-1 shows the various types of lightning discharges.

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25 Figure 2-1. Different types of lightning discharges. Taken from Schoene (2007). 2.1.1 Cloud Discharges Cloud discharges or ICs, cons tituting of approximately th ree-quarters of all lightning discharges, do not contact ground. ICs have been less well studied than CGs because of the difficulty of securing photographic records of in -cloud channels and inability to obtain direct measurements of currents and charge transfers associated with ICs, though some in situ electric field and acoustic measurements and radar obser vations have provided in formation about ICs. However, ground-based singleand multiple-station electric field measurements have been the primary means of studying ICs. In general, cloud discharges are most likely to begin in the high electric field regions of the upper and lower boundaries of the main negative charge, bridging the main negative and upper positive charge regions. Cloud discharges can typically be viewed as being composed of an early or active stage and a late or final stage. The be ginning of a cloud discha rge is typically marked by the largest microsecond scale pulses in its wideband electric field record. According to

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26 Villanueva et al. (1994), the larger pulses tend to occur early in a flash and presumably are related to the flash initiating brea kdown process. These early stage pulses are often referred to as initial-breakdown pulses. The transition from the ear ly stage to the late st age of a cloud discharge is thought to be related to the disintegration of the negative channel existing between the main negative and upper positive charge regions. The late stage, also called the J-stage, is physically similar to the J-process (junction process) in ground discharges. In the late stage, charge transp ort supposedly takes place from remote sources in the main negative charge regi on to the partially (or completely) neutralized negative charge center from where the negative channel originated during the initial stage. Various transient processes occurr ing during the late stage are refe rred to as K-processes. Kitagawa and Brook (1960) portrayed the el ectric field signature of cloud lightning discharges as being composed of three stages, in itial, very active and fina l. However, the threestage structure was later replaced by the pres ently accepted two-stage model (described above) proposed by Villanueva et al. (1994). Villanueva et al. analyzed microsecond-scale pulses in wideband electric field records of cloud flashes in Florida and New Mexico acquired using a 12bit digitizing system with a 500 ns sampling inte rval. The average of the peak-to-peak amplitude of the five largest pulses in a flash was found. All pulses with peak-to-peak amplitude greater than 50% of that average amplitude were labele d “large pulses”, pulses with amplitudes between 25% and 50% of the average amplitude were labeled “medium pulses”, and pulses between 12.5% and 25% of the average amplitude were labe led “small pulses”. The results of the analysis showed that about 60% and 50% of the large pul ses occurred within the first 20 ms and 5 ms, respectively, of the flash suggesting that the large pulses were associated with the initialbreakdown process. Occurrence statistics of pulses in one of the electric fi eld records in this

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27 Figure 2-2. Occurrence statistics of electric field pul ses in a cloud discharge. a. (i) Overall electric field record and hist ograms of occurrence of (ii) la rge, (iii) medium, and (iv) small electric field pulses in different part s of this record for cloud flash 64 on day 231 in 1991 at Kennedy Space Center, Florida. b. Same as (a) but for the first 25 ms of the flash. Adapted from Villanueva et al. (1994). study is shown in Figure 2-2. Bodhika et al. ( 2006) conducted a similar study of cloud flashes recorded in Sri Lanka and reporte d that about 80% of electric fiel d pulses occurred in the early stage of the flash thus suppor ting the two stage model of cl oud discharges proposed by Villanueva et al. (1994). However, in both the above cited studies, pulses that were smaller than 12.5% of the average amplitude (“in some flas hes there were hundreds of them”) were not included. The work presented in Section 4.1.2 of this thes is includes all detectab le pulses, including the smaller pulses ignored by Villanueva et al. ( 1994) and Bodhika et al. (2006), in the initial ( i ) ( iv ) ( iii ) ( ii ) ( iv ) ( iii ) ( ii ) ( i ) (a) (b)

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28 Figure 2-3. Four types of cl oud-to-ground lightning discharge. Taken from Rakov and Uman (2003). stage of cloud discharges. In addition to the histogram of o ccurrence of pulses of different amplitude in different parts of the flash, histog ram of occurrence of pulses of different total duration in different parts of th e flash, distribution of pulses of different amplitude and polarity, and distribution of pulses of different total dur ation and polarity have also been presented. 2.1.2 Cloud-to-Ground Discharges Cloud-to-ground discharges or CGs, as the name suggests, involves charge transfer between cloud and ground via a high conduc tivity channel. The overall cloud-to-ground lightning discharge, often termed ground flash, cons ists of typically three to five component strokes or just strokes (Rakov and Uman, 2003). Each stroke is composed of a leader/return stroke sequence. Cloud-to-ground di scharges can be classified in to four types (Figure 2-3)

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29 Figure 2-4. Various processes comprising a ne gative cloud-to-ground ligh tning flash. Adapted from Uman (1987) and Rakov and Uman (2003). according to the polarity of the charge transfe rred to ground and the direction of propagation of the initial leader. Types (a) and (b) effectively lower negati ve charge to ground, while types (c) and (d) effectively lower positive charge to ground. Downward negative lightning comprises about 90% of all cloud-to-ground flashes, wh ile downward positive lightning accounts for only about 10% of cloud-to-ground discharges. Upward negative and positive lightning is relatively rare as compared to the other two categories and are most often observed on tall structures or short structures on mountain tops. The various processes associated with a negative cloud-to-ground lightning flash are shown in Figure 2-4. A downwardnegative cloud-to-ground stroke is composed of a downwardmoving leader and an upward-moving return stroke. Leader initiati ng the first stroke in a flash exhibits stepping and is preceded by the initial or preliminary breakdown, which can be defined

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30 as the in-cloud process that initi ates or leads to the initiation of the downward-moving stepped leader. The stepped leader serves to form a ne gatively charged plasma channel extending from the cloud toward the ground at an average speed of 2 x 105 m/s. As the leader approaches ground, one or several upward leaders, having po sitive charge, are initiated from the ground or other grounded objects (e.g., trees or other structur es) and one of these upward leaders attaches to a branch of the downward-movi ng stepped leader at tens of meters above the ground surface. Once the two leaders have connect ed, a large surge of current, known as the first return stroke, travels at about one third to one half the speed of light (with speed de creasing with increasing height) from the ground toward the cloud charge source along the plasma channel neutralizing the negative leader charge. When the first-retu rn stroke reaches the cloud, in-cloud discharge activity known as J (for junction) and K-processe s occur in the cloud. The J-processes result in redistribution of cloud charge in response to th e preceding return-stroke and lasts for tens of milliseconds. K-processes are transients th at occur during the slower J-process. Following this in-cloud activity, often a new l eader, known as a dart leader, follows the path of the previous leader ch annel at an average speed of 107 m/s and does not exhibit stepping. As the dart or dart-stepped leader approaches ground, an attachment process similar to that for the first stroke takes place. However, the attach ment process for subseque nt strokes occurs over a shorter distance, takes less time and typically occu rs when the upward leader is a few meters in height. This is followed by the subsequent return -stroke wave that again neutralizes the leader charge deposited along the channel. The peak currents associated with the first and subsequent return strokes are about 30 kA and 15 kA respectively. Downward-positive flashes (type c), accounti ng for roughly 10% of the total cloud-toground discharges, transport positive charge fr om cloud to ground. The leader/return stroke

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31 process in downward positive discharges is similar to that of negative flashes. However, the peak currents and charge transfers associated with pos itive first strokes can be much higher than for negative first strokes. Also, single-stroke posit ive flashes are much more common than singlestroke negative flashes. Upward lightning discharges (types b and d) are thought to occur only from tall objects (higher than 100 m or so) or from objects of m oderate height on mountain tops. The initiation mechanism involved in an upward discharge is co mpletely different from that in a downward flash. The first leader in an upward flash is initiated from the ground-based object and moves towards the cloud. This upward directed leader bridges the gap between the object and the cloud charge source or in-cloud discharge channel, forming a continuous pa th to ground through which a current, with a magnitude of several hundred amperes lasting for several hundred milliseconds called the initial continuous curre nt (ICC) , flows. This process is followed by subsequent dart leader/return stroke sequences which are similar to subsequent strokes of downward flashes. It may be mentioned here that rocket-triggere d lightning is similar in phenomenology to the upward lightning discharge initia ted from tall grounded objects. 2.2 Preliminary Breakdown One of the less understood areas in lightning research is th e initiation mechanism of a lightning discharge. This is mainly because of the fact that the maximum electric fields typically measured in thunderclouds are 1 to 2 x 105 V/m which is an order of magnitude lower than the expected air breakdown fi eld of the order of 106 V/m. Also, there is a lack of optical records due to the in-cloud nature of this process. Preliminary brea kdown, often called initial breakdown, may be viewed as an in-cloud phenomenon that pr ecedes and leads to the initiation of both cloud discharges and downward cloud-to -ground discharges. The detailed physics involved in this process is not yet understood. It may be thought of as a process which results in the formation of

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32 the first plasma channel within the cloud that may lead to ei ther a cloud or a ground discharge depending on the types of charges involved and region of the cloud in which the breakdown is taking place. 2.2.1 Preliminary Breakdown in Cloud Discharges Cloud discharges, as described in Section 2.1.1, can be viewed as being composed of an early (or active) stage having a duration of so me tens to a few hundreds of milliseconds and a late (or final) stage that cons titutes the remainder of the flash. In general, cloud discharges are most likely to begin in the high electric field regions of the upper and lower boundaries of the main negative charge, bridging the main nega tive and upper positive ch arge regions with a negatively charged channel extending in an inte rmittent manner with an average speed of the order of 105 m s-1. Overall, the early-stage processes are probably similar to the initial breakdown and stepped leader processes in negative cloud-to-ground lightning. The largest microsecond-scale electric field pul ses tend to occur at the beginning of a cloud discharge (Villanueva et al., 1994). These pulse s, in analogy to the preliminary breakdown pulses in ground flashes (described later in Sec tion 2.2.2), are usually referr ed to as initial or preliminary breakdown pulses. These pulses are e ither relatively slow-rising, wide bipolar waveforms with several small pulses superimposed on the initial half cycl e or relatively narrow or smooth singly-peaked or multiply-peaked bipo lar waveforms (Rakov, 2006) as illustrated in Figure 2-5a. The initial polarity of the bipolar elect ric field pulses in clou d discharges is usually opposite to that of the preliminary-breakdown pulses in negative cloud-to-ground discharges (discussed in Section 2.2.2), the latter pulses being usually of the same pola rity as that of the following return-stroke pulse. Individual pulses in the early stage of cloud discharges are characterized by a typica l total duration of 50-80 s and the typical time interval between pulses

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33 Figure 2-5. Examples of electric field pulse waveforms characteris tic of (a) the active stage in cloud flashes, (b) preliminary breakdown in negative ground flashes. The waveforms have been recorded, from a distant stor m, by D.E. Crawford at Camp Blanding, Florida. Adapted from Rakov (1999). is 600-800 s (Rakov et al., 1996). Kitagawa and Brook (1960) reported that cloud-flash pulses appeared in groups separated by in tervals ranging from 0.3 to 10 ms. 2.2.2 Preliminary Breakdown in Ground Discharges In the electric field records of some cloud-to -ground discharges a bipolar pulse-train with pulses having the same initial polarity as the foll owing return-stroke, durations of the order of tens of microseconds and preceding the first-return stroke pulse by tens of milliseconds is commonly attributed to preliminary breakdown. The leader initiating the fi rst stroke in a flash exhibits stepping and is preceded by the initial or preliminary breakdown, which is the in-cloud process that initiates or lead s to the initiation of the dow nward-moving stepped leader. The preliminary breakdown often involves th e formation of a sequence of channels extending in seemingly random directions from the cloud charge source with one of these events evolving into the stepped leader which bridge s the cloud charge source and the ground (see Rakov (2006) for a recent review). The characteris tic features of preliminary-breakdown pulse (a) ( b )

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34 trains (see Figure 2-5b) in negative cloud-to-gro und flashes (based on information found in the literature) are as follows: Duration of the pulse train: The entire duration of the pr eliminary-breakdown pulse train is of the order of 1 ms (e.g., Rakov, 1999). Regularity of pulses in a train: This is a subjective featur e, but it has been noted by many researchers. According to Kitagawa and Brook (1960) and Weidman and Krider (1979), regularity of preliminary-breakdown pulses and uniformity of time in tervals between them in the case of cloud-to-ground flashe s is higher than for cloud flashes. Overall pulse shape: Individual preliminary-breakdown pulse s in the train are bipolar, as reported by many investigators (e.g., K itagawa, 1957; Clarence and Malan, 1957; Kitagawa and Kobayashi, 1959; Kitagawa and Brook, 1960; Krider and Radda, 1975; Weidman and Krider, 1979; Beasley et al., 1982; Gomes et al., 1998; Rakov, 1999). Polarity of the initial half cycle: The initial polarity of bipolar pulses in the train is the same as that of negative re turn-stroke pulses (e .g., Weidman and Krider, 1979). For the atmospheric electricity sign convention (e.g., Rakov and Uman, 2003, pp. 8-9) this polarity is positive. In contrast, for the initial brea kdown in cloud flashes, the dominant polarity of the initial half cycle of individual pulses is negative (e.g., Rakov, 1999). Overall pulse duration: According to Rakov et al. (1996) , the typical total duration of individual pulses in the trai n is in the range of 20 to 40 s. In contrast, for the preliminary breakdown in cloud flashes the typica l total pulse duration is 50 to 80 s. Interpulse interval: The typical time interval between individual pulses in the train is 70 to 130 s, versus 600 to 800 s for initial breakdown in cloud flashes (Rakov et al., 1996). 2.3 Lightning Initiation Mechanisms Over the years a number of theories have been proposed in order to explain the physics behind the initiation mechanism of lightning. In this section, a brief review of the proposed lightning initiation mechanisms th at involve either conventional or runaway air breakdown has been presented. Both of these two mechanisms at tempt to explain the formation of an ionized region sometimes referred to as lightning seed in the cloud that is capable of locally enhancing the electric field at its extremities thus leading to the formation of a self-propagating leader channel.

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35 2.3.1 Conventional Breakdown According to the conventiona l breakdown mechanism, light ning is initiated via the emission of positive corona from the surface of precipitation partic les, highly deformed by strong electric fields in the case of raindrops, coupled with some mech anism whereby the electric field is locally enhanced to suppor t the propagation of corona streamers. The most detailed hypothetical scenario of lightning initiation via conven tional breakdown is de scribed by Griffiths and Phelps (1976b) who consider a system of positive streamers developing from a point on a hydrometeor where the electric field exceeds the corona onset value of 2.5 to 9.5 x 105 V m-1 (2.5 to 9.5 kV cm-1). The developing streamers are assumed to form a conical volume that grows longitudinally. The ambient electric field in th e thundercloud required to support the propagation of corona streamers, E0, was found by Griffiths and Phelps (1976a) from laboratory experiments to be 1.5 x 105 V m-1 (1.5 kV cm-1) at about 6.5 km and 2.5 x 105 V m-1 (2.5 kV cm-1) at about 3.5 km. If the ambient electric field is higher than E0, the streamer system will intensify, carrying an increasing amount of positive charge on the pr opagating base of the cone and depositing an equally increasing amount of negative charge in th e conical volume. As a result, an asymmetric conical dipole is formed, which presumably can se rve to enhance the existing electric field at the cone apex (Rakov, 2006). Another hypothetical mechanism proposed by Nguyen and Michnowski (1996) involves a bidirectional streamer development assisted by a ch ain of precipitation particles, as opposed to the propagation of positive streamers alone. 2.3.2 Runaway Breakdown Gurevich et al. (1999) suggested that runaway electrons may play an important role in lightning initiation. Energy gained by a runawa y electron from the electric field between collisions with air particles, must be more th an it looses in a collision. The runaway breakdown

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36 Figure 2-6. Submicrosecond-scale pulses that oc curred in a cloud discharge. Taken from Rakov and DeCarlo (2005) and Rakov (2006). mechanism is associated with a current pulse having an amplitude of 100-200 A leading to the formation of a field-enhancing io nized region (“lightning seed”) by a cosmic ray particle with an energy of 1016 eV (Gurevich et al. 2003). The current pu lse is predicted to generate a bipolar electric field pulse with a char acteristic full width of 0.2-0.4 s (Gurevich et al. 2002) which is more than an order of magnitude shorter th an the shortest preliminary breakdown pulses, including so-called na rrow bipolar pulses (e.g., Rakov 2006) (see Section 2.4) that have characteristic full widths of a few tens of microseconds. The occurrence of such submicrosecond-scale pulses can be determined experimentally. Submicrosecond-scale electric field pulses some what similar to the “lightning initiation pulses” predicted by Gurevich et al. (2002, 2003) have been observ ed as a part of preliminary breakdown in cloud and ground discharges by Gurevi ch et al. (2003) and by the University of

