M. A. Uman

University of Florida, Gainesville, Florida, United States

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Publications (269)558.6 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: We present 63 high-speed video frames (108 kfps, 9.26 µs per frame) showing the development of the downward negative stepped leader in the initial stage of an altitude-triggered flash. The downward negative stepped leader initiated from the bottom of the triggering wire at a height of about 128 m above ground and, 553 µs later, it struck a lightning rod located at a distance of about 50 m from the launch tower. During the leader's development, electric-field-derivative pulses were detected associated with leader stepping. The interpulse intervals ranged from 3 to 27 µs with a mean value of 13 µs. Distinct segments of luminosity were observed ahead of the main leader channel that appear similar to space leaders were observed in the high speed video frames. A total of eight luminous segments were observed that were 1 m to 6 m in length, and were centered at distances from the main leader channel ranging from 3 m to 8 m. The new leader steps that appeared in the frames following the luminous segments were 5 m to 8 m in length. Two of the observed segments apparently never connected to the leader channel, and thus failed to produce a new leader step.
    Journal of Geophysical Research: Atmospheres. 04/2014;
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    ABSTRACT: [1] Dart-stepped-leader step formation in triggered lightning is documented with high-speed video recorded at 648 kilo-frames per second (1.16 µs exposure time, 380 ns dead time) and linear streak film with a temporal resolution of about 1 µs. Locally luminous points and segments of channel both separate and below the main descending leader tip were recorded on the high-speed video. Bidirectional leaders were imaged initiating at the locally luminous points below the main channel tip, points that remain stationary during the inter-step process. The average speed of five bidirectional leaders was 8.4x105 m/s upward and 4.8x105 m/s downward, assuming 1.5 µs between successive images. The main dart-stepped leader channel tip moved downward between steps. Leader steps extended below the bottom of the previous bidirectional leader. Processes that can be seen between steps on high-speed video are generally below the noise threshold of the streak film, which shows primarily the newly-formed steps.
    Geophysical Research Letters. 03/2014;
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    ABSTRACT: We present a detailed evaluation of performance characteristics of the U.S. National Lightning Detection Network (NLDN) using, as ground-truth, Florida rocket-triggered lightning data acquired in 2004–2012. The overall data set includes 78 flashes containing both the initial stage and leader/return stroke sequences and 2 flashes composed of the initial stage only. In these 80 flashes, there are a total of 326 return strokes (directly-measured channel-base currents are available for 290 of them) and 173 kiloampere-scale (≥1 kA) superimposed pulses, including 58 initial continuous current pulses and 115 M-components. All these events transported negative charge to ground. The NLDN detected 245 return strokes and 9 superimposed pulses. The resultant NLDN flash detection efficiency is 94%, return stroke detection efficiency is 75%, and detection efficiency for superimposed pulses is 5% for peak currents ≥1 kA and 32% for peak currents ≥5 kA. For return strokes, the median location error is 334 m and the median value of absolute peak current estimation error is 14%. The percentage of misclassified events is 4%, all of them being return strokes. The median value of absolute event-time mismatch (the difference in times at which the event is reported by the NLDN and recorded at the lightning triggering facility) for return strokes is 2.8 μs. For two out of the nine superimposed pulses detected by the NLDN, we found optical evidence of a re-illuminated branch (recoil leader) coming in contact with the existing grounded channel at an altitude of a few hundred meters above ground.
    Journal of Geophysical Research 03/2014; · 3.17 Impact Factor
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    ABSTRACT: Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the process for stepped, dart, and dart-stepped leaders in an “anomalous” rocket-triggered lightning flash that terminated on a 10 m grounded utility pole. The initiation of the first return stroke was found to occur at a height of 23 ± 3 m above the top of the utility pole and was associated with three “slow-front” dE/dt pulses. A time of 1.5 µs later, a fast rise in luminosity at 18 ± 2 m was associated with a “fast-transition” dE/dt pulse. The first return stroke propagated bi-directionally from its initiation height, as did subsequent return strokes from their initiation heights of 8 ± 1 m to 16 ± 2 m above the top of the utility pole. The initial upward speed of the first return stroke was 1.4 × 108 m/s, while its initial downward speed was 2.2 × 107 m/s. The channel-bottom luminosity of the first return stroke rose more slowly to a two-or-more-times larger amplitude than that of the subsequent stroke luminosities. In contrast, the NLDN-derived first-return-stroke peak current is smaller than that of the second and the third strokes, and our electric field records at 45 km show similar behavior for the initial field peaks of the first and subsequent strokes.