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37 Florida Lightning Research Group (see Figur e 2-6) (Rakov and DeCarlo, 2005; Rakov, 2006). However, the relation between the runaway br eakdown mechanism proposed by Gurevich et al. (1999) and the occurrence of multiple submicrosec ond-scale pulses is still unclear. Further, not all the recorded flashes contained submicros econd-scale pulses, bringing into question the indispensability of such pulses in the process of lightning initiation a nd atmospheric conditions needed for their occurrence. 2.4 Isolated Narrow Bipolar Pulse Narrow bipolar pulses have characteristic full widths of a few tens of microseconds, often appear temporally isolated from any other di scharge activity, and are accompanied by large HF emissions that are ten times larger than the HF emissions from typical cloud-to-ground and other cloud discharges. Smith et al. (1999a, b) (see Figure 2-7) has attributed the narrow bipolar pulse to “compact intracloud discharges”, a term based on the inference from a simple model that the spatial extent of this in-cloud process must be relatively small, 300 to 1000 m. Observations of narrow bipolar el ectric field pulses that have been recorded in conjunction with their associated HF-VHF radiation were first reported by Le Vine (1980) who used an electric field measuring system triggered when th e associated HF-VHF sign al, in the range from 3 to 300 MHz, exceeded a relatively high threshol d level. The initial pol arity of the bipolar pulses was negative (atmospheric elec tricity sign convention), that is, opposite to that of electric field pulses due to return strokes in negativ e cloud-to-ground lightning. The pulse amplitudes were of the order of one-third t hose of the return stroke peaks recorded at about the same time. Le Vine (1980) attributed the observed narro w bipolar pulses accompanied by strong HF-VHF radiation to K processes, a hypothesis not confir med by subsequent studies. Willett et al. (1989), working at KSC in 1985 and 1987, obt ained recordings of both elec tric field (E) and electric field derivative (dE/dt) for narrow bipolar puls es. Their measuring system was triggered by the

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38 Figure 2-7. Narrow bipolar pulses recorded by the Los Alamos Sfer ic Array (LASA). (a) Narrow positive bipolar pulses (positive NBPs) from a discharge that occurred 32 km northwest of Tampa, Florida. Physics sign convention is used here. (b) Narrow negative bipolar pulses (negative NBPs) from a discharge that occurred in Oklahoma. Adapted from Smith et al. (2002). output of an HF receiver that could be tuned to any center frequency between 3 and 18 MHz. Both polarities of the initial half -cycle of the narrow bipolar pulses were observed, with negative polarity (atmospheric electricity sign conventio n) being more frequent. For 18 waveforms, Willett et al. (1989) estimated mean values of E and dE/dt peaks normalized to 100 km of 8 V m-1 and 20 V m-1 s-1, respectively. Both values are comparable to their counterparts for first return strokes observed in the same experi ment. The overall pulse width was 20 to 30 s. Spectral analysis indicated that the sources of the narrow bipola r pulses radiated much more strongly than first return str okes at frequencies from 10 to at least 50 MHz. At 18 MHz the energy-spectral density (measured in (V m-1 Hz-1)2) for these pulses was nearly 16 dB higher than that for first return strokes at the same distance. Time, s (b) (a) 34 Hz – 0.5 MHz, ~ 1 ms Time, s

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39 Table 2-1. Characteristics of narro w bipolar electric fi eld pulses and associated HF (3-30 MHz) radiation reported by Sm ith et al. (1999a). Mean Std. Dev. Electric Field Pulse Characteristics Risetime (10-90 percent) 2.3 0.8 s Half-peak width 4.7 1.3 s Pulse duration 25.8 4.9 s Initial peaka 9.5 3.6 V m-1 Opposite polarity overshoota -3.9 1.6 V m-1 Ratio of initial peak to o pposite polarity overshoot 2.7 Ratio of peaks for narrow bi polar pulses and returnstroke pulses 0.71 Ratio of peaks for narrow bipolar pulses and cloud-flash pulses 2.6 HF Radiation Characteristics Duration 2.8 0.8 s Peak b 2.4 1.1 mV m-1 Ratio of peaks for narrow bi polar pulses and returnstroke pulses 9.9 Ratio of peaks for narrow bipolar pulses and cloud-flash pulses 29 a Normalized to 100 km; physics sign convention. b Normalized to 10 km and 1 kHz bandwidth. Smith et al. (1999a), who used a multiple-station electric field change measuring system in concert with an HF (3 to 30 MHz) time-of-a rrival (TOA) lightning locat ing system, presented a detailed characterization of the narrow bipolar pu lses in three thunderstorm s at distances greater than 80 km in New Mexico and west Texas, incl uding locations of their sources in the cloud. The characteristics of the 24 narrow bipolar electric field pulses and associated HF (3 to 30 MHz) radiation studied by Smith et al. (1999a) are su mmarized in Table 2-1. Smith et al. (1999a) reported that the pulse peaks were comparable to those of return-stroke waveforms. Nearly all pulses studied by Smith et al. (1999a) were the on ly events within the field record having a length of typically 4 to 10 ms. The HF (3 to 30 MHz) emissions associated with narrow bipolar pulses had a duration of only a few microseconds and were typically 10 times more powerful than the HF emissions from “ normal lightning discharges.”

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40 The reason behind the occurrence of these isol ated pulses is not yet known. Note that a necessary feature of the narrow bipo lar pulses discussed earlier in this section is strong HF-VHF radiation. Eight isolated narrow bipolar pulses, although without simultaneous HF-VHF records, will be presented in Section 4.2 of this thesis.

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41 CHAPTER 3 EXPERIMENTAL SETUP This chapter discusses the instrumentation us ed in 2006 for the Electric Field Measurement Station (herein referred to as EM S) in Gainesville, Florida. Sec tion 3.1 presents an overview of the experimental setup at the EMS. In Secti on 3.2, the antenna system, fiber-optic link, the digitizer and the associated electronics are discussed. 3.1 Overview of the 2006 Experiment The electric field measuring system in Gaines ville was installed on the roof and third floor of Benton Hall (29 38’ 37.77” N, 82 20’ 50.00” W), a three-storey building on the University of Florida campus. 3.1.1 Theory Lightning discharges, both cloud and cloud-to-g round, have durations that may extend to a second or more. However, the individual physical pr ocesses in a particular discharge can vary at microsecond and submicrosecond time scales. Thus cu rrents associated with lightning discharges produce wideband electric and magnetic fields ha ving frequency content from below 0.1 Hz to above 10 MHz (Uman, 1987). A sensor that is commonly used to detect the lightning electric fi eld is a metallic flat plate. Figure 3-1 shows a flat plate antenna system. CG is the capacitance between the antenna plate and the ground, R0 terminates the coaxial cable in its characteristic impedance. C is the integrating capacitor and R is the relatively large resistor (usually input resistance of associated electronics) that provides a path for the capacitor C to discharge so that the output voltage decays exponentially with a time constant RC. The value of RC should be chosen such that it is much larger than the variation time of the signal of interest. In order to satisfy boundary conditions on th e surface of a perfect conductor,

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42 Figure 3-1. Electric field antenna sy stem. Adapted from Uman (1987). the electric field vector will only have a vertical component. Any ch ange in the vertical electric field E on the antenna plate having a surface area A will cause a change in the charge Q induced on the antenna plate and is given by Equation 3-1. 0()AE(t) Qt (3-1) The current I produced by the charge Q is obtained as shown in Equation 3-2. 0()() ()A dQtdEt It dtdt (3-2) Therefore, the voltage across the resistor R (and capacitor C) and hen ce the output voltage (Vout) is given by Equation (3-3). 0 01() ()t outAEt VItd CC (3-3) The output voltage can be obtained from Equati on (3-3) for time t << RC, where RC is the time constant of the system. Hence the output voltage is directly proportional to the electric field on the antenna plate. So any change in the electri c field due to lightning discharge will cause a voltage change at the output. 3.1.2 Experimental Setup at the EMS in 2006 Figure 3 shows a sketch of the experimental setup at the EMS. The electric field measuring system at the EMS consisted of a circular flat-plate antenna followed by an Vout Coaxial Cable

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43 integrating capacitor and a unity-gain, high input-impedance amplifier. The antenna was installed on the roof of Benton Hall. A Nicolet ISOBE 3000 fiber-opt ic link was used to transmit signals from the antenna and associated electr onics to a LeCroy 8-bit digitizing oscilloscope situated in a closet on the third floor of Bent on Hall. The digitizer recorded and stored the incoming signals in a 200 GB memory unit. The system had a useful frequency bandwidth of 16 Hz to 10 MHz, the lower and upper limits being determined by the RC time constant of the integrator and the high input-impedance amplifier, respectively. The tim e constant (9.89 ms) was long enough for faithful reproduction of microsecondand submicrosecond-scale pu lses examined in this thesis. Using thunder ranging and the characteristic features of return-s troke electric field waveforms at known distances in the 50 to 250 km range (Pavlick et al., 2002; Figure 5), it was estimated that the majority of data recorded by th e electric field measuring system at the EMS in 2006 are due to lightning discharges occurring at distances ranging from a few to tens of kilometers. Also, as the antenna in this expe riment was placed on the roof of a building an enhancement of the lightning induced electric fiel d is expected. However, as all electric field peaks and amplitudes are measured in relative units in this thesis, the effect of the field enhancement due to heig ht can be ignored. 3.2 Instrumentation In this section a description of each block of the experimental setup including the antenna, the high input-impedance amplifier, fiberoptic link and digitizer is presented. While the amplifier and the fiber-optic li nk transmitter was placed inside a shielded metallic Hoffman enclosure (also called Hoff man box) on the roof, the digitizer was placed inside the closet. Figure 3-3 shows th e electronics inside the Hoffman box.

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44 Figure 3-2. Experimental setup at th e EMS in 2006, individual components of which are described in Section 3.2.

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45 Figure 3-3. Electronics following the Antenna on the roof of Benton Hall. 3.2.1 The Antenna The circular flat plat e electric field antenna used at the 2006 EMS had a diameter of 46.5 cm and an area of 0.170 m2. The flat plate is raised from the roof of the building by a height of 1.33 meters. At the base of the antenna and w ithin the Hoffman box was an integrating capacitor of 1.94 nF connected via a short (about 80 cm long) shielded 50 coaxial cable. 3.2.2 High Input-Impedance Amplifier The integrating capacitor was c onnected to a high impedance unity gain amplifier placed within the Hoffman box. Figure 3-4 shows a schematic of the amplifier circuit. It consists of an AD825 op-amp which is used to implement a vo ltage follower. The input impedance of the amplifier is 5.1 M and the output impedance of the feedback circuit an d hence the amplifier is a fraction of an Ohm. Hence, the RC time consta nt of the antenna system equals 9.89 ms. The amplifier is powered by a 12 V battery placed inside the Hoffman box. The amplifier ground is

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46 Figure 3-4. Schematic of the high input-impedan ce amplifier used in 2006 EMS. Taken from Jerauld (2003). located at 6 V above the negative battery terminal in order to bias the AD825 op-amp with V (Jerauld, 2003). The amplifier ground being 6 V above the co mmon ground, a separate 12 V battery was used to power the amplifier. The re lay circuit of the amplifier block was however powered by the same 12 V battery as the Nicole t Isobe 3000 transmitter. The frequency response of the amplifier is shown in Figure 3-5. Figure 3-5. Frequency response of the high input-impedance amplifier. Taken from Jerauld (2003).

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47 3.2.3 Fiber-Optic Link A fiber-optic link was used to transmit the anal og data from the sensor on the roof to the digitizer in the closet. The fiber-optic link co nsisted of two elements , a Nicolet Isobe 3000 transmitter/receiver pair and a fiber-optic cable. The input resistance of the Nicolet Isobe 3000 transmitter is 1 M and its input capacitance is 50 pF. At the EMS, in order to ma tch the input impedance of the Isobe transmitter to the 50 characteristic impedance of the coaxial cable between the amplifier and the Isobe transmitter input, a 50 terminator was used at the input of the transmitter. Nicolet Isobe 3000 utilizes a combination of am plitude modulation (AM) and pu lse width modulation (PWM) to transmit the analog signal to the re ceiver unit via a fiber-optic cable pair. The input range of the transmitter is selectable from .1 V, V, and V. For an input range of .1 V the Isobe fiber-optic link introduces a gain of 20 dB to the input signal and for an input range of V the input signal is attenuated by -20 dB. The V input range which passes the input signal without any gain or attenuation is the setting that was used at the EM S in 2006. The output range of the receiver is thus fixed at V regardless of the selected input range. The Isobe transmitter was powered by a 12 V battery in the Hoffman box. The gain and offset of the link can be manually adjusted at the receiver end. The output impedance at the receiver end is 50 . The manufacturer specified -3 dB bandwidth of the Nicolet Isobe 3000 fiber-optic link is 15 MHz. The fiber-optic link included a 200 m multim ode Kevlar reinforced duplex fiber-optic cables with SMA connectors. The fiber was manu factured by OFS Fitel Corporation and had a refractive index of 1.429. The lengt h of the fiber-optic cable between the roof and the closet that was used for the EMS experiment in 2006 was de termined to be 194.7 m using an optical time domain reflectometer (OTDR).

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48 3.2.4 LeCroy WavePro 7100 Digitizer The LeCroy WavePro 7100 is a four-channel DSO with a maximum bandwidth of 1 GHz at a maximum sampling rate of 10 GHz with an 8-bit vertical resolution. The maximum sampling rate is 20 GHz when operating in the 2-ch annel mode. The WavePro 7100 is capable of recording 24 megabyte per channe l when all four channels are in use. Hence with an 8-bit resolution (that is one byte re quired to store each sample) the total record length is 24 megasamples for each of the 4 channels. At a maximum sampling rate of 10 GHz, the maximum record length is 2.4 ms. The WavePro 7100 uses the Microsoft Wi ndows 2000 Professional (SP 4) operating system and has a flat panel touch-screen disp lay. The processor memory is 1024 MB and the processor speed is 1.70 GHz. For the 2006 EMS expe riment, 2 channels were used to record simultaneous electric field records with different volts/division setting (usually 300 mV/div for Channel 1 and 600 mV/div for Channel 2) for each channel. The input im pedances on channels 1 and 2 of the scope were set at 50 and 1 M , respectively, thus eff ectively terminating the coaxial cable into 50 which is its characteristic impedan ce. The other end of the coaxial cable was terminated in 50 which is the output impedance of th e Isobe fiber-optic link receiver. A positive edge-trigger (with voltage level usually set at 300 mV) corresponding to positive (atmospheric electricity sign conve ntion) electric field change along with an appropriate pretrigger time (80 or 120 ms for a sampling rate of 100 MHz and 40 or 60 ms for a sampling rate of 250 MHz) was used to collect all data described in this thesis. The recorded data were stored in an external hard disk manufactured by Seagat e with a capacity of 200 GB via a USB port.

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49 CHAPTER 4 DATA PRESENTATION AND ANALYSIS 4.1 Microsecondand Submicrosecond-Scale Pulses This section presents a detailed descripti on of the microsecondand submicrosecond-scale electric field pulse activity in lightning disc harges. Electric field r ecords from both cloud and cloud-to-ground flashes have been analyzed. The data, methodology and analyses for each of the two types of lightning are described in the following subsections. 4.1.1 Analysis of Pulses in Cloud-to-Ground Discharges The primary focus of this study is to closel y examine pulse activity associated with the predominantly bipolar pulse train that sometime s appears a few to several tens of milliseconds before the first return-stroke of a cloud-to-ground discharge. This pulse train is commonly attributed to preliminary breakdown and hence referred to as the preliminary breakdown pulse train. 4.1.1.1 Data Summary The data used in this section were acquired in the months of May, June and July, 2006 at the EMS in Gainesville, Florida. The datase t which consists of 12 negative cloud-to-ground flashes is characterized in Table 4-1. The firs t six digits of the fl ash identification number represent the date on which the record was acquire d and all the following di gits give the record number of a particular flash. 4.1.1.2 Methodology The electric field pulse activ ity of each flash was examined thoroughly. A manual, nonalgorithmic approach was adopted to locate, identify and character ize each individual pulse in a particular record in order to minimize inaccuraci es that would have been associated with the

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50 Table 4-1. Characterization of el ectric field records selected for analysis of pulses in cloud-toground discharges. Storm ID (mm/dd/yy) Number of records Flash ID Sampling Interval, ns Record Length, ms 05/24/06_224 05/24/06_228 05/24/06 3 05/24/06_1078 10 200 05/28/06_1152 05/28/06 2 05/28/06_1360 4 96 06/01/06 1 06/01/06_21 4 96 06/02/06_120 06/02/06_139 06/02/06_207 06/02/06 4 06/02/06_212 4 96 07/15/06 1 07/15/06_23 10 200 07/17/06 1 07/17/06_54 10 200 All data combined 12 4 or 10 96 or 200 automated detection and characte rization of different pulse type s, some of which were being examined in detail for the first time. Given the sufficiently large frequency bandwid th of the measuring system, short sampling interval ( 10 ns), and adopted data processing met hod, the ability to de tect and properly characterize pulses was primarily limited by the electromagnetic noise level at the position of antenna. Each electric field record was examined thr oughout its entire length using different time windows (mostly tens of microseconds). Only puls es with peak-to-peak amplitudes equal to or exceeding twice that of the local average noise le vel were considered. On detecting a pulse, it

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51 was examined using different time-windows va rying from few microseconds to tens of microseconds, depending upon the duration and amplit ude of the pulse. All detected pulses were classified depending on their pola rity (positive or negative) an d detectable opposite polarity overshoot (bipolar or unipolar). In order to classify a pulse as either bipolar or unipolar, each pulse was first examined for any visually detectab le overshoot using different time windows. If a pulse was found to have an oppos ite polarity overshoot which was more than half the average local background noise level, it was classified as bipolar. All other pulses, having either an insignificant opposite polarity oversh oot or no detectable opposite po larity overshoot at all, were classified as unipolar. Note that, due to the limiti ng effect of noise, some pulses that were labeled unipolar in this study could act ually be bipolar but with unde tectable opposite polarity overshoots. The possible consequences of bipol ar pulses with undetect able opposite polarity overshoot being classified as unipolar pulses in cloud-to-ground discharges are discussed in Section 5.1.1. Bipolar pulses were further cla ssified as positive or ne gative bipolar depending upon the polarity of their initial half-cycle. Similarl y, unipolar pulses were classified as positive or negative unipolar depending upon their polarity. The peak-to-peak amplitude and overall duration of each pulse were measured and tabul ated. Examination of relative magnitudes of initial half-cycle and opposite po larity overshoot of bipolar pulses is outside the scope of this study. Only non-return-stroke-type pulse s, excluding irregular pulse tr ains (an example of which is shown in Figure 4-1) we re included in this analysis. Irregular pulse trains, which are bursts of pulses of random polarity and waveshape surrounded by relatively quiet regions, constituted less than 1% of the total electric field pulse activity in cloud-to-ground discharges reported in this study. Further, the pulses characterized herein ar e only the ones detectable above the noise level

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52 Figure 4-1. Example of an irregul ar pulse train that was not included in the pulse analysis performed in this study. of the recorded data. As noted above, the loca l electromagnetic environment at the time of recording and the digitiza tion of the data contribute to the noise level of each record. Smaller electric field pulses masked by the noise and, hence, not included in the an alysis may still exist. As a result, this study cannot be viewed as a complete characterizati on of pulses in lightning discharges, but it does cove r a very large class of pulses that ha ve not been analyzed in previous studies. In this study, 12 cloud-to-gr ound discharges showing relatively large preliminary breakdown pulse trains were select ed for analysis. The time at which the first return-stroke of each cloud-to-ground discharge occurred was relabele d as the zero of the time scale (t = 0) and positions of all other pulses on the time axis were determined with respect to it. Only pulses prior

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53 Table 4-2. Classification of pulses ac cording to normalized amplitude. Normalized Pulse Amplitude Category 0.25 Very Small > 0.25 and 0.5 Small > 0.5 and 0.75 Medium > 0.75 and 1.0 Large to the first return stroke in each flash were examined. The peak-to-peak amplitude of each bipolar pulse and zero-to-peak amplitude of eac h unipolar pulse in a pa rticular cloud-to-ground discharge was normalized with respect to that of the largest preliminary breakdown pulse (which was also the largest pulse prior to the first return stroke) in that flash. Pulses were classified into four different categories depending upon the value of their normalized amplitude as shown in Table 4-2. Si milar classification of pulses of different amplitude, but in cloud discharges, by Villanueva et al. (1994) et al. is discussed in Section 2.1.1. 4.1.1.3 Analysis The analyses carried out for each of the 12 cloud-to-ground discharges examine the following characteristics: Occurrence of pulses of different amplitu de in different parts of the flash. Occurrence of pulses of different total dur ation in different parts of the flash. Statistical distribution of pulses of different amplitude and polarity. Statistical distribution of pulses of different total duration and polarity. Figures 4-2 to 4-61 show measured electric fiel d waveforms of each of the 12 flashes examined here along with the corresponding histograms.

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54 Figure 4-2. A 150 ms time-window of the el ectric field record of flash 05/24/06_224.

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55 Time (ms) -60-55-50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 100 120 Large Medium Small Very Small N = 169 NL = 1 NM = 4 NS = 11 NVS = 153 Figure 4-3. Occurrence of pulses of different amplitude in di fferent parts of flash 05/24/06_224. Time (ms) -60-55-50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 100 120 Submicrosecond 1-4 microseconds Above 4 microseconds N = 169 N (<1 s) = 71 N (1-4 s) = 78 N (>4 s) = 20 Figure 4-4. Occurrence of pulses of different total duration in different parts of flash 05/24/06_224.

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56 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 120 140 160 180 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar N = 169 NBP = 146 NUP = 23 Figure 4-5. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_224. Duration ( s) 01248163264 Number of Pulses 0 20 40 60 80 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar N = 169 NBP = 146 NUP = 23 Figure 4-6. Histogram of occurrence of pulses of different total duration and polarity in flash 05/24/06_224.

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57 Figure 4-7. A 150 ms time-window of the elec tric field record of flash 05/24/06_228.

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58 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 30 Large Medium Small Very Small N = 27 NL = 1 NM = 1 NS = 14 NVS = 11 Figure 4-8. Occurrence of pulses of different amplitude in di fferent parts of flash 05/24/06_228. Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 30 Submicrosecond 1-4 microseconds Above 4 microseconds N = 27 N (<1 s) = 1 N (1-4 s) = 10 N (>4 s) = 16 Figure 4-9. Occurrence of pulses of different total duration in different parts of flash 05/24/06_228.

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59 Duration ( s) 01248163264 Number of pulses 0 1 2 3 4 5 6 7 Positive Bipolar Positive Unipolar N = 27 NBP = 21 NUP = 6 Figure 4-10. Histogram of distri bution of pulses of different amplitude and polarity in flash 05/24/06_228. Duration ( s) 01248163264 Number of pulses 0 1 2 3 4 5 6 7 Positive Bipolar Positive Unipolar N = 27 NBP = 21 NUP = 6 Figure 4-11. Histogram of distribu tion of pulses of different total duration and polarity in flash 05/24/06_22.

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60 Figure 4-12. A 140 ms time-window of the el ectric field record of flash 05/24/06_1078.

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61 N = 97 NL = 1 NM = 1 NS = 3 NVS = 92 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 100 120 Large Medium Small Very Small Figure 4-13. Occurrence of pulses of different amplitude in different parts of flash 05/24/06_1078. N = 97 N (<1 s) = 13 N (1-4 s) = 69 N (>4 s) = 15 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 100 120 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-14. Occurrence of pulse s of different total duration in different parts of flash 05/24/06_1078.

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62 N = 97 NBP = 85 NUP = 12 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-15. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_1078. N = 97 NBP = 85 NUP = 12 Durations ( s) 01248163264 Number of Pulses 0 10 20 30 40 50 60 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-16. Histogram of occurrence of pulses of different total durati on and polarity in flash 05/24/06_1078.

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63 Figure 4-17. Time-window of about 75 ms of th e electric field reco rd of flash 05/28/06_1152.

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64 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 2 4 6 8 10 12 14 16 Large Medium Small N = 23 NL = 5 NM = 5 NS = 13 NVS = 0 Figure 4-18. Occurrence of pulses of different amplitude in different parts of flash 05/28/06_1152. Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 2 4 6 8 10 12 14 16 Submicrosecond 1-4 microseconds Above 4 microseconds N = 23 N (<1 s) = 3 N (1-4 s) = 2 N (>4 s) = 18 Figure 4-19. Occurrence of pulse s of different total duration in different parts of flash 05/28/06_1152.

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65 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 2 4 6 8 10 12 14 Positive Bipolar N = 23 N BP = 23 N UP = 0 Figure 4-20. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/28/06_1152. Duration ( s) 01248163264 Number of Pulses 0 2 4 6 8 10 Positive Bipolar N = 23 N BP = 23 N UP = 0 Figure 4-21. Histogram of occurrence of pulses of different total durati on and polarity in flash 05/28/06_1152.

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66 Figure 4-22. A 70 ms time-window of the el ectric field record of flash 05/28/06_1360.

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67 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 10 20 30 40 Large Medium Small Very Small N = 41 NL = 2 NM = 3 NS = 22 NVS = 14 Figure 4-23. Occurrence of pulses of different amplitude in different parts of flash 05/28/06_1360. Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 10 20 30 40 Submicro second 1-4 microseconds Above 4 microseconds N = 41 N (<1 s) = 11 N (1-4 s) = 23 N (>4 s) = 7 Figure 4-24. Occurrence of pulse s of different total duration in different parts of flash 05/28/06_1360.

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68 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 5 10 15 20 25 Positive Bipolar Negative Unipolar N = 41 N BP = 40 N UP = 1 Figure 4-25. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/28/06_1360. Duration (s) 01248163264 Number of Pulses 0 2 4 6 8 10 12 14 16 18 Positive Bipolar Negative Unipolar N = 41 N BP = 40 N UP = 1 Figure 4-26. Histogram of occurrence of pulses of different total durati on and polarity in flash 05/28/06_1360.

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69 Figure 4-27. A 70 ms time-window of the el ectric field record of flash 06/01/06_21.

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70 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 10 20 30 40 Large Medium Small Very small N = 44 NL = 1 NM = 3 NS = 5 NVS = 35 Figure 4-28. Occurrence of pulses of different am plitude in different parts of flash 06/01/06_21. Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 10 20 30 40 Submicrosecond 1-4 microseconds Above 4 microseconds N = 44 N (<1 s) = 9 N (1-4 s) = 22 N (>4 s) = 13 Figure 4-29. Occurrence of pulse s of different total duration in different parts of flash 06/01/06_21.

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71 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 10 20 30 40 Positive Bipolar Positive Unipolar N = 44 N BP = 42 N UP = 2 Figure 4-30. Histogram of occurrence of pulses of different amplitude and polarity in flash 06/01/06_21. Duration ( s) 01248163264 Number of Pulses 0 5 10 15 20 Positive Bipolar Positive Unipolar N = 44 N BP = 42 N UP = 2 Figure 4-31. Histogram of occurrence of pulses of different total durati on and polarity in flash 06/01/06_21.

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72 Figure 4-32. Time-window of about 70 ms of th e electric field reco rd of flash 06/02/06_120.

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73 Time (ms) -60-55-50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 Large Medium Small Very Small N = 25 NL = 2 NM = 2 NS = 13 NVS = 8 Figure 4-33. Occurrence of pulses of different am plitude in different parts of flash 06/02/06_120. Time (ms) -60-55-50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 Submicrosecond 1-4 microseconds Above 4 microseconds N = 25 N (<1 s) = 1 N (1-4 s) = 8 N (>4 s) = 16 Figure 4-34. Occurrence of pulse s of different total duration in different parts of flash 06/02/06_120.

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74 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 2 4 6 8 10 12 14 Positive Bipolar Negaitive Bipolar Positive Unipolar N = 25 N BP = 23 N UP = 2 Figure 4-35. Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_120. Duration ( s) 012481632 Number of Pulses 0 2 4 6 8 10 Positive Bipolar Negative Bipolar Postive Unipolar N = 25 N BP = 23 N UP = 2 Figure 4-36. Histogram of occurrence of pulses of different total durati on and polarity in flash 06/02/06_120.

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75 Figure 4-37. A 70 ms time-window of the el ectric field record of flash 06/02/06_139.

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76 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 Large Medium Small Very small N = 72 NL = 4 NM = 4 NS = 31 NVS = 33 Figure 4-38. Occurrence of pulses of different am plitude in different parts of flash 06/02/06_139. Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 Submicrosecond 1-4 microseconds Above 4 microseconds N = 72 N (<1 s) = 1 N (1-4 s) = 63 N (>4 s) = 8 Figure 4-39. Occurrence of pulse s of different total duration in different parts of flash 06/02/06_139.

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77 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 5 10 15 20 25 30 35 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar N = 72 N BP = 66 N UP = 6 Figure 4-40. Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_139. Duration (s) 01248163264 Number of Pulses 0 10 20 30 40 Postive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar N = 72 N BP = 66 N UP = 6 Figure 4-41. Histogram of occurrence of pulses of different total durati on and polarity in flash 06/02/06_139.

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78 Figure 4-42. A 60 ms time-window of the el ectric field record of flash 06/02/06_207.

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79 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 10 20 30 40 Large Medium Small Very Small N = 48 NL = 1 NM = 5 NS = 6 NVS = 36 Figure 4-43. Occurrence of pulses of different am plitude in different parts of flash 06/02/06_207. Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 10 20 30 40 Submicrosecond 1-4 microseconds Above 4 microseconds N = 48 N (<1 s) = 2 N (1-4 s) = 38 N (>4 s) = 8 Figure 4-44. Occurrence of pulse s of different total duration in different parts of flash 06/02/06_207.

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80 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 10 20 30 40 Positive Bipolar Positive Unipolar N = 48 N BP = 42 N UP = 6 Figure 4-45. Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_207. Duration (s) 01248163264 Number of Pulses 0 5 10 15 20 25 30 Positive Bipolar Positive Unipolar N = 48 N BP = 42 N UP = 6 Figure 4-46. Histogram of occurrence of pulses of different total durati on and polarity in flash 06/02/06_207.

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81 Figure 4-47. A 60 ms time-window of the el ectric field record of flash 06/02/06_212.

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82 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 30 35 Large Medium Small Very Small N = 65 NL = 3 NM = 5 NS = 23 NVS = 34 Figure 4-48. Occurrence of pulses of different am plitude in different parts of flash 06/02/06_212. Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 30 35 Submicrosecond 1-4 microseconds Above 4 microseconds N = 65 N (<1 s) = 24 N (1-4 s) = 28 N (>4 s) = 13 Figure 4-49. Occurrence of pulse s of different total duration in different parts of flash 06/02/06_212.

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83 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 10 20 30 40 Positive Bipolar Negative Bipolar Negative Unipolar N = 65 N BP = 56 N UP = 9 Figure 4-50. Histogram of occurrence of pulses of different amplitude and polarity in flash 06/02/06_212. Duration ( s) 01248163264 Number of Pulses 0 5 10 15 20 25 30 Positive Bipolar Negative Bipolar Negative Unipolar N = 65 N BP = 56 N UP = 9 Figure 4-51. Histogram of occurrence of pulses of different total durati on and polarity in flash 06/02/06_212.

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84 Figure 4-52. A 100 ms time-window of the el ectric field record of flash 07/15/06_23.

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85 N = 73 NL = 1 NM = 3 NS = 20 NVS = 49 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 Large Medium Small Very Small Figure 4-53. Occurrence of pulses of different am plitude in different parts of flash 07/15/06_23. N = 73 N (<1 s) = 13 N (1-4 s) = 48 N (>4 s) = 12 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 20 40 60 80 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-54. Occurrence of pulse s of different total duration in different parts of flash 07/15/06_23.

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86 N = 73 N BP = 62 N UP = 11 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 10 20 30 40 50 60 Positive Bipolar Positive Unipolar Figure 4-55. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/15/06_23. N = 73 N BP = 62 N UP = 11 Duration ( s) 01248163264 Number of Pulses 0 5 10 15 20 25 30 35 Positive Bipolar Positive Unipolar Figure 4-56. Histogram of occurrence of pulses of different total durati on and polarity in flash 07/15/06_23.

PAGE 87

87 Figure 4-57. A 90 ms time-window of the el ectric field record of flash 07/17/06_54.

PAGE 88

88 N = 22 NL = 3 NM = 3 NS = 8 NVS = 8 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 Large Medium Small Very Small Figure 4-58. Occurrence of pulses of different am plitude in different parts of flash 07/17/06_54. N = 22 N (<1 s) = 0 N (1-4 s) = 15 N (>4 s) = 7 Time (ms) -50-45-40-35-30-25-20-15-10-50 Number of Pulses 0 5 10 15 20 25 1-4 microseconds Above 4 microseconds Figure 4-59. Occurrence of pulse s of different total duration in different parts of flash 07/17/06_54.

PAGE 89

89 N = 22 N BP = 20 N UP = 2 Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 2 4 6 8 10 Positive Bipolar Negative Bipolar Positive Unipolar Figure 4-60. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_54. N = 22 N BP = 20 N UP = 2 Duration ( s) 012481632 Number of Pulses 0 2 4 6 8 10 Positive Bipolar Negative Bipolar Positive Unipolar Figure 4-61. Histogram of occurrence of pulses of different total durati on and polarity in flash 07/17/06_54.