    Journal of Geophysical Research 01/2014; · 3.17 Impact Factor
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    ABSTRACT: Return-stroke peak current is one of the most important measures of lightning intensity needed in different areas of atmospheric electricity research. It can be estimated from the corresponding electric or magnetic radiation field peak. Electric fields of 89 strokes in lightning flashes triggered using the rocket-and-wire technique at Camp Blanding (CB), Florida, were recorded at the Lightning Observatory in Gainesville, about 45 km from the lightning channel. Lightning return-stroke peak currents were estimated from the measured electric field peaks using the empirical formula of Rakov et al. (1992) and the field-to-current conversion equation based on the transmission line model (Uman and McLain, 1969). These estimates, along with peak currents reported by the U.S. National Lightning Detection Network (NLDN), were compared with the ground-truth data, currents directly measured at the lightning channel base. The empirical formula, based on data for 28 triggered-lightning strokes acquired at the Kennedy Space Center (KSC), tends to overestimate peak currents, whereas the NLDN-reported peak currents are on average underestimates. The field-to-current conversion equation based on the transmission line model gives the best match with directly measured peak currents for return-stroke speeds between c/2 and 2c/3 (1.5 and 2 × 108 m/s, respectively). Possible reasons for the discrepancy in the peak current estimates from the empirical formula and the ground-truth data include an error in the field calibration factor, difference in the typical return-stroke speeds at CB and at the KSC (considered here to be the most likely reason), and limited sample sizes, particularly for the KSC data. A new empirical formula, I = − 0.66–0.028rE, based on data for 89 strokes in lightning flashes triggered at CB, is derived.
    Atmospheric Research 01/2014; 135-136:306-313. · 2.20 Impact Factor
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    ABSTRACT: [1] We investigate the structure of x-ray emissions from downward rocket-triggered lightning leaders using a pinhole-type x-ray camera (XCAM) located at the International Center for Lightning Research and Testing (ICLRT). This study builds on the work of Dwyer et al. [2011], which reported results from XCAM data from the 2010 summer lightning season. Additional details regarding the 2010 data are reported here. During the 2011 summer lightning season, the XCAM recorded 12 out of 17 leaders,five of which show downward leader propagation. Of those five leaders, one dart-stepped leader and two “chaotic” dart leaders are the focus of this paper. These three leaders displayed unique x-ray emission patterns: a “chaotic” dart leader displayed a diffuse structure (i.e., a wide lateral “spraying” distribution of x-rays spanning at least 12 m across) and a dart-stepped leader and a “chaotic” dart leader exhibited compact emission (i.e., a narrow lateral distribution of strong x-rayemission spanning about four m across). These two distinct x-ray emission patterns (compact and diffuse) illustrate the variability of lightning leaders. Using Monte Carlo simulations, we show that the diffuse x-ray source must originate from a diffuse source of energetic electrons or possibly emission from several sources. The compact x-ray sources originate from compact electron sources and the x-ray source region radius and electric charge contained within the x-ray source region were between 2-3 m and on the order of 10− 4 C, respectively. For the leaders under investigation, the x-ray source region average currents were determined to be on the order of 102 A.