PAGE 90

90 4.1.2 Analysis of Pulses in Cloud Discharges Initial breakdown pulses in cloud discharges ha ve a dominant negative polarity (of the initial half-cycle in the case of bipolar pulses) and typical total pulse durat ions in the range of 5080 s, with the larger pulses tending to appear early in the flash. While characterizing pulse activity in cloud discharges, Villanueva et al. (1994) considered only la rger pulses and ignored pulses that were smaller than 12.5% of the average value of the five largest pulses in a flash (“in some flashes there were hundreds of them”). Pr esent study attempts to extend the study of Villanueva et al. (1994) to addi tionally include these smaller pulses by examining their total duration, amplitude, and occurrence primarily duri ng the active (initial) stage of cloud flashes. The final stage of cloud flashes generally could not be examined due to limited record length (200 ms). 4.1.2.1 Data Summary The data used in this section were acquire d in the months of Ma y and July, 2006 at the EMS in Gainesville, Florida. The dataset which c onsists of 12 cloud flashe s is characterized in Table 4-3. The first six digits of the flash id entification number represen t the date on which the record was acquired and all the fo llowing digits give the record num ber of a particular flash. 4.1.2.2 Methodology The methodology adopted in order to examine in detail and classify pulse activity in cloud discharges is identical to that used for analyzing cloud-to-gro und discharges and described in Section 4.1.1.2. Due to the limiting effect of noise , some pulses that were labeled unipolar in this study could actually be bipolar but with undete ctable opposite polarity overshoots. The possible consequences of bipolar pulses w ith undetectable opposite polarity overshoot being classified as unipolar pulses in cloud discharges ar e discussed in Section 5.1.2. In the case of bipolar pulses,

PAGE 91

91 Figure 4-62. Example of a complex waveform not included in the analysis performed in this study. only peak-to-peak amplitudes were measured. Exam ination of relative magnitudes of initial halfcycle and opposite polarity overs hoot of bipolar pulses is outsi de the scope of this study. Irregular pulse trains, which are bursts of pulses of apparently random polarity and waveshape surrounded by relatively quiet regions (e.g. Figure 4-1) were not included in this analysis. Complex waveforms (e.g. Figure 4-62), probably resulting from the overlap of two or more electric field pulses of similar durations and amplitudes, were also excluded. However, these constituted less than 1% of the total el ectric field pulse activ ity in cloud discharges reported in this study. Further, the pulses charac terized herein are only th e ones detectable above the noise level of the recorded data. Smaller elec tric field pulses masked by the noise and, hence, not included in the analysis may still exist. The local electromagnetic environment at the time of recording and the digitizati on of the data contribute to the nois e level of each record. As a result, this study cannot be viewed as a complete charact erization of pulses in lightning discharges, but

PAGE 92

92 it does cover a very large class of pulses that have not been analyzed in previous studies. Given the sufficiently large frequency bandwidth of th e measuring system, short sampling interval (10 ns), and adopted data processing method, the ab ility to detect and pr operly characterize pulses was primarily limited by the electromagnetic noise level at the position of antenna. In this study, 12 cloud discharges showing re latively large amount of pulse activity were selected for analysis. The time at which the puls e having the largest peak-to-peak amplitude in a particular flash occurred was relabeled as the zer o of the time scale (t = 0) and positions of all other pulses on the time axis were determined wi th respect to it. The peak-to-peak amplitude of each bipolar pulse and zero-to-peak amplitude of each unipolar pulse in a particular cloud discharge was normalized with respect to that of the largest pulse in the flash. The pulses were classified into four different categories dependi ng upon the value of its normalized amplitude as shown in Table 4-2. Similar classification of pulses of different amplitude by Villanueva et al. (1994) is discussed in Section 2.1.1. 4.1.2.3 Analysis The analyses carried out for each of the 12 cloud discharges examine the following characteristics: Occurrence of pulses of different amplitude in different parts of the flash. Occurrence of pulses of different total dur ation in different parts of the flash. Statistical distribution of pulses of different amplitude and polarity. Statistical distribution of pulses of different total duration and polarity. Figures 4-63 to 4-122 show measured electric field waveforms of each of the 12 flashes examined here along with the corresponding histograms.

PAGE 93

93 Table 4-3. Characterization of electric field re cords selected for analysis of pulses in cloud discharges. Storm ID (mm/dd/yy) Number of records Flash ID Sampling Interval, ns Record Length, ms 05/24/06_49 05/24/06_52 05/24/06_54 05/24/06_57 05/24/06_226 05/24/06 6 05/24/06_299 10 200 07/17/06_555 07/17/06_559 07/17/06 3 07/17/06_565 10 200 07/21/06_1013 07/21/06_1015 07/21/06 3 07/21/06_1018 10 200 All data combined 12 10 200

PAGE 94

94 Figure 4-63. A 140 ms time-window of elect ric field record of flash 05/24/06_49.

PAGE 95

95 N = 105 NL = 1 NM = 0 NS = 0 NVS = 104Time (ms) -100102030405060708090100110120 Number of Pulses 0 5 10 15 20 25 30 35 Large Very Small Figure 4-64. Occurrence of pulses of different am plitude in different parts of flash 05/24/06_49. N = 105 N(<1 s) = 15 N(1-4 s) = 72 N(>4 s) = 18Time (ms) -100102030405060708090100110120 Number of Pulses 0 5 10 15 20 25 30 35 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-65. Occurrence of pulse s of different total duration in different parts of flash 05/24/06_49.

PAGE 96

96 N = 105 NBP = 86 NUP = 19Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 120 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-66. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_49. N = 105 NBP = 86 NUP = 19Duration (s) 01248163264128 Number of Pulses 0 10 20 30 40 50 60 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-67. Histogram of occurrence of pulses of different total durati on and polarity in flash 05/24/06_49.

PAGE 97

97 Figure 4-68. Time-window of about 140 ms of electric field record of flash 05/24/06_52.

PAGE 98

98 N = 129 NL = 2 NM = 2 NS = 3 NVS = 122Time (ms) -20-100102030405060708090100110 Number of Pulses 0 5 10 15 20 25 30 35 Large Medium Small Very Small Figure 4-69. Occurrence of pulses of different am plitude in different parts of flash 05/24/06_52. N = 129 N(<1 s) = 23 N(1-4 s) = 75 N(>4 s) = 31Time (ms) -20020406080100 Number of Pulses 0 5 10 15 20 25 30 35 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-70. Occurrence of pulse s of different total duration in different parts of flash 05/24/06_52.

PAGE 99

99 N = 129 NBP = 114 NUP = 15Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 120 140 Positive Bipolar Negatve Bipolar Positive Unipolar Negative Unipolar Figure 4-71. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_52. N = 129 NBP = 114 NUP = 15Duration ( s) 01248163264128264 Number of Pulses 0 10 20 30 40 50 60 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-72. Histogram of occurrence of pulses of different total durati on and polarity in flash 05/24/06_52.

PAGE 100

100 Figure 4-73. A 140 ms time-window of el ectric field record of flash 05/24/06_54.

PAGE 101

101 N = 187 NL = 1 NM = 0 NS = 4 NVS = 182Time (ms) -100102030405060708090100110120 Number of Pulses 0 10 20 30 40 50 Large Small Very Small Figure 4-74. Occurrence of pulses of different am plitude in different parts of flash 05/24/06_54. N = 187 N(<1 s) = 61 N(1-4 s) = 108 N(>4 s) = 18Time (ms) -100102030405060708090100110120 Number of Pulses 0 10 20 30 40 50 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-75. Occurrence of pulse s of different total duration in different parts of flash 05/24/06_54.

PAGE 102

102 N = 187 NBP = 136 NUP = 51Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 120 140 160 180 200 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-76. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_54. N = 187 NBP = 136 NUP = 51Duration (s) 01248163264128 Number of Pulses 0 20 40 60 80 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-77. Histogram of occurrence of pulses of different total durati on and polarity in flash 05/24/06_54.

PAGE 103

103 Figure 4-78. Time-window of about 160 ms of electric field record of flash 05/24/06_57.

PAGE 104

104 N = 152 NL = 2 NM = 0 NS = 1 NVS = 149Time (ms) -100102030405060708090100110120 Number of Pulses 0 5 10 15 20 25 Large Small Very Small Figure 4-79. Occurrence of pulses of different am plitude in different parts of flash 05/24/06_57. N = 152 N(<1 s) = 22 N(1-4 s) = 106 N(>4 s) = 24Time (ms) -100102030405060708090100110120 Number of Pulses 0 5 10 15 20 25 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-80. Occurrence of pulse s of different total duration in different parts of flash 05/24/06_57.

PAGE 105

105 N = 152 NBP = 129 NUP = 23Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 120 140 160 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-81. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_57. N = 152 NBP = 129 NUP = 23Duration ( s) 01248163264128 Number of Pulses 0 20 40 60 80 100 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-82. Histogram of occurrence of pulses of different total durati on and polarity in flash 05/24/06_57.

PAGE 106

106 Figure 4-83. Time-window of about 135 ms of electric field record of flash 05/24/06_226.

PAGE 107

107 N = 183 NL = 1 NM = 1 NS = 22 NVS = 159Time (ms) -40-30-20-100102030405060708090 Number of Pulses 0 5 10 15 20 25 30 35 Large Medium Small Very Small Figure 4-84. Occurrence of pulses of different am plitude in different parts of flash 05/24/06_226. N = 183 N(<1 s) = 97 N(1-4 s) = 72 N(>4 s) = 14Time (ms) -40-30-20-100102030405060708090 Number of Pulses 0 5 10 15 20 25 30 35 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-85. Occurrence of pulse s of different total duration in different parts of flash 05/24/06_226.

PAGE 108

108 N = 183 NBP = 121 NUP = 62Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 120 140 160 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-86. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_226. N = 183 NBP = 121 NUP = 62Duration ( s) 01248163264 Number of Pulses 0 20 40 60 80 100 120 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-87. Histogram of occurrence of pulses of different total duration and polarity in flash 05/24/06_226.

PAGE 109

109 Figure 4-88. Time-window of about 150 ms of electric field record of flash 05/24/06_299.

PAGE 110

110 N = 185 NL = 3 NM = 5 NS = 25 NVS = 152Time (ms) -100102030405060708090100110120130 Number of Pulses 0 5 10 15 20 25 Large Medium Small Very Small Figure 4-89. Occurrence of pulses of different amplitude in different parts of flash 05/24/06_299. N = 185 N(<1 s) = 43 N(1-4 s) = 92 N(>4 s) = 50Time (ms) -100102030405060708090100110120130 Number of Pulses 0 5 10 15 20 25 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-90. Occurrence of pulse s of different total duration in different parts of flash 05/24/06_299.

PAGE 111

111 N = 185 NBP = 122 NUP = 63Normalized Amplitude 0.000.250.500.751.00 Number of Pulss 0 20 40 60 80 100 120 140 160 Positive Bipolar Negative Bipolar Positive Unpolar Negative Unipolar Figure 4-91. Histogram of occurrence of pulses of different amplitude and polarity in flash 05/24/06_299. N = 185 NBP = 122 NUP = 63Duration (s) 01248163264128 Number of Pulses 0 10 20 30 40 50 60 70 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-92. Histogram of occurrence of pulses of different total duration and polarity in flash 05/24/06_299.

PAGE 112

112 Figure 4-93. Time-window of about 110 ms of electric field record of flash 07/17/06_555.

PAGE 113

113 N = 124 NL = 1 NM = 4 NS = 50 NVS = 69Time (ms) -20-10010203040506070 Number of Pulses 0 10 20 30 40 50 60 70 80 Large Medium Small Very Small Figure 4-94. Occurrence of pulses of different amplitude in different parts of flash 07/17/06_555. N = 124 N(<1 s) = 32 N(1-4 s) = 80 N(>4 s) = 12Time (ms) -20-10010203040506070 Number of Pulses 0 10 20 30 40 50 60 70 80 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-95. Occurrence of pulse s of different total duration in different parts of flash 07/17/06_555.

PAGE 114

114 N = 124 NBP = 78 NUP = 46Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-96. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_555. N = 124 NBP = 78 NUP = 46Duration (s) 01248163264 Number of Pulses 0 10 20 30 40 50 Positive Bipolar Negative Bipolar Postive Unipolar Negative Unipolar Figure 4-97. Histogram of occurrence of pulses of different total duration and polarity in flash 07/17/06_555.

PAGE 115

115 Figure 4-98. A 100 ms time-window of elect ric field record of flash 07/17/06_559.

PAGE 116

116 N = 143 NL = 2 NM = 3 NS = 14 NVS = 124Time (ms) -1001020304050607080 Number of Pulses 0 10 20 30 40 50 60 70 80 90 Large Medium Small Very Small Figure 4-99. Occurrence of pulses of different amplitude in different parts of flash 07/17/06_559. N = 143 N(<1 s) = 29 N(1-4 s) = 92 N(>4 s) = 22Time (ms) -1001020304050607080 Number of Pulses 0 10 20 30 40 50 60 70 80 90 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-100. Occurrence of pulse s of different total duration in different parts of flash 07/17/06_559.

PAGE 117

117 N = 143 NBP = 89 NUP = 54Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 20 40 60 80 100 120 140 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-101. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_559. N = 143 NBP = 89 NUP = 54Duration (s) 01248163264 Number of Pulses 0 10 20 30 40 50 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-102. Histogram of occurrence of pulses of different total duration and polarity in flash 07/17/06_559.

PAGE 118

118 Figure 4-103. A 130 ms time-window of el ectric field record of flash 07/17/06_565.

PAGE 119

119 N = 49 NL = 4 NM = 7 NS = 36 NVS = 2Time (ms) -100102030405060708090100 Number of Pulses 0 2 4 6 8 10 12 14 16 Large Medium Small Very Small Figure 4-104. Occurrence of pul ses of different amplitude in different parts of flash 07/17/06_565. N = 49 N(<1 s) = 5 N(1-4 s) = 41 N(>4 s) = 3Time (ms) -100102030405060708090100 Number os Pulses 0 2 4 6 8 10 12 14 16 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-105. Occurrence of pulse s of different total duration in different parts of flash 07/17/06_565.

PAGE 120

120 N = 49 NBP = 29 NUP = 20Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 10 20 30 40 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-106. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/17/06_565. N = 49 NBP = 29 NUP = 20Duration (s) 01248163264 Number of Pulses 0 5 10 15 20 25 30 Positive Bipolar Negative Bipolar Positive Unipolar Negative Unipolar Figure 4-107. Histogram of occurrence of pulses of different total duration and polarity in flash 07/17/06_565.

PAGE 121

121 Figure 4-108. A 100 ms time-window of el ectric field record of flash 07/21/06_1013.

PAGE 122

122 N = 19 NL = 3 NM = 6 NS = 10 NVS = 0Time (ms) -20-1001020304050 Number of Pulses 0 2 4 6 8 10 12 14 16 Large Medium Small Figure 4-109. Occurrence of pul ses of different amplitude in different parts of flash 07/21/06_1013. N = 19 N(<1 s) = 2 N(1-4 s) = 16 N(>4 s) = 1Time (ms) -20-1001020304050 Number of Pulses 0 2 4 6 8 10 12 14 16 Submicrosecond 14 microseconds Above 4 microseconds Figure 4-110. Occurrence of pulse s of different total duration in different parts of flash 07/21/06_1013.

PAGE 123

123 N = 19 NBP = 17 NUP = 2Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 2 4 6 8 10 12 Positive Bipolar Negative Bipolar Negative Unipolar Figure 4-111. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/21/06_1013. N = 19 NBP = 17 NUP = 2Duration ( s) 01248163264 Number of Pulses 0 2 4 6 8 10 12 Positive Bipolar Negative Bipolar Negative Unipolar Figure 4-112. Histogram of occurrence of pulses of different total duration and polarity in flash 07/21/06_1013.

PAGE 124

124 Figure 4-113. A 100 ms time-window of el ectric field record of flash 07/21/06_1015.

PAGE 125

125 N = 23 NL = 5 NM = 6 NS = 12 NVS = 0Time (ms) -100102030405060708090100 Number of Pulses 0 2 4 6 8 10 Large Medium Small Figure 4-114. Occurrence of pul ses of different amplitude in different parts of flash 07/21/06_1015. N = 23 N(<1 s) = 3 N(1-4 s) = 18 N(>4 s) = 2Time (ms) -100102030405060708090100 Number of Pulses 0 2 4 6 8 10 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-115. Occurrence of pulse s of different total duration in different parts of flash 07/21/06_1015.

PAGE 126

126 N = 23 NBP = 13 NUP = 10Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 2 4 6 8 10 12 14 Positive Bipolar Negative Bipolar Negative Unipolar Figure 4-116. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/21/06_1015. N = 23 NBP = 13 NUP = 10Duration (s) 01248163264 Number of Pulses 0 2 4 6 8 10 12 Positive Bipolar Negative Bipolar Negative Unipolar Figure 4-117. Histogram of occurrence of pulses of different total duration and polarity in flash 07/21/06_1015.

PAGE 127

127 Figure 4-118. A 100 ms time-window of el ectric field record of flash 07/21/06_1018.

PAGE 128

128 N = 24 NL = 4 NM = 4 NS = 16 NVS = 0Time (ms) -100102030405060708090100 Number of Pulses 0 2 4 6 8 10 12 14 16 Large Medium Small Figure 4-119. Occurrence of pul ses of different amplitude in different parts of flash 07/21/06_1018. N = 24 N(<1 s) = 1 N(1-4 s) = 20 N(>4 s) = 3Time (ms) -100102030405060708090100 Number of Pulses 0 2 4 6 8 10 12 14 16 Submicrosecond 1-4 microseconds Above 4 microseconds Figure 4-120. Occurrence of pulse s of different total duration in different parts of flash 07/21/06_1018.

PAGE 129

129 N = 24 NBP = 17 NUP = 7Normalized Amplitude 0.000.250.500.751.00 Number of Pulses 0 2 4 6 8 10 12 14 16 18 Positive Bipolar Negative Bipolar Negative Unipolar Figure 4-121. Histogram of occurrence of pulses of different amplitude and polarity in flash 07/21/06_1018. N = 24 NBP = 17 NUP = 7Duration (s) 01248163264 Number of Pulses 0 2 4 6 8 10 12 14 Positive Bipolar Negative Bipolar Negative Unipolar Figure 4-122. Histogram of occurrence of pulses of different total duration and polarity in flash 07/21/06_1018.

PAGE 130

130 4.2 Narrow Bipolar Pulses This section presents a description of the so-called narrow bipolar pulses (NBPs) which often appear temporally isolated from any other discharge activity. Note that a necessary feature of the narrow bipolar pulses disc ussed earlier in Section 2.4 is strong HF-VHF radiation. Narrow bipolar pulses, although without simultaneous HF-VHF records, are described next. 4.2.1 Data Summary The data used in this section were acquire d in the months of Ma y and July, 2006 at the EMS in Gainesville, Florida. The dataset which c onsists of eight NBPs is characterized in Table 4-4. The first six digits of the flash identifica tion number represent the date on which the record was acquired and all the following digits give the record number of a particular flash. 4.2.2 Methodology Narrow bipolar pulses with durations of the order of a few tens of microseconds, temporally isolated from any other li ghtning discharge activ ity by at least 500 s and showing a relatively smooth initial rise to peak are included in this analysis. The electric field record of each NBP was examined to determine the polarity of initial half-cycle, the 10-90% risetime, total pulse duration including overshoot, total width of the initial half -cycle, the half-peak width of initial half-cycle, and the ratio of initial peak to opposite polarity overshoot. It should be noted that, since a positive voltage level at the osci lloscope corresponding to positive (atmospheric electricity sign convention) electric field change was used as trig ger, the experimental setup was biased towards recording NBPs having a positiv e dominant polarity and a negative polarity overshoot. Due to this limitation all the NBPs in this dataset have a dominant positive polarity and occurrence of NPBs with a dominant negative polarity might not have been recorded by the measuring system.

PAGE 131

131 Table 4-4. Characterization of electric field records of narrow bipolar pulses. Storm ID (mm/dd/yy) Number of records Event ID Sampling Interval, ns Record Length, ms 05/24/06 1 05/24/06_543 10 200 05/28/06_1146 05/28/06_1149 05/28/06_1150 05/28/06 4 05/28/06_1154 4 96 07/17/06 1 07/15/06_115 10 200 07/27/06 1 07/17/06_1874 10 200 07/28/06 1 07/18/06_1937 10 200 All data combined 8 4 or 10 96 or 200 4.2.3 Analysis Table 4-5 summarizes the electric field charact eristics of each of the eight NBPs. Figure 4123 shows the electric field records of the NPBs in this dataset. Table 4-5. Summary of electric field char acteristics of each of the eight NBPs. Event ID Risetime (10-90%), s Total pulse duration including overshoot, s Total width of initial half-cycle, s Half-peak width of initial halfcycle, s Ratio of initial peak to opposite polarity overshoot 05/24/06_543 1.5 6.5 3.8 1.9 1.9 05/28/06_1146 0.81 12 3.4 1.8 3.8 05/28/06_1149 0.82 14 3.1 1.6 5.3 05/28/06_1150 0.88 12 2.6 1.5 3.8 05/28/06_1154 0.76 12 2.7 1.5 3.6 07/15/06_115 1.0 14 3.7 1.6 3.1 07/17/06_1874 0.39 11 2.8 1.4 3.0 07/18/06_1937 0.53 14 2.5 1.1 4.2 All data combined (GM values) 0.78 12 3.1 1.5 3.5

PAGE 132

132 Figure 4-123. Electric fiel d records of eight NBPs analyzed in this study. NBP Pulse activity following NBP Pulse activity preceding NBP (a)

PAGE 133

133 Figure 4-123. Continued NBP Pulse activity following NBP (b)

PAGE 134

134 Figure 4-123. Continued NBP Pulse activity preceding NBP (c)

PAGE 135

135 Figure 4-123. Continued (d) (e)

PAGE 136

136 Figure 4-123. Continued (f) (g)

PAGE 137

137 Figure 4-123. Continued 4.3 Leader Duration This section presents a description of th e data analyzed and methodology adopted to estimate the stepped-leader duration in negative cloud-to-ground flashes. 4.3.1 Data Summary The data used in this section were acq uired on July 15 and 17, 2006 at the EMS in Gainesville, Florida and are su mmarized in Table 4-6. 4.3.2 Methodology In this analysis it is assumed that the begi nning of stepped leader is marked by preliminary breakdown pulses (when these puls es are detectable). Thus, th e time interval between the beginning of the preliminary breakdown pulse train and the first return-stroke pulse in electric field record gives the du ration of stepped leader, which is usua lly of the order of a few tens of milliseconds (see for example Figure 4-124). A sim ilar approach to finding stepped-leader duration was used by Brook (1992) and Heavner et al. (2002). Only those pul se trains having at (h)

PAGE 138

138 least three distinct pulses with peak-to-peak amp litudes equal to or exceeding twice that of the average noise level were consid ered. Electric field records of a total of 325 negative cloud-toground discharges were examined out of whic h 59 (about 18%) were found to be showing pronounced preliminary breakdown pulse trains. These 59 negative cloud-to-ground flashes were used to obtain an estimate of stepped-leader duration based on the a pproach described above. Table 4-6. Summary of electric field records of cloud-to-ground flashes used to find the duration of the stepped leader. Storm ID (mm/dd/yy) Number of cloud-toground flashes examined Number of records containing preliminary breakdown pulse train 07/15/2006 8 7 07/17/2006 317 52 All data combined 325 59 4.3.3 Analysis The statistical distribution of stepped-lead er durations for 59 ne gative cloud-to-ground flashes is shown in Figure 4-125. Figure 4-124. Electric field record s howing the stepped-leader duration (TL).

PAGE 139

139 Leader Duration (ms) 0102030405060708090100110120 Occurrence 0 5 10 15 20 25 AM = 30 ms GM = 23 ms Min = 3.4 ms Max = 119 ms n = 59 Figure 4-125. Histogram of the stepped-le ader duration for individual flashes. 4.4 Ratio of Preliminary Breakdown to Firs t Return Stroke Electric Field Peaks In this section, a description of the data and analysis method adopted to find the ratio of the largest electric field peak of the preliminary breakdown pulse train and the corresponding first return-stroke pulse peak has been presented. 4.4.1 Data Summary The data used in this section were acq uired on July 15 and 17, 2006 at the EMS in Gainesville, Florida and are summar ized in Table 4-6. The electric field records of a total of 325 negative cloud-to-ground discharges were examined out of which 59 (about 18%) were found to be showing pronounced prelimin ary breakdown pulse trains.

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140 4.4.2 Methodology Preliminary breakdown pulse trains having at least three distin ct pulses with peak-to-peak amplitudes equal to or exceeding tw ice that of the average noise level were selected for this analysis. An example of a prel iminary breakdown pulse train follo wed by a return-stroke pulse is shown in Figure 4-127. The electr ic field peaks of the initia l half-cycle of the largest preliminary-breakdown pulse in a train and the corresponding firs t return-stroke pulse in the flash were measured and their ratio was found. 4.4.3 Analysis The statistical distribution of the ratios of preliminary breakdown to first return-stroke electric field peak found in the 59 flashes is shown in Figure 4-126. AM = 0.62 GM = 0.45 Min = 0.16 Max = 5.1 n = 59Preliminary Breakdown to First Return Stroke Field Peak Ratio 00.250.51248 Occurrence 0 5 10 15 20 25 30 Figure 4-126. Histogram of the ratio of preliminar y breakdown to first return stroke field peaks for individual flashes.

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141 Figure 4-127. Electric field peak s of the largest PB pulse (APB) and corresponding first return-stroke pulse (ARS) in flash 07/17/2006_77.

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142 4.5 Ratio of Subsequent to First Retu rn Stroke Electric Field Peaks This section presents a description of the data used to find the ratio of subsequent to first return stroke electric field peaks in multiple -stroke cloud-to-ground flashes and also discusses the methodology used for the purposes of the analyses performed. 4.5.1 Data Summary The data used in this section were acqui red on July 15, 16 and 17, 2006 at the EMS in Gainesville, Florida. The dataset which consis ts of 176 multiple-stroke negative cloud-to-ground flashes is summarized in Table 4-7. An example of electric field record of a multiple-stroke cloud-to-ground discharge in this dataset is shown in Figure 4-128. The first six digits of the flash identification number represent the date on which the record wa s acquired and all the following digits give the record number of a particular flash. 4.5.2 Methodology A total of 600 electric field records of lightning were collect ed during thunderstorms over the three days among which 373 records were f ound to be that of cloud-to-ground flashes and 176 among them were found to contain multiple (m ore than one) return strokes within the individual record having a total length of 200 ms. It should be not ed that, since a positive voltage level at the oscilloscope corre sponding to positive (atmospheri c electricity sign convention) electric field change was used as trigger, the experimental setup was biased towards recording more cloud-to-ground flashes than cloud discharges which usually cause a ne gative electric field change. Hence, conclusions about the ratio of cloud and ground discharges should not be drawn from these data. Also, due to a record length of 200 ms and a post-trigger time of 80 ms, only one or few subsequent return strokes of multiple-stroke cloud-to-ground flashes could be recorded. Each of these 176 records was individually examined to measure the amplitude of the

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143 initial electric field peak in di gitizer units of each return stro ke in the record. The ratio of subsequent to first return st roke electric field peaks was found using two different methods. Table 4-7. Summary of electric field records used for finding the ratio of subsequent to first return stroke electric field peaks. Storm ID (mm/dd/yy) Number of records Number of records of cloudto-ground flashes Number of records containing multiple returnstrokes 07/15/06 28 8 4 07/16/06 2 2 0 07/17/06 570 317 172 All data combined 600 327 176 Table 4-8. Number of strokes of different order. Stroke order 1 2 3 4 Number of strokes 176 176 60 3 Figure 4-128. Multiple-stroke cloud-to-ground flash. a. Typical electric fiel d record with three return strokes (RS) of a multiple-stroke cl oud-to-ground flash. b. Electric field peak (EP1) of the first return stroke. c. Electric field peak (EP2) of the second return stroke. d. Electric field peak (EP3) of the third return stroke. (a)

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144 Figure 4-128. Continued (c) (b)

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145 Figure 4-128. Continued Method 1: The arithmetic mean of the electric field peaks of all th e subsequent return strokes in each flash was calculated. Then, ratio of the average subsequent return stroke field peak to the first return stroke field peak was f ound for each flash. Finally, the arithmetic mean of the ratio for all the flashes was calculated. Method 2: The ratio of the average subsequent re turn stroke field peak to the first return stroke field peak was found, as described in Met hod 1. Then, the geometric mean of the ratio for all the records was calculated. In addition to this, the arithmetic and geometri c means of the ratio of first to subsequent return stroke field peaks was calculated in or der to compare with the value the reported by Schulz et al. (2005). Also, for completeness, the arithmetic a nd geometric means of the ratio of the second return stroke field peak to the first return stroke fiel d peak for all records was also found using the two methods described above. (d)

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146 In order to compare the electric field peaks of strokes of diffe rent orders, the electric field peak for each stroke order in a particular flash was normalized with respect to the field peak of the first return-stroke in that flash. Next, for each stroke order, the geometric mean of the normalized field peaks in all flashes was calculated. 4.5.3 Analysis Table 4-9 summaries the values of the ratio of the subsequent to firs t return stroke field peaks obtained using the two met hods described in Section 4.5.2. In Table 4-10 the arithmetic and geometric mean values of the ratio of second to first return stroke field peaks are shown. The distribution of the ratio of the mean subsequent return stroke field peak to the first return stroke field peak is shown in Figure 4-129. Table 4-11 s hows the values of the ratio of the first to subsequent return st roke field peaks. The geometric mean of the normalized electric field peak for each stroke order is shown in Figure 4-130, which additionally also shows si milar data presented by Rakov and Uman (1990a), Diendorfer et. al (1998), a nd Schulz et al. (2005). Table 4-9. Arithmetic and geometric mean values of the ratio of subsequent to first return stroke field peaks. Parameter Arithmetic Mean (Method 1)Geometric Mean (Method 2) Ratio of subsequent to first return stroke field peak 0.75 0.59 Table 4-10. Arithmetic and geometric mean values of the ratio of second to first return stroke field peaks. Parameter Arithmetic Mean (Method 1) Geometric Mean (Method 2) Ratio of second to first return stroke field peaks 0.78 0.58

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147 Table 4-11. Arithmetic and geometric mean values of the ratio of first to s ubsequent return stroke field peaks. Parameter Arithmetic Mean (Method 1) Geometric Mean (Method 2) Ratio of first to subsequent return stroke field peaks 2.0 1.7 AM = 0.75 GM = 0.59 Min = 0.13 Max = 8.3 n = 176Subsequent to First Return Stroke Field Peak Ratio 00.5124816 Occurrence 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Figure 4-129. Histogram of the ratio of the mean s ubsequent return stroke field peak to the first return stroke field peak for individual flashes.

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148 Stroke Order 1234 Geometric Mean Field Peak (relative units) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Rakov and Uman [1990a] Diendorfer et al. [1998] Schulz et al. [2005] Present study Figure 4-130. Histogram showing the geometric mean of the normalized (to the field peak of the first return stroke in each multiple-stroke flash) field peaks for each stroke order found in the present study (yel low column). Also shown are data from other studies (black, red, and green columns). For othe r studies, the geometric mean (mean for Schulz et al. (2005)) field peaks of subs equent strokes are normalized to the geometric mean (mean for Schulz et al. (2005) ) field peak for first strokes (including single stroke flashes). 4.6 Attempted Leaders In this section, electric field pulse trains that are characteristic of preliminary breakdown in negative cloud-to-groun d discharges, but are not followe d by return-stroke waveforms are identified and examined. These events are he rein referred to as “attempted cloud-to-ground leaders”, although some of them are followed by fu ll-fledged cloud discharges. Note that in the latter case polarity of the initial half-cycle of preliminary-breakdown-type pulses are the same as that of negative return-stroke pulses and opposite to the polarity of pulses characteristic of the initial breakdown in cloud discharges.