    Journal of Geophysical Research: Atmospheres. 01/2014;
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    ABSTRACT: [1] Lightning Mapping Array source locations, channel base currents, and electric field waveforms are presented for a lightning flash triggered in the rainbands of 2012 tropical storm Debby. The National Lightning Detection Network reported no natural cloud-to-ground discharges within 60 km of the North Florida triggering site for at least 20 h before and 8 h after the triggered flash. Additionally, local electric field mill and wideband antenna networks show no close cloud or cloud-to-ground flashes. The triggering rocket was launched with negative charge overhead producing an electric field at the ground of 5 kV m−1 and in coordination with X-band, dual-polarimetric radar observations of streamers of enhanced precipitation descending from the melting level as they approached the site. The Debby flash consisted of an initial stage (IS) followed by eleven leader/return stroke sequences. The flash exhibited all the processes of normal triggered and natural cloud-to-ground lightning: leader/return stroke sequences, continuing currents, K events, and M components. Additionally, the flash exhibited several exceptional characteristics: three return stroke peak currents greater than 25 kA, one very long, 352 ms, continuing current that transferred about 35 C of charge to ground, and a relatively short, 202 ms, IS containing no initial continuous current pulses. Following a near-vertical upward positive leader attaining 2.8 km height, the IS branched and propagated horizontally at 3.5 km altitude. The flash, exhibiting strokes and continuing current, then ascended to and propagated horizontally at 5.5 km, extending about 25 km south and 15 km east. The 0°C level was near 4.5 km above sea level. It follows from the above that clouds that are not producing natural lightning can represent a triggered lightning hazard to launch vehicles and aircraft.
    Journal of Geophysical Research: Atmospheres. 12/2013; 118(23).
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    ABSTRACT: [1] We investigate individual X-ray bursts from lightning leaders to determine if energetic electrons at the source (and hence X-rays) are emitted isotropically or with some degree of anisotropy. This study was motivated by the work of Saleh et al. (2009), which found the falloff of X-rays in concentric radial annuli, covering all azimuthal directions in each annulus, from the lightning channel to be most consistent with an isotropic electron source. Here we perform a statistical analysis of angular and spatial distributions of X-rays measured by up to 21 NaI/PMT detectors at the International Center for Lightning Research and Testing site for 21 leader X-ray bursts from five leaders (including four dart-stepped leaders and one dart leader). Two procedures were used to complete this analysis. Procedure 1 found the first-order anisotropy, and procedure 2 tested whether or not the angular distribution was consistent with an isotropic distribution. Because higher-order anisotropies could be present in the data, a distribution that is not isotropic does not necessarily have a significant first-order anisotropy. Using these procedures, we find that at least 11 out of 21 X-ray bursts have a statistically significant first-order anisotropy, and hence those 11 are inconsistent with an isotropic emission. The remaining 10 bursts do not have significant first-order anisotropy. However, of those 10 bursts, 9 are inconsistent with isotropic emission, since they exhibit significant higher-order anisotropies. Since Saleh et al. (2009) did not consider anisotropies in the azimuthal direction, these new measurements of anisotropy do not necessarily contradict that work. Indeed, our analysis supports the finding that the X-ray emissions from lightning are inconsistent with a vertically downward beam. The level of anisotropy of the runaway electrons is important because it provides, in principle, information on the streamer zone in front of the leader and the electric field near the lightning leader tip.
    Journal of Geophysical Research: Atmospheres. 10/2013; 118(20).
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    ABSTRACT: We have estimated the ENTLN (formerly WTLN) performance characteristics using data for 245 negative return strokes in 55 flashes triggered from June of 2009 to August of 2012 at Camp Blanding, Florida. Performance characteristics are presented for both originally reported lightning data and reprocessed lightning data (the new processor was introduced in November of 2012). After the reprocessing, the ENTLN performance characteristics changed (relative to those corresponding to the originally reported data) as follows. Flash detection efficiency increased from 80% to 89%, stroke detection efficiency increased from 49% to 67%, percentage of misclassified events decreased from 60% to 52%, median location error increased from 621 m to 687 m, and median absolute current estimation error decreased from 51% to 17%.
    2013 International Symposium on Lightning Protection (XII SIPDA), Belo Horizonte, Brazil; 10/2013
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    ABSTRACT: [1] We present ground-level electric field intensity and trigger-wire-base current measurements in Florida during 33 successful rocket-and-wire triggered lightning attempts, those which initiated a sustained upward leader, and 20 unsuccessful attempts. The electric field changes during wire ascent were measured at eight stations between 35 m and 208 m from the launch site while the electric fields produced by precursor discharges at the ascending wire tip were measured at 120 m and 220 m. Both relatively steady trigger wire currents in the milliampere-range and fast precursor currents in the ampere to hundred-ampere range were measured at the wire base. A total of 2196 individual precursors were measured in 45 launches with negative charge overhead, with 0 to 225 precursors per launch and each precursor depositing 1 μC to 157 μC of charge at the wire tip. With negative charge overhead, slowly varying currents measured during the wire ascent increased to a maximum value between 3 mA and 10 mA. Positive line charge densities on the trigger wire were inferred from both the wire-base current and the ground-level electric field reduction during wire ascent for 38 launches, ranging from 1 μC m−1 near ground to about 100 μC m−1 at 200 m to 300 m aloft. A comparison of successful launches and unsuccessful launches with negative charge overhead shows that successful launches tend to have larger trigger wire line charge densities and larger precursor charge magnitudes, implying larger electric fields aloft. Three unsuccessful triggering attempts were made with positive charge overhead.