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149 4.6.1 Data Summary The dataset contains 2475 electr ic field records of lightning di scharges that were acquired at the EMS in Gainesville, Flor ida (on May 24 and 28, June 1, 2 and 3, and July 15, 16 and 17) in 2006 out of which 33 were found to satisfy crit eria set for attempted cloud-to-ground leaders (see Section 4.6.2). Table 4-12 presents a characte rization of electric field records used in this study. 4.6.2 Methodology The preliminary breakdown process in ground fl ashes sometimes (in 18% of events in the data set of 325 negative cloud-to-ground discharges used in Section 4.3) produces a train of relatively large microsecond-scale electric field pulses. An example of pronounced preliminary breakdown pulse train in a cloud-to -ground flash containing at leas t five strokes is shown in Figure 4-131. The time interval between the puls e train and the return stroke waveform is typically several milliseconds or more (geometric mean of 23 ms as discussed in Section 4.3). The amplitude of the initial breakdown pulses can be comparable to or even exceed that of the first return-stroke pulse (as discussed in Section 4.4). Table 4-12. Characterization of electric field records acquired dur ing eight thunderstorms in Gainesville, Florida, in 2006. Storm ID (mm/dd/yy) Number of records Number o f attempted leaders (%) Sampling interval, ns Record length, ms Pretrigger/Posttrigger, ms/ms Number of attempted leaders for different pretrigger/ posttrigger settings (%) 05/24/06 1073 25 (2.3%) 10 200 80/120 25 (76%) 05/28/06 287 1 (0.4%) 4 96 40/56 06/01/06 21 0 (0%) 4 96 40/56 06/02/06 234 0 (0%) 4 96 40/56 06/03/06 260 3 (1.2%) 4 96 40/56 4 (12%) 07/15/06 27 0 (0%) 10 200 120/80 07/16/06 2 0 (0%) 10 200 120/80 07/17/06 571 4 (0.7%) 10 200 120/80 4 (12%) All data combined 2475 33 (1.3%) 4 or 10 96 or 200 40/56, 120/80 or 80/120 33 (100%)

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150 In this study, it is assumed that the beginning of stepped leader is marked by preliminary breakdown pulses (when these puls es are detectable). Thus, th e time interval between the beginning of the preliminary breakdown pulse train and the first return-stroke pulse in electric field record gives the du ration of stepped leader, which is usua lly of the order of a few tens of milliseconds. A similar approach to finding stepped-leader duration was used by Brook (1992) and Heavner et al. (2002). In Section 4.3 of this th esis it has been shown that only 12% (7 out of 59) of the leaders had durations greater than 60 ms (none of the leaders ha d duration in the range from 55 to 60 ms) and only about 5% (3 out of 59) of them had durations greater than 90 ms. Rakov and Uman (1990a) measured leader durat ions using their overall electric field waveforms and reported that the overwhelming majority of leaders initiating negative Figure 4-131. Electric field record of a negati ve cloud-to-ground lightning flash. a. A negative cloud-to-ground lightning flash showing a pronounced preliminary breakdown pulse train followed by five return-stroke pul ses. b. Pronounced preliminary breakdown pulse train and the first retu rn-stroke pulse of the cloud-to -ground flash shown in (a).

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151 Figure 4-131. Continued first strokes in Florida had durat ions in the range from 20 to 90 ms. According to their Figure 10a, only about 3% of Florida stepped leaders are expected to have dura tions longer than 90 ms. This result is consistent with the finding in Sec tion 4.3 of this thesis th at stepped leaders having durations longer than 90 ms are ve ry rare. Note that geometric mean first-stroke leader durations in Section 4.3 and Rakov and Uman’s studies we re 23 ms (59 events) and 35 ms (71 events), respectively. Based on the above, it is assumed that the leader duration is unlikely to exceed 90 ms. Accordingly, if a pulse train characteristic of preliminary breakdown in negative ground flashes is identified, but it is found not to be followed by return-s troke pulses within 90 ms after the beginning of the train, the event is classified as an attempted cloud-to-ground leader. This criterion could be applied to a significant fraction (1073 out of 2475 records) of the data set, for which the posttrigger time was 120 ms. However, when the records were not long enough to

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152 examine 90 ms after the beginning of the pre liminary breakdown pulse train, the assumed “maximum” leader duration (the time interval after the beginning of preliminary-breakdown pulse train, during which we searched for return -stroke waveforms) had to be reduced. Such reduced “maximum” leader durations were 80 or 56 ms. This should not introduce a significant error, since only about 12 to 18% of stepped leaders in Florida are expected to be longer than 55 ms, according to Rakov and Uman (1990a) and the anal ysis described in Section 4.3. Over threequarters (76%; see Table 4-12) of attempted leader s were identified in records with posttrigger time of 120 ms. None of the conclu sions of this study would change if records with 80 and 56 ms posttriggers were excluded. Figure 4-132. Electric field record illustrating definition of (a) overall preliminary-breakdown pulse train duration (TPB) and (b) pulse duration (TPW) and interpulse interval (TIP) for preliminary-breakdown pulses of an attempted leader.

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153 Figure 4-132. Continued The criteria for identification of preliminary breakdown pulse trains of attempted leaders are based on the characteristic features (describ ed in Section 2.2.2) of preliminary-breakdown pulse trains in negative cloud-to-ground flashe s. Definitions of the various pulse-train characteristics are illustrated in Figures 4-132a an d b. Only those pulse trains, which had at least three distinct pulses that satisfi ed the criteria and peak-to-peak amplitudes equal to or exceeding twice that of the average noise le vel, have been considered in th is study. Note that besides the characteristic pulses described above, preliminary breakdown pulse trains often contain other types of pulses, usually of considerably smalle r amplitude and duration (see, for example, pulses occurring between 1.3 and 1.4 ms in Figure 4132a and “narrow” pulses shown in Figure 4137b). These “non-characteristic” pulses were no t used for identification of preliminary breakdown pulse trains, but were included in the overall character ization of pulse trains and individual pulses pres ented in Section 4.6.3.

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154 4.6.3 Analysis Waveforms in 33 electric field records were found to satisfy the crite ria set for attempted cloud-to-ground leaders. These are classified in Table 4-13, depending upon the pulse activity (or lack of such) following the preliminary-brea kdown pulse train and presence (or absence) of electrostatic field ramp. Pulses were treated as followi ng the pulse train (as opposed to being part of the train) if they were separated from the last pulse of the train by at least 2 ms. Two of the 33 records each contained two trains of preliminarybreakdown-type pulses, so that the total number of pulse trains analyzed here was 35. Table 4-13. Classification of da ta according to pulse activity (or lack of such) following the preliminary breakdown (PB) pulse train and presence (or absence) of static ramp. Type Number of events without static ramp (Figure 4-133) 7 PB pulse train followed by no pulse activity with static ramp (Figure 4-134) 3 without static ramp (Figure 4-135) 19 PB pulse train followed by nonreturn-stroke-type pulses with static ramp (Figure 4-136) 4 The electric field associated with lightning discharges can be viewed as composed of the electrostatic, induction, and radi ation field components. At larger distances (beyond several tens of kilometers) the radiation field component is the dominant one. Micr osecond-scale pulses can be viewed as being entirely due to this component. When the distance between the source and observer is relatively small (of the order some k ilometers), the electrostatic field component can be dominant at later times. The millisecond-scale ramp seen in 7 of the 33 electric field records exhibiting preliminary-breakdowntype pulses analyzed here is due to this electrostatic component.

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155 Figure 4-133. Electric field record of an attempte d leader. a. An attempted leader with no pulse activity following the preliminary breakdow n pulse train. b. Preliminary breakdownlike pulses of the attempted leader shown in (a). Figure 4-133. Continued

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156 Figure 4-134. Electric field record of an atte mpted leader. a. An attempted leader whose preliminary breakdown pulse train is follo wed by static ramp but no other pulse activity. b. Preliminary breakdown pulses of the attempted leader shown in (a). Figure 4-134. Continued

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157 Figure 4-135. Electric field record of an atte mpted leader. a. An attempted leader whose preliminary breakdown pulse train is fo llowed by non-return-stroke-type pulses (see inset) without static ramp. b. Preliminary breakdown pulses of the attempted leader shown in (a). Figure 4-135. Continued

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158 Figure 4-136. Electric field record of an atte mpted leader. a. An attempted leader whose preliminary breakdown pulse train is fo llowed by non-return-stroke-type pulses (see inset) and static ramp. b. Preliminary breakdown pulses of the attempted leader shown in (a). Figure 4-136. Continued

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159 Figure 4-137. Pulses in a preliminary-breakdown pul se train of an attempted leader. a. A typical “classical” pulse. b. Typical “narrow” pulses. Figure 4-137. Continued

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160 Preliminary-breakdown pulse trains of atte mpted cloud-to-ground leaders examined here typically contained two types of pul ses, larger “classical” pulses with durations of the order of tens of microseconds (see Figure 4-137a) and “na rrow” pulses whose durations were as short as a few microseconds, with many being in the 1 to 2 s range (see Figure 4-137b). Smaller and narrower pulses tended to occur at the onset and towards the end of each pulse train. Similar narrow and often low amplitude pulses in “norma l” preliminary-breakdown pulse trains have been observed in negative cloud-to-ground and cloud discharges as discussed in Section 5.1 of this thesis. A total of 35 preliminary-breakdown pulse trains were found in 33 electr ic field records of attempted leaders, that is, th ere were two records each contai ning two distinct pulse trains. Histogram of the total pulsetrain duration is shown in Fi gure 4-138. Figures 4-139 and 4-140 show ranges of variation (vertical bars) of pulse duration and interpulse interval in individual pulse trains, respectively. Figure 4-138. Histogram of preliminary-breakdown pulse-train duration for attempted leaders. Note that a total of 35 preliminary-breakdow n pulse trains were found in 33 electric field records.

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161 Figure 4-139. Ranges of variati on (vertical bars) and mean valu es (diamonds) of pulse duration in individual preliminary breakdown pulse trains. See also caption of 4-138. Figure 4-140. Ranges of variati on (vertical bars) and mean valu es (diamonds) of interpulse interval in individual prel iminary breakdown pulse trains. See also caption of 4-138.

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162 4.7 Positive Cloud-to-Ground Lightning This section presents a description of various characteristic features of three positive cloudto-ground discharges recorded in th e course of this study. Clearly, the sample size is limited and, therefore, the results should be viewed as preliminary. 4.7.1 Data Summary Three positive cloud-to-ground discharges were recorded at the EMS in Gainesville, Florida during a storm on July 17, 2006. Figures 4141 to 4-143 show the elec tric field records of the three flashes, each of which contained a single positive return-stroke pulse. The first six digits of the flash identification number represen t the date on which the record was acquired and all the following digits give the reco rd number of a particular flash. 4.7.2 Methodology Figures 4-144 to 4-146 show th e three return-stroke pulses from the flashes shown in Figures 4-141 to 4-143, respectively, over shorte r time-windows. The electric field waveform of each positive return-stroke pulse is seen to be co mposed of two distinct portions, an initial slow rising portion, often called the slow front, and a portion showing a fast rise to peak, often called fast transition. 4.7.3 Analysis The zero-to-peak risetime of each return-stroke pulse, 10-90% risetime of each returnstroke pulse, 10-90% risetime of fast transition, and the slow fr ont duration and amplitude were measured. Various parameters of the microsec ond-scale electric field waveforms produced by the three positive return-strokes are described in Table 4-14.

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163 Table 4-14. Parameters of microsecond-scale el ectric field waveforms produced by the three positive cloud-to-ground discharges. Flash ID Zero-to-peak risetime, s 10-90% risetime, s Slow front duration, s 10-90% risetime for fast transition, ns Slow front amplitude relative to peak, % 07/17/06_567 6.7 3.7 5.3 920 28 07/17/06_568 11 4.7 9.9 1160 19 07/17/06_583 8.1 6.5 6.2 1120 12 Geometric Mean 8.4 4.8 6.9 1061 19 Figure 4-141. A 90 ms portion of showing the electric field record containing a positive cloud-toground discharge 07/17/06_567.

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164 Figure 4-142. A 200 ms portion of showing the el ectric field record c ontaining a positive cloudto-ground discharge 07/17/06_568. Figure 4-143. A 140 ms portion of showing the electric field reco rd showing a positive cloud-toground discharge 07/17/06_583.

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165 Figure 4-144. Slow front (ES1) and fast transition (EF1) of the return stroke waveform of positive cloud-to-ground discharge 07/17/06_567. Figure 4-145. Slow front (ES2) and fast transition (EF2) of the return stroke waveform of positive cloud-to-ground discharge 07/17/06_568. ES2EF2 ES1 EF1 10 s

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166 Figure 4-146. Slow front (ES3) and fast transition (EF3) of the return stroke waveform of positive cloud-to-ground discharge 07/17/06_583. EF3ES3 20 s

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167 CHAPTER 5 DISCUSSION This chapter presents a detailed discussion of the various analyses pe rformed in Chapter 4. Each section in this chapter contains discussi ons relevant to the co rresponding section in Chapter 4. 5.1 Microsecondand Submicrosecond-Scale Pulses Results obtained from the an alyses of microsecondand su bmicrosecond-scale pulses in cloud-to-ground and cloud discharges are discussed in Sections 5.1.1 and 5.1.2, respectively. 5.1.1 Analysis of Pulses in Cloud-to-Ground Discharges Table 5-1 summarizes results ob tained from the histograms in Figures 4-2 to 4-61. In addition to “classical” preliminary breakdown pulse s (e.g. Figure 5-1) having durations of the order of tens of microseconds, pr eviously unreported “narrow” pulses with durations less than 4 s (e.g. Figure 5-2) are also seen. Typically, th e majority of pulses in a flash are found to be small or very small in amplitude and have durations less than 4 s. It is to be noted that the upper limit for the duration of “narrow” pulses is somewhat arbitrarily chosen to be 4 s in order to indicate that their durations are at least an order of magnitude smaller than that of the “classical” (previously documented) preliminary breakdown pulses. From the occurrence of pulses on the time-scale for each of the 12 ground discharges analyzed in Section 4.1.1, it can be seen that typically, the overwhelming majority of the pulses in each cloud-to-ground discharge examined here are associated with the preliminary breakdown pulse train occurring tens of milliseconds before th e first return-stroke of the flash and lasting for a few milliseconds. 57 to 98% of the pulses in the 12 flashes belong to the small and very small categories of pulse amplitude. 22 to 89% of the pul ses occurring in the flashes have durations

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168 Figure 5-1. “Classical” preliminary breakdown pulse from flash 05/24/06_1078. less than 4 s. Figure 5-3 shows the distribution of the total duration of pulses in all the 12 flashes taken together. 78% (553 out of 706) of th e pulses have durations le ss than 4 s, of which 87% (479 out of 553) are bipolar. From Table 5-1 it can be seen that a significan t fraction of the small and very small pulses and pulses having durations less than 4 s are bipolar in nature. Hence, conclusions drawn from this analysis about the amplitude and durati on of preliminary breakdown pulses in cloud-toground discharges would remain unchanged if the apparently unipolar pulses were not included in the analysis. Next, the existence of a possible relationship between the amplitude and duration of pulses in individual flashes is examined. For this purpos e a sample of four flashes is randomly chosen

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169 from the twelve cloud-to-ground flashes analyzed he re. Figure 5-4 shows th e scatter plots for the normalized amplitude versus duration of pulses along with the regression lines and linear regression equations. It can be seen from the plots that a moderate linear correlation exists between the amplitude and duration of pulses. Hence, from the above analysis it can be conc luded that the majority of pulses associated with the preliminary breakdown pulse train in a cloud-to-ground discharge that occurs a few to tens of milliseconds prior to the first return stroke in a flash are small in amplitude and duration, the durations being an order of magnitude sm aller than those of “cl assical” preliminary breakdown pulses. Figure 5-2. “Narrow” pulses from flashes (a) 05/24/06_1078 and (b), (c) 05/24/06_224.

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170 Figure 5-2. Continued

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171 Table 5-1. Summary of occurrence of smaller a nd narrower pulses observed in the 12 selected cloud-to-ground discharges. Number of pulses in the Small and Very Small categories (%) Number of pulses having durations less than 4 s (%) Flash ID Total Number of Pulses Bipolar Unipolar Total Bipolar Unipolar Total 05/24/06_224 169 141 (83) 23 (14) 164 (97) 127 (75) 22 (13) 149 (88) 05/24/06_228 27 19 (70) 6 (22) 25 (93) 7 (26) 4 (15) 11 (41) 05/24/06_1078 97 83 (86) 12 (12) 95 (98) 70 (72) 12 (12) 82 (85) 05/28/06_1152 23 13 (57) 0 (0) 13 (57) 5 (22) 0 5 (22) 05/28/06_1360 41 35 (85) 1 (2.4) 36 (88) 33 (80) 1 (2.4) 34 (83) 06/01/06_21 44 38 (86) 2 (4.6) 40 (91) 29 (66) 2 (4.6) 31 (71) 06/02/06_120 25 19 (76) 2 (8.0) 21 (84) 8 (32) 1 (4.0) 9 (36) 06/02/06_139 72 58 (81) 6 (8) 64 (89) 58 (81) 6 (8.3) 64 (89) 06/02/06_207 48 36 (75) 6 (13) 42 (88) 34 (71) 6 (13) 40 (83) 06/02/06_212 65 48 (74) 9 (14) 57 (88) 43 (66) 9 (14) 52 (80) 07/15/06_23 73 58 (79) 11 (15) 69 (95) 52 (71) 9 (12) 61 (84) 07/17/06_54 22 14 (64) 2 (9.1) 16 (73) 13 (59) 2 (9.1) 15 (68) Total 706 562 (80) 80 (11) 642 (91) 479 (68) 74 (10) 553 (78)

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172 N = 706 NBP= 626 NUP= 80Duration ( s) 01248163264 Number of Pulses 0 50 100 150 200 250 300 350 Bipolar Unipolar Figure 5-3. Histogram of total durat ion of unipolar and bipolar pulses. Figure 5-4. Scatter plots of normalized amplitude versus duration of pulses in flashes (a) 05/24/06_224 (b) 07/15/06_23 (c) 05/28/06_1360 and (d) 06/01/06_21.

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173 Figure 5-4. Continued

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174 Figure 5-4. Continued 5.1.2 Analysis of Pulses in Cloud Discharges This section contains discussion relevant to the analysis of microsecondand submicrosecond-scale pulses in cloud discharg es presented in Section 4.1.2. Table 5-2 summarizes the results obtained from the histog rams in Figures 4-63 to 4-122. In addition to “classical” initial breakdown pulses (e.g. Figure 55) having durations of the order of tens of microseconds, “narrow” pulses with durations less than 4 s (e .g. Figure 5-6) are also seen. Typically, the majority of the pulses in a flash ar e found to be small or very small in amplitude and have durations less than 4 s. It is to be noted that the upper li mit for the duration of “narrow” pulses is somewhat arbitrarily chosen to be 4 s in order to indi cate that their durations are at least an order of magnitude smaller than that of the “classical” (previously documented) initial breakdown pulses. It is to be noted that the time window exam ined for pulse activity before and after the trigger pulse (the largest pulse in a flash) is no t the same for all flashes as shown in Table 5-2.

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175 Figure 5-5. “Classical” preliminary br eakdown pulse from flash 05/24/06_49. However, none of the pulse characteristics desc ribed in this study and conclusions drawn depend upon the record length examined. 52 to 99% of the pulses in the 12 flashes belong to the small and very small categories of pulse amplitude. 73 to 95% of the pulses occurring in the flashes have durations less than 4 s. Fi gure 5-7 shows the dist ribution of the total duration of pulses in all the 12 flashes taken together . 85% (1125 out of 1323) of the pul ses have durations less than 4 s in which 70% (783 out of 1125) are bipolar. From Table 5-3 it can be seen that a significan t majority of the small and very small pulses and pulses having durations less than 4 s are bipolar in nature. Hence, conclusions drawn from this analysis about the amplitude and durati on of pulses in cloud discharges would remain unchanged if the apparently unipolar pulse s were not included in the analysis.

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176 Next, the existence of a possi ble relationship between amplit ude and duration of pulses in individual flashes is examined. For this purpose a sample of four flashes is randomly chosen from the twelve cloud flashes analyzed here. Figure 5-8 shows the scatter plots for the normalized amplitude versus duration of pulses along with the regression lines and linear regression equations. It can be seen from the plots that a moderate linear correlation exists between the amplitude and duration of pulses. Hence, from the above analysis it can be conclu ded that the majority of pulses in the active or initial stage of a cloud disc harge are small in amplitude and duration, the durations being an order of magnitude smaller than those of the “classical” initial breakdown pulses. Figure 5-6. “Narrow” pulses from flashes (a) 07/17/06_555 (b) 05/24/06_57 (c) 05/24/06_226 and (d). 05/24/06_57 (a)

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177 Figure 5-6. Continued (d) (c) (b)

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178 Table 5-2. Summary of length of record examined before and after the trigger pulse and total number of pulses in each of the 12 selected cloud discharges. Flash ID t1 (t<0) t2 (t>0) t = t1 + t2 N 05/24/06_49 80.0 120 200 105 05/24/06_52 97.6 102.4 200 129 05/24/06_54 80.0 120 200 187 05/24/06_57 81.0 119 200 152 05/24/06_226 112.7 87.3 200 183 05/24/06_299 65.7 134.3 200 185 07/17/06_555 130.8 69.2 200 124 07/17/06_559 118.1 81.9 200 143 07/17/06_565 91.6 108.4 200 49 07/21/06_1013 120.0 80 200 19 07/21/06_1015 113.8 86.2 200 23 07/21/06_1018 118.7 81.3 200 24 N = 1323 NBP = 945 NUP = 378Duration ( s) 01248163264128264 Number of Pulses 0 50 100 150 200 250 300 350 400 450 500 550 Bipolar Unipolar Figure 5-7. Histogram of total durat ion of unipolar and bipolar pulses.

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179 Table 5-3. Summary of occurrence of smalle r and narrower pulses in the selected 12 cloud discharges. Number of pulses in the Small and Very Small categories (%) Number of pulses having durations less than 4 s (%) Flash ID Total Number of Pulses Bipolar Unipolar Total Bipolar Unipolar Total 05/24/06_49 105 85 (81) 19 (18) 104 (99) 70 (67) 17 (16) 87 (83) 05/24/06_52 129 110 (85) 15 (12) 125 (97) 85 (66) 13 (10) 98 (76) 05/24/06_54 187 135 (72) 51 (27) 186 (99) 122 (65) 47 (25) 169 (90) 05/24/06_57 152 121 (80) 29 (19) 150 (99) 100 (66) 28 (18) 128 (84) 05/24/06_226 183 119 (65) 62 (34) 181 (99) 108 (59) 61 (33) 169 (92) 05/24/06_299 185 115 (62) 62 (34) 177 (96) 88 (48) 47 (25) 135 (73) 07/17/06_555 124 74 (60) 45 (36) 119 (96) 70 (56) 42 (34) 112 (90) 07/17/06_559 143 84 (59) 54 (38) 138 (97) 70 (49) 51 (36) 121 (85) 07/17/06_565 49 21 (43) 17 (35) 38 (78) 27 (55) 19 (39) 46 (94) 07/21/06_1013 19 8 (42) 2 (11) 10 (53) 16 (84) 2 (11) 18 (95) 07/21/06_1015 23 7 (30) 5 (21) 12 (52) 13 (57) 8 (35) 21 (91) 07/21/06_1018 24 11 (46) 5 (21) 16 (76) 14 (59) 7 (29) 21 (88) Total 1323 890 (67) 366 (28) 1266 (96) 783 (59) 342 (26) 1125 (85)

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180 Figure 5-8. Scatter plots of normalized amplitude versus duration of pulses in flashes (a) 05/24/06_226 (b) 05/24/06_299 (c) 07/17/06_559 and (d) 07/17/06_565. (b) (a)

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181 Figure 5-8. Continued (c) (d)

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182 5.2 Narrow Bipolar Pulses This section contains discussion of the analys is of narrow bipolar pulses (NBPs) performed in Section 4.2. Figure 4-123 and Table 5-4 show that while five of the NBPs are isolated in time from any other discharge activity by at least 40 ms (maximum pret rigger time for some records), three of them are accompanied by some other pulse activity. Table 5-4. Summary of other pu lse activities accompanying NBPs. Event ID Time-length of record prior to NBP, ms Pulse activity preceding NBP (time interval, if any) Time-length of record following NBP, ms Pulse activity following NBP (time interval, if any) 05/24/06_543 97 Yes (8.7 ms) 103 Yes (697 s) 05/28/06_1146 40 No 56 Yes (15 ms) 05/28/06_1149 40 Yes (4.1 ms) 56 No 05/28/06_1150 120 No 80 No 05/28/06_1154 40 No 56 No 07/15/06_115 40 No 56 No 07/17/06_1874 120 No 80 No 07/18/06_1937 120 No 80 No All eight NPBs have positive initial half-cycle polarity. However, as discussed in Section 4.2.2, this may be due to the use of a positive voltage level at the osci lloscope corresponding to positive (atmospheric electricity sign convention) electric field change as trigger at the EMS because of which the occurrence of NPBs with a dominant negative polarity might not have been recorded by the measuring system. The geomet ric mean 10-90% risetime is 780 ns with minimum and maximum values of 390 ns and 1.5 s respectively. The mean total pulse duration including overshoot is 12 s with minimum and maximum values of 6.5 s and 14 s, respectively. The ratio of the in itial peak to opposite polarity ov ershoot, which varies within a range of 1.9 to 5.3, has a mean value of 3.5. Electric field characteristics of NBPs reported by different researches are discusse d in Section 2.4. Rakov et al. ( 1996) reported the typical total

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183 pulse durations of NBPs to be in the range of 10-20 s which is similar to that found in this study. However, as the sample size is limited, results obtained in th e present study should be viewed as preliminary. 5.3 Leader Duration This section contains discussion relevant to Section 4.3 of this thesis in which the duration of the stepped leader has been estimated. The arithmetic and geometric mean stepped-leader durations are 30 ms and 23 ms, respectively. The maximum and minimum stepped-leader durations are 3.4 ms and 119 ms, respectively. Fr om the histogram in Figure 4-125 we see that only 12% (7 of 59) of the steppe d leaders had durations greater th an 60 ms and only about 5% (3 of 59) of them had durations greater than 90 ms. Rakov and Uman (1990b) measured leader durat ions using their ove rall electric field waveforms and reported that the overwhelming ma jority of leaders initiating negative first strokes in Florida had durations in the range from 20 to 90 ms. According to their Figure 10a, only about 3% of Florida stepped leaders are expected to have dur ations longer than 90 ms. This result is consistent with the result obtained from the present study that stepped leaders having durations longer than 90 ms are ve ry rare. Note that geometric mean first-stroke leader durations in Rakov and Uman’s studies was 35 ms (71 events ). Also, interestingly, our geometric mean of 23 ms is equal to the median time interval be tween the preliminary br eakdown pulse train and the first stroke reported by Schulz and Diendorfe r (2006) for 92 negative multiple-stroke flashes in Austria. 5.4 Ratio of Preliminary Breakdown to Fi rst Return-Stroke El ectric Field Peaks This section contains discussion of the ratio of the largest elect ric field peak of preliminary breakdown pulse train to that of the corresponding first return str oke analyzed in Section 4.5. The geometric and arithmetic means of the ratio are 0.45 and 0.62, respectively, with minimum

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184 and maximum values being 0.16 and 5.1, respec tively. From the histogram in Figure 4-127, it can be seen that 11 (about 19%) of the 59 preliminary breakdown pul se trains have electric field peaks that are greater than thos e of the corresponding first return strokes. Also, 21 (about 36%) of the 59 preliminary breakdown pulse trains have electric field peaks that are comparable to (50% or larger) that of the corresponding firs t return-stroke pulse. The preliminary breakdown pulses have the same polarity as return-stroke pulses in negative cloud-to-ground flashes, and durations of these two type s of pulses are comparable (tens of microseconds). This has important implications for lightni ng locating systems such as the U.S. National Lightning Detection Network (NLDN) which can misinterpret the electric field peak of the preliminary breakdown pulse train as that of the first return stroke of the flash. 5.5 Ratio of Subsequent to First Re turn Stroke Electric Field Peaks This section contains discussion of the analysis of first and su bsequent return stroke field peaks performed in Section 4.5. The ratio of the s ubsequent to first return stroke electric field peaks has been found to be equal to 0.59 when the geometric mean of the ratio for all the flashes is considered. The minimum and the maximum valu es of the ratio for this dataset are 0.13 and 8.3, respectively. The arithmetic mean ratio of the subsequent to the first return stroke electric field peaks has been found to be equal to 0.75. The arithmetic and geomet ric means of the ratio of the first to subsequent return stroke fiel d peaks are 2.0 and 1.7, respectively. Hence, on average, the electric field peak of the first stroke is roughly 2 time s larger than the field peak of the subsequent stroke. According to Rakov and Uman (2003), the electric fi eld peak of the first return stroke is approximately 2 to 3 times larger than that of the subsequent stroke. Also, Berger et al. (1975) found the median return -stroke current peak for first strokes in Switzerland to be 2.5 times larger than that for subsequent strokes. Diendorfer et al. (1998) examined return strokes from multiple-stroke flashes recorded by the Au strian Lightning Detection Network (ALDIS)

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185 and found the value of the field peaks of the firs t and subsequent strokes to be approximately equal. Using an independent electric field measur ement system in Austri a, Schulz et al. (2005) found the mean ratio of the first to the subsequent return stroke electric field peaks for multiplestroke flashes to be equal to 1.3. More recently, Saba et al. (2006) using data from the Brazilian Lightning Detection Network (RINDAT) found the m ean peak current of 193 subsequent return strokes (13.5 kA) to be 0.48 times (about 50% of) th e mean peak current of 55 first return strokes (28.3 kA). Table 5-5 summarizes the values of the subsequent to first stroke electr ic field (or current) peak ratio reported by researchers working in diffe rent parts of the world. All but one (based on ALDIS data) of the geometric mean ratios lie between 0.4 and 0.6. Table 5-5. Summary of the subsequent to first str oke electric field (or curr ent) peak ratio reported in different studies. Study Location Arithmetic Mean (Method 1) Geometric Mean (Method 2) Number of Subsequent Strokes Rakov and Uman (1990a) Florida 0.49b 270 Thottappillil et al. (1992) Florida 0.42 199 Cooray and Perez (1994) Sweden 0.63 0.51 314 Cooray and Jayaratne (1994) Sri Lanka 0.55 0.43 284 Diendorfer et al. (1998) Austria 1.0b 53443 Saba et al. (2006) Brazil 0.48a 193 Present study Florida 0.75 0.59 239 a The value indicates the ratio of the means of the subsequent and first stroke current peaks (including single stroke flashes), not the mean of the ratio of subsequent to first stroke field peaks (in multiple stroke flashes), as in other studies in this column. b The value indicates the ratio of the geometric means of the subsequent and first stroke electri c field peaks (including single stroke flashes), not the geometric mean of the ratio of subsequent to first stroke field peaks (in multiple stroke flashes), as in other studies in this column.

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186 Table 5-6. Summary of multiple-stroke flash characteristics reported in different studies. Study Total number of flashes Percentage of flashes with at least one subsequent stroke field peak greater than the first Percentage of subsequent strokes with field peaks greater than the first Thottappillil et al. (1992) (Florida) 46 33 13 Cooray and Perez (1994) (Sweden) 276 24 15 Cooray and Jayaratne (1994) (Sri Lanka) 81 35 12 Diendorfer et al. (1998) (ALDIS, Austria) 15905 51 Present study* (Florida) 176 24 21 * In this study the maximum number of recorded strokes per flash was four, due to limited record length of 200 ms. In this study, 21% (49 out of 239) of the subs equent strokes were found to have field peaks greater than that of the first stroke. 24 % (42 out of 176) of the flashes were found to contain at least one subsequent stroke with field peak greater than that of the first stroke. Table 5-6 presents a summary of multiple-stroke flash characteristics in different parts of the world reported by researchers. It should be noted that in this st udy the maximum number of strokes per flash is four, due to limited record le ngth of 200 ms. Therefore, each flash in this data set might have more subsequent return strokes than actually recorded in the time window of 200 ms. Since higher order return-strokes are expected to have smaller peak fields (Rakov and Uman, 1990a), the ratio of the subsequent to first return stroke field p eaks found in this study should be viewed as upper bound (the actu al value can be lower). From Figure 4-129 it can be seen that 31 out of the 176 mean subsequent stroke field peaks are greater than the corresponding first stroke pe ak field. Thus, on average, 82% of the first stroke peak fields are greater th an the mean of the corresponding subsequent stroke field peaks, and 39% of the first stroke field peaks are at least twice the mean of the corresponding

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187 subsequent stroke field peak. Also, among the 176 flashes in this dataset, 34 (19%) were found to have second stroke field peak greater than that of the corre sponding first stroke, and 14 (8%) were found to have third stroke field peak greater than that of the corresponding first stroke. The geometric mean normalized electric field peak for each stroke order found in the present study and the geometric mean electric fiel d peak for each stroke order normalized to the geometric mean field peak of the first stroke reported by Rakov and Uman (1990a), Diendorfer et. al (1998), and Schulz et al. (2005) are shown in Figure 4130. The normalized field peaks for the subsequent strokes in the 1990 and 2006 Florid a data are found to be approximately equal. From the above discussion, it can be seen that lightning peak currents reported for subsequent return strokes by li ghtning locating systems such as the U.S. NLDN (e.g., Rakov and Uman, 2003) and the Austrian ALDIS (Diendorfer et al., 1998) are not much different from their first-stroke counterparts. Howeve r, some recent studies indicate that the ratio of first to subsequent stroke peaks can be close to 2 for lightning locating system reports (Saba et al., 2006). The reasons for the discrepancy are pres ently not known, but may include regional peculiarities of the lightning electromagnetic envi ronment. In the case that the actual ratio is close to 2 (as suggested by most studies empl oying continuous field records), the NLDN and ALDIS results could be due to poor detection of relatively small s ubsequent strokes, rejection of the first stroke by the waveform discrimination algorithm and acceptance of the second stroke as the first stroke, and misclassificat ion of a preliminary-breakdown pul se as the first return stroke (37% of the preliminary breakdow n pulse trains may contain pulse s that are comparable to or greater than the first return stroke, as discussed in Sections 4.4 and 5.4). 5.6 Attempted Leaders This section contains discussi on of the attempted cloud-to-leade rs analyzed in Section 4.6. Lightning events exhibiting pulse trains that are characteristic of preliminary breakdown in