    Journal of Geophysical Research: Atmospheres. 09/2013; 118(17).
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    ABSTRACT: a high speed optical imaging system operated at a time resolution of either 10 ns or 100 ns, we have documented the initiation process of 14 return strokes in four rocket-triggered lightning flashes. Of the 14 strokes, nine occurred following dart leaders and five following dart-stepped leaders. The return strokes are found to initiate at heights ranging from 7.2 ± 1.4 to 21.0 ± 4.6 m above the lightning termination point. Return strokes with larger peak current tend to initiate higher. All the return strokes show initial bidirectional (upward and downward from their initiation height) propagation. We have been able to estimate the initial upward propagation speeds below 60 m for all of the return strokes. The resultant speeds range from 0.4 × 108 to 2.5 × 108 m/s. For the downward propagation speeds, only six strokes among the 14 strokes allow us to perform a reasonable estimation. Those downward speeds range from 0.6 × 108 to 1.9 × 108 m/s.
    Journal of Geophysical Research 09/2013; 118(17):9880-9888. · 3.17 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: [1] Correlated Lightning Mapping Array and vertical-scan radar images are presented for three rocket-and-wire triggered lightning flashes that occurred sequentially within 17 min in the presence of a decaying multicellular convective storm system over north-central Florida. The initial stage (IS) of each flash propagated generally vertically to the altitude of the 0°C melting level, about 5 km, and then subsequently propagated for many kilometers horizontally along the melting level contour. Radar images suggest that the propagation paths of the IS channels below and above the melting level were heavily influenced by precipitation gradients. Flash UF 11-24 exhibited a 12.6 km unbranched IS channel, the longest unbranched channel observed in the study by a factor of three. During flash UF 11-25 (119 ms following the cessation of the measured IS current at ground and prior to the first return stroke), a natural cloud-to-ground discharge, perhaps induced by the IS, initiated between 2.5 and 4 km altitude and struck ground 5 to 7 km from the launching facility. The IS of flash UF 11-26 propagated upward through a descending precipitation packet and apparently induced a naturally appearing bi-level intracloud discharge via an upward-negative leader that initiated within the IS breakdown region 3.5 km from the launching facility. The upward-negative leader propagated from 5.6 to 9.3 km altitude in a time of 11 ms. The electrical current measured at ground during the IS of flash UF 11-26 exhibited a 57 ms polarity reversal, transferring 19 C of positive charge to ground.
    Journal of Geophysical Research: Atmospheres. 08/2013; 118(15).
  • [show abstract] [hide abstract]
    ABSTRACT: We have estimated the NLDN performance characteristics using data for 326 negative return strokes in 78 flashes triggered in 2004–2012 at Camp Blanding, Florida. The flash and stroke detection efficiencies are 94% and 75%, respectively. Out of the 245 NLDN-detected strokes, 9 (4%) were misclassified as cloud discharges. The median location error is 334 m. The median absolute current estimation error is 14%.
    8th 2013 Asia-Pacific International Conference on Lightning (APL), Seoul, Korea; 06/2013
  • Joseph R. Dwyer, Martin A. Uman
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    ABSTRACT: Despite being one of the most familiar and widely recognized natural phenomena, lightning remains relatively poorly understood. Even the most basic questions of how lightning is initiated inside thunderclouds and how it then propagates for many tens of kilometers have only begun to be addressed. In the past, progress was hampered by the unpredictable and transient nature of lightning and the difficulties in making direct measurements inside thunderstorms, but advances in instrumentation, remote sensing methods, and rocket-triggered lightning experiments are now providing new insights into the physics of lightning. Furthermore, the recent discoveries of intense bursts of X-rays and gamma-rays associated with thunderstorms and lightning illustrate that new and interesting physics is still being discovered in our atmosphere. The study of lightning and related phenomena involves the synthesis of many branches of physics, from atmospheric physics to plasma physics to quantum electrodynamics, and provides a plethora of challenging unsolved problems. In this review, we provide an introduction to the physics of lightning with the goal of providing interested researchers a useful resource for starting work in this fascinating field.