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188 negative cloud-to-groun d discharges, but are not followed by return-stroke waveforms, are assumed to be manifestations of attempted cloud-to-ground leaders. Preliminary-breakdown pulse trains of a ttempted cloud-to-ground leaders typically contained two types of pulses, larger “classical” pulses with durations of the order of tens of microseconds and “narrow” pulses whose duratio ns were about a few microseconds, with many being in the 1 to 2 s range. Almost half (46%) of the pul se trains were found to have minimum pulse durations in the range of 1-2 s. Smaller and narrower pulses tended to occur at the onset and toward the end of each pulse train. In a ddition to bipolar pulses with positive (atmospheric electricity sign convention) initia l half-cycle, negative unipolar a nd negative (initial half-cycle) bipolar pulses were sometimes seen toward the e nd of the train. Characteristics of preliminarybreakdown pulse trains in attempted leaders can be summarized as follows: (1) The range of variation and arithmetic mean of total durations of pulse trains are 0.8-7.9 ms and 2.7 ms, respectively, with 74% of the pulse trains having total durations less than or equal to 3 ms (2) The range of variation and the weighted arithme tic mean of individual pulse durations are 1-91 s and 17 s, respectively. (3) The range of variati on and the weighted arithmetic mean of interpulse intervals are 1-530 s and 73 s, respectively. Temporally isolated short-duration (sub-millis econd) discharges were observed using the VHF Lightning Mapping Array by Kr ehbiel et al. (2003). These di scharges were described as occurring either as precursors of full-fledged lightning flashes or as “spatially limited or attempted breakdown events” and apparently we re more commonly associated with the upper negative charge region and convective surges in anomalous (inverted polar ity) storms. Maier et al. (1996) reported that 9.3% of their flashes detected by the VHF Lightning Detection and Ranging System (LDAR) at the Kennedy Space Cent er (KSC) had durations of less than 101 s,

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189 the minimum time between consecutive LDAR sources. Defer et al. (2001), using a VHF interferometric lightning mapper, observed a particular type of li ghtning flashes (15% of the total number of flashes imaged during a storm in Nebraska-Colorado on July 10, 1996) that were characterized by durations less th an 1 ms. These short-duration flashes were recorded in cells where high (50 dBz) reflectivity r eached high altitude (8 km above mean sea level) and vertical updraft velocity exceeded 10 m/s. At least some of the isolated short-duration discharges reported from VHF lightning mappi ng studies and reviewed above might be attempted cloud-toground leaders consider ed in this study. Since the preliminary-breakdown-type pulses co nsidered here have the same polarity as return-stroke pulses in negative cloud-to-ground flashes and typical durations of these two types of pulses are comparable (tens of microseconds), some of the attempte d cloud-to-ground leaders can be misclassified by the NLDN as low intensit y negative cloud-to-ground discharges. If it is assumed that about 25% of the 2475 records exam ined here were due to negative cloud-toground flashes, and that 25% of these cloud-to-gr ound flashes had peak currents equal to or less than 10 kA, the expected nu mber of low-intensity ( 10 kA) negative clou d-to-ground events will be 155. If the NLDN record ed all these 155 negative cloud-to-ground events plus all 35 attempted leaders (all assumed to have NLDN intensities 10 kA), about 18% of reported lowintensity negative cloud-to-ground flashe s would be misclassified events. 5.7 Positive Cloud-to-Ground Lightning This section contains discussion of the thre e positive cloud-to-ground discharges analyzed in Section 4.7 of this thesis. From the electric field records of the two flashes shown in Figures 4141 and 4-142 it can be seen that these positive cloud-to-ground discharges were preceded by incloud discharge activity. Also, the leaders of th e two positive return strokes shown in Figures 4145 and 4-146 display apparent stepping.

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190 The geometric mean zero-to-peak and 10-90% ri setimes for the return -stroke pulses are 8.4 s and 4.8 s respectively. Rust et al. (1981) a nd Cooray (1986) reported zero-to-peak risetimes in the range 4-10 s and 4-12 s respectively. Beasley et al. (1983) and Cooray (1986) found the 10-90% risetimes for positive return-stroke pulses to be in the range of 1.2-4.0 s and 3-9 s, respectively. In the present study, the slow fr ont whose geometric mean amplitude 19% of the overall return-stroke pulse peak has geometric mean duration of 6.9 s. Also, the geometric mean 10-90% risetime of fast transition is 1. 1 s. Cooray et. al ( 1986) reported the mean duration of the slow front (whose amplitude relative to the peak was 3-60%) and the 10-90% risetime of the fast transition to be in the range of 3-11 s a nd 400-800 ns respectively. From the above discussion it can be seen th at the electric field ch aracteristics of the three positive return strokes analyzed in this study are, in general, similar to those found in the li terature. However, as the sample size is limited the re sults presented here should be viewed as preliminary.

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191 CHAPTER 6 RECOMMENDATIONS FOR FUTURE RESEARCH Results of the present study i ndicate the presence of a large number of “narrow” pulses in both cloud and ground discharges. These pulses have durations that are an order of magnitude smaller than the tens of microseconds long “classi cal” pulses that are thought to be characteristic of initial breakdown. Further study needs to be undertaken in order to explain the physical mechanisms behind the occurrence of these “narrow” pulses, as well as behind other types of pulses examined here. A few modifications are recommended to the existing EMS in Gainesville, Florida in order to expand the scope of the experiment and help achieve the above objective. 1) GPS timing: The experimental setup which was used to acquire the data analyzed in this study did not include GPS timing. As a result, th e acquired data could not be correlated with those of the NLDN and other li ghtning detection systems. Having GPS timestamps for obtained data will enable estimation of distances and peak currents of return strokes, as well as evaluation of possible NLDN response to pr eliminary breakdown and narrow bipolar pulses. Also, this will help answer the question regard ing the magnitude of preliminary breakdown pulses relative to corresponding first return stroke pulses. Moreover, correlation between terrestrial gamma-ray flashes (TGFs), which are detected by satellites such as RHESSI and intracloud discharges (e.g. Stanley et. al, 2006; Inan et. al, 2006) such as th e so-called narrow bipolar pulses (a small sample was examined in this study), is of interest and can be examined using GPS timestamps. A GPS antenna and receiver card will soon be installed at the EMS. 2) HF/VHF radiation measurements: HF and VHF radiation associated with the microsecondand submicrosecond-scale pulses in both cloud and ground discharges, in cluding the so-called narrow bipolar pulses is of inte rest and may prove to be importa nt in better unders tanding of the

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192 physics behind lightning initiation. It is presently unknown if th ere exist narrow bipolar pulses that are not accompanied by significant HF radiati on. It is thought that HF/VHF radiation is associated with breakdown of virgin air, whil e wideband field pulses are due to current waves propagating along conducting channe ls. The author plans to set up an HF measurement station co-located with the EMS in Gainesville, Florida as a part of his Ph.D. research. 3) High Speed Video Records: Optical records obtained usi ng a high speed (1000 frames per second) video camera in correlation with field measurements will help identify channels to ground in both negative and positive cloud to ground lightning (a small sample of the latter was examined in this study). Optical images of positive (and bipolar) lightning ch annels are very rare. Further, high-speed video record can help evalua te the performance characteristics of the NLDN. 4) Opticomm Fiber Optic Links: Replacement of the deteriorating and frequently failing Nicolet Isobe 3000 fiber-optic li nk, which is presently being used at the EMS, by Opticomm MMV-120C fiber-optic link will in crease the reliability of the field measuring system. This replacement is planned to be made before the 2007 thunderstorm season.

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193 LIST OF REFERENCES Beasley, W.H., M.A. Uman, and P.L. Rustan ( 1982), Electric fields pr eceding cloud-to-ground lightning flashes, J. Geophys. Res., 87, 4883-4902. Beasley, W.H., M.A. Uman, D.M. Jordan, an d C. Ganesh (1983), Positive cloud to ground lighning return strokes, J. Geophys. Res., 88, 8475-8482. Berger, K., R.B. Anderson, and H. Kroninge r (1975), Parameters of lightning flashes, Electra 80: 223-37. Bodhika, J.P., M. Fernando, and V. Cooray (2006), Characteristics of cloud flashes inferred from measurements of electric fields over Sr i Lanka a tropical country, Proc. of 28th Intl. Conf. on Lightning Protection, Kanazawa, Japan, 249-252. Brook, M. (1992), Breakdown electri c fields in winter storms, Res. Lett. Atmos. Electr., 12, 4752. Cooray, V. (1986a), A novel method to identify the radiation fields produced by positive return strokes and their submicrosecond structure, J. Geophys. Res., 91, 7907-79011. Cooray, V. (1986b), Correction to “A novel method to identify the radiation fields produced by positive return strokes and thei r submicrosecond structure”, J. Geophys. Res., 91, 13,318. Cooray, V., and K.P.S.C. Jayara tne (1994), Characteristics of li ghtning flashes observed in Sri Lanka in the tropics, J. Geophys. Res., 99(D10), 21,051-21,056. Cooray, V., and H. Perez (1994) , Some features of lightning flashes observed in Sweden, J. Geophys. Res.,99(D5), 10,683-10,688. Clarence, N.D., and D.J. Malan (1957), Preliminary discharge processes in lightning flashes to ground, Q.J.R. Meteor. Soc., 83, 161-172. Defer, E., P. Blanchet, C. Thery, P. Laroch e, J. Dye, and M. Venticinque (2001), Lightning activity for the July 10, 1996, storm during the St ratosphere-Troposphere Experiment: Radiation, Aerosol, and Ozone-A (STERAO-A) experiment, J. Geophys. Res., 106(D10), 10151-10172. Diendorfer, G., W. Schulz, and V. A. Rakov (1998), Lightning Charac teristics based on data from the Austrian Lightning Locating System, IEEE Trans. on Electromagn. Compat., 40(4), 452-464. Gomes, C., V. Cooray, and C. Jayaratne (1998) , Comparison of preliminary breakdown pulses observed in Sweden and in Sri Lanka, J. Atmos. Solar-Terr. Phys., 60, 975-60,979. Griffiths, R.F. and C.T. Phelps, (1976a), The e ffects of air pressure a nd water vapour content on the propagation of positive corona streamers, a nd their implications to lightning initiation. Q. J. Roy. Meteor. Soc. 102: 419-426.

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194 Griffiths, R.F., and C.T. Phelps (1976b), A mo del of lightning initiation arising from positive corona streamer development. J. Geophys. Res., 31 : 3671-3676. Gurevich, A.V., L.M. Duncan, A.N. Karashtin, and K.P. Zybin (2003), “Radio emission of lightning initiation”, Phys. Lett. A , 312: 228-37. Gurevich, A.V., L.M. Duncan, Yu. V. Medvedev, and K.P Zybin (2002), “Radio emission due to simultaneous effect of runaway breakdown and extensive atmospheric showers” Phys. Lett. A , 301: 320-6. Gurevich, A.V., K.P. Zybin, and R.A. Roussel-Dupre (1999) , “Lightning initiation by simultaneous effect of runaway breakdown and cosmic ray showers”, Phys. Lett. A , 254: 79-87. Heavner, M. J., D.A. Smith, A. R. Jacobs on, and R. J. Sheldon (2002), LF/VLF and VHF lightning fast-stepped leader observations, J. of Geophys. Res. , 107 (D24), 4791, doi: 10.1029/2001JD001290. Inan, U. S., M. B. Cohen, R. K. Said, D. M. Smith, and L. I. Lopez (2006), Terrestrial gamma ray flashes and lightning discharges, Geophys. Res. Lett. , 33 , L18802, doi: 10.1029/2006GL027085. Jerauld, J.E (2003), A multiple station experiment to examine the close tromagnetic environment of natural and triggered lightning, masters thes is, University of Florida, Gainesville. Kitagawa, N. (1957), On the electr ic field change due to the leader processes and some of their discharge mechanism, Pap. Meteor. Geophys. (Tokyo), 7, 4147,424. Kitagawa, N., and M. Brook (1960), A comparis on of intracloud and cloud-to-ground lightning discharges, J. Geophys. Res., 65 , 1189-1201. Kitagawa, N., and M. Kobayashi (1959), Field ch anges and variations of luminosity due to lightning flashes, Recent Advances in Atmo spheric Electricity, Oxford: Pergamon, 485-501. Krehbiel, P., W. Rison, R.J. Thomas, T. Hamli n, and J. Harlin (2003), Lightning modes in thunderstorms, Eos. Trans. AGU, 84 (46), Fall Meet. Suppl., AE31A-06. Krider, E.P., and G.J. Radda (1975), Radiati on field waveforms produ ced by lightning stepped leaders, J. Geophys. Res., 80, 2653-2657. Le Vine, D.M. (1980), Sources of the strongest RF radiation from lightning, J. Geophys. Res., 85 , 4091-4095. Maier, L., C. Lennon, P. Krehbiel, and M. Ma ier (1996), Lightning as observed by a fourdimensional lightning location system at Kenne dy Space Center, Proc. of 10th Intl. Conf. on Atmospheric Electricity, Osaka, Japan.

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195 Nguyen, M.D., and S. Michnowski (1996), On the initiation of li ghtning discharge in a cloud 2. The lightning initiation on precipitati on particles, J. Geophys. Res., 101, 26,675-26,680. Pavlick, A., D.E. Crawford, and V.A. Rakov (2002), Characteristics of dist ant lightning electric fields, Proc. of 7th Intl. Conf. on Probalisti c Methods Applied to Power Systems (PMAPS), Naples, Italy. Rakov, V.A. (1999), Lightning electri c and magnetic fields. In Proc. 13th Int. Zurich Symp. on Electromagn. Compat., Zuri ch, Switzerland, 561-566. Rakov, V.A. (2006), Initiation of lightning in thunderclouds, Proc. of SPIE , Vol. 5975 , 5975-12. Rakov, V.A., and B.A. DeCarlo (2005), Lightning Initiation Mechanisms: A Review and New Data on Submicrosecond "Lightning Initiation Pulses ", Eos Trans. AGU , 86 (52), Fall Meeting Suppl., Abstract AE32A-06, 2005. Rakov, V.A., and M.A. Uman (1990a), Some prope rties of negative clo ud-to-ground lightning flashes versus stroke order, J. Geophys. Res ., 95 (D5), 5447-5453. Rakov, V.A., and M.A. Uman (1990b), Waveforms of first and subsequent leaders in negative lightning flashes, J. Geophys. Res., 95 (D10), 16,561-16577. Rakov, V.A., and M.A. Uman (2003), Lightning: P hysics and Effects, Cambridge University Press, Cambridge. Rakov, V.A., M.A. Uman, G.R. Hoffman, M.W. Mast ers, and M. Brook (1996), Bursts of Pulses in Lightning Electromagnetic Radiation: Observ ations and Implications for Lightning Test Standards, IEEE Trans. on EMC, 38 (2), 156-164. Rust, W.D., D.R. MacGorman, and R.T. Arno ld (1981), Positive cloud to ground lightning flashes in severe storms, Geophys. Res. Lett.,8, 791-794. Saba, M.M.F., O. Pinto Jr., and M.G. Ballarotti (2006), Relation between lig htning return stroke peak current and following continuing current, Geophys. Res. Lett. , 33 , L23807, doi: 10.1029/2006GL027455. Schoene, J.D. (2007), Direct and nearby lightning interaction with test power distribution lines, PhD dissertation, University of Florida, Gainesville. Schulz, W., and G. Diendorfer (2006), Flash multip licity and interstroke intervals in Austria, Proc. of 28th Intl. Conf. on Lightning Protection, Kanazawa, Japan. Schulz, W., B. Lackenbauer, H.Pichler, and G.Diendorfer (2005), LLS data and correlated continuous e-field measurements, In Proc. VIII Intl. Symp. on Lightning Protection, Sao Paulo, Brazil, 21-25 Nov., 2005, 383-386.

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197 BIOGRAPHICAL SKETCH Amitabh Nag was born December 1, 1982 in India. In June, 2005, he received a Bachelor of Technology in electronics and communication e ngineering from West Bengal University of Technology, Kolkata. In August 2005, he was ad mitted to the Department of Electrical and Computer Engineering at the University of Fl orida as a Ph.D. student. There he joined the Lightning Research Group as a research assi stant. He was responsible for running and maintaining a lightning electric fiel d measurement station in Gainesvi lle, Florida which is part of the International Center for Lightning Res earch and Testing. In December 2006, Mr. Nag presented a paper of which he is the first au thor at the Fall American Geophysical Union Meeting in San Francisco. An expanded version of that paper was submitt ed to the Journal of Geophysical Research – Atmospheres. He has also co-authored three other papers submitted to international conferences.