    Physics Reports. 01/2013;
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    ABSTRACT: The performance characteristics of the U.S. National Lightning Detection Network (NLDN) are evaluated for “classical” return strokes (RS) and kiloampere-scale pulses superimposed (SIP) on steady currents (initial continuous-current or ICC pulses and M-components) using, as ground-truth, data for lightning triggered at Camp Blanding, Florida. The data set consists of 78 flashes with 245 RS and 173 SIP (58 ICC and 115 M) for the period of 2004-2012. The flash and stroke detection efficiencies were 94% and 75%, respectively. In addition to return strokes, the NLDN detected 9 superimposed pulses (7 ICC and 2 M). The NLDN detection efficiencies for SIP with peaks of ≥ 1 kA was 5%. When SIP with peaks ≥ 5 kA were considered, the detection efficiency was 32%. Results have important implications for detectability by the NLDN of tower-initiated flashes, which often do not contain return strokes.
    2012 AGU Fall Meeting, San Francisco, CA; 12/2012
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    ABSTRACT: We examine initial rising portions of electric field waveforms of negative first return strokes in natural cloud-to-ground lightning recorded simultaneously at near and far distances from the lightning channel. The near and far field-measuring stations are located at Camp Blanding and in Gainesville, Florida, respectively, separated by a distance of about 45km. A total of five return strokes had been recorded in 2007–2008, four of which were analyzed in detail (one was not suitable for analysis due to saturation of electric field waveform at the far station). Field waveform characteristics, including overall zero-to-peak and 10-to-90% risetimes, duration of slow front, fast transition 10-to-90% risetime, and magnitude of slow front relative to the peak, were found to be similar to those reported from other studies, in which the field propagation path was over ground (as opposed to sea water). It is shown, via modeling, that the slow front in electric field waveforms at far distances is primarily due to the radiation field component, while at near distances it is composed of comparable contributions from all three components of electric field. For both measured and model-predicted waveforms, the durations of the slow front appear to be similar at near and far distances from the lightning channel.
    Atmospheric Research 11/2012; · 2.20 Impact Factor
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    ABSTRACT: Terrestrial gamma-ray flashes (TGFs) are bright, sub-millisecond bursts of gamma-rays, originating within the Earth's atmosphere. Most TGFs have been detected by spacecraft in low-Earth orbit. Only two TGFs have previously been observed from within our atmosphere: one at ground level and one from an aircraft at 14.1 km. We report on a new TGF-like gamma-ray flash observed at ground level, detected by the 19-station Thunderstorm Energetic Radiation Array (TERA) at the University of Florida/Florida Tech International Center for Lightning Research and Testing (ICLRT). The gamma-ray flash, which had a duration of 52.7 μs, occurred on June 30, 2009 during a natural negative cloud-to-ground lightning return stroke, 191 μs after the start of the stroke. This event is the first definitive association of a gamma-ray flash with natural CG lightning and is among the most direct links to a specific lightning process so far. For this event, 19 gamma-rays were recorded, with the highest energy exceeding 20 MeV. The high-energy radiation exhibited very different behavior from the typical x-ray emission from lightning. Specifically, the gamma-ray flash had a much harder energy spectrum, consistent with relativistic runaway electron avalanche (RREA) multiplication; it did not arrive in sub-microsecond bursts, typical of leader emission from lightning, and it occurred well after the start of the return stroke, which has not been previously observed for the x-ray emission from lightning. Nevertheless, we present evidence that the source region for the gamma ray flash was the same as that for the preceding leader x-ray bursts.
    Journal of Geophysical Research 10/2012; 117(A10):10303-. · 3.17 Impact Factor
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    ABSTRACT: Energetic radiation is known to be produced by lightning. To investigate these emissions, ground-based observations are being conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL where measurements of energetic radiation from both natural and rocket-triggered lightning discharges are recorded. In the present study, data from two natural negative cloud-to-ground stepped leaders and one rocket-triggered "chaotic" dart leader are analyzed in detail to investigate X-ray energy spectra and spatial X-ray distributions around the source. These measurements are compared with Monte Carlo simulations of runaway electron propagation with the goal of understanding the underlying mechanism of runaway electron production and their role in lightning initiation and propagation. We show that the energetic electrons that produce X-rays exhibit a characteristic energy less than 3 MeV for two natural and one rocket-triggered leaders investigated. In addition to studying these three leaders, energetic electron luminosity, total energy, and energetic electron per meter are compared to the following return stroke currents for 28 leaders from 12 different triggered flashes. Electron luminosity is found to increase exponentially with return stroke current up to about 10 kA and to be roughly constant for larger currents. The maximum electron luminosity, which is determined indirectly through X-ray luminosities, is on the order of 1017 electrons/s, which is less than the value found from theoretical calculations.
    Journal of Geophysical Research 08/2012; 117(D15):15201-. · 3.17 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We characterize the geometrical and electrical characteristics of the initial stages of nine Florida triggered lightning discharges using a Lightning Mapping Array (LMA) and measured channel-base currents. We determine initial channel and subsequent branch lengths, average initial channel and branch propagation speeds, and channel-base current at the time of each branch initiation. The channel-base current is found to not change significantly when branching occurs, an unexpected result. The initial stage of Florida triggered lightning typically transitions from vertical to horizontal propagation at altitudes of 3-6 km, near the typical freezing level of 4 km and several kilometers below the expected center of the negative cloud-charge region at 7-8 km. The data presented potentially provide information on thunderstorm electrical and hydrometeor structure and discharge propagation physics. LMA source locations were obtained from VHF sources of positive impulsive currents as small as 10 A, in contrast to expectations found in the literature.
    Geophysical Research Letters 05/2012; 39(9):9807-. · 3.98 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We examine VHF interferometric images, channel-base currents, and broadband electric field waveforms of the initial stage (IS) in two rocket-and-wire triggered lightning flashes. Two types of negative leaders, termed "long-duration" and "short-duration" leaders, were imaged by the VHF interferometers during the IS of the two flashes. There were three leaders that had relatively long durations of more than a few milliseconds. These three leaders were not accompanied by a significant change of channel-base current during their early stages of development, indicating that they corresponded to intracloud (IC) discharges that were not connected to the grounded triggered-lightning channel. Two of these three leaders eventually connected to the triggered-lightning channel and initiated initial continuous current (ICC) pulses. The third long-duration leader apparently developed from the vicinity of an isolated negative charge region toward an upper-level positive charge region and toward a branch of the grounded channel; it served to bridge the positive charge region and the triggered-lightning channel, resulting in the opposite polarity portion of the bipolar ICC. The short-duration negative leaders had durations of some hundreds of microseconds. These negative leaders apparently recoiled along the conductive channels created by branches of the upward positive leader (UPL); they initiated ICC pulses when the grounded channel was sufficiently conductive. It follows that ICC pulses can be initiated either by recoil leaders or via interception of separate in-cloud leaders by a grounded current-carrying channel.
    Journal of Geophysical Research 05/2012; 117(D9):9119-. · 3.17 Impact Factor

Publication Stats

3k Citations
558.60 Total Impact Points

Institutions

  • 1029–2014
    • University of Florida
      • Department of Electrical and Computer Engineering
      Gainesville, Florida, United States
  • 2005–2009
    • University of Bologna
      • "Guglielmo Marconi" Department of Electrical, Electronic and Information Engineering DEI
      Bologna, Emilia-Romagna, Italy
  • 2005–2007
    • University of Toronto
      • Department of Electrical and Computer Engineering
      Toronto, Ontario, Canada
  • 2003
    • Kennedy Space Center
      Gainesville, Florida, United States
  • 1995–1999
    • The University of Arizona
      Tucson, Arizona, United States
  • 1992
    • Louisiana State University
      • Department of Electrical Engineering
      Baton Rouge, LA, United States