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  • 1
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    Do Cosmic Ray Air Showers Initiate Lightning?: A Statistical Analysis of Cosmic Ray Air Showers and Lightning Mapping Array Data (2017)
    Wiley
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    Publication Date: 2017-07-22
    Description: It has been argued in the technical literature, and widely reported in the popular press, that cosmic ray air showers (CRASs) can initiate lightning via a mechanism known as relativistic runaway electron avalanche (RREA), where large numbers of high energy and low energy electrons can, somehow, cause the local atmosphere in a thundercloud to transition to a conducting state. In response to this claim, other researchers have published simulations showing that the electron density produced by RREA is far too small to be able to affect the conductivity in the cloud sufficiently to initiate lightning. In this paper, we compare 74 days of cosmic ray air shower data collected in north central Florida during 2013, 2014, and 2015, the recorded CRASs having primary energies on the order of 10 16 eV to 10 18 eV and zenith angles less than 38 degrees, with Lightning Mapping Array (LMA) data, and we show that there is no evidence that the detected cosmic ray air showers initiated lightning. Furthermore, we show that the average probability of any of our detected cosmic ray air showers to initiate a lightning flash can be no more than 5 percent. If all lightning flashes were initiated by cosmic ray air showers, then about 1.6 percent of detected CRASs would initiate lightning, therefore we do not have enough data to exclude the possibility that lightning flashes could be initiated by cosmic ray air showers.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Return Stroke Current Reflections in Rocket-Triggered Lightning (2016)
    Wiley
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    Publication Date: 2016-03-08
    Description: Ten upward-propagating return stroke currents in eight triggered lightning flashes have been observed to reflect downward from 140 to 300 m altitude by way of measurements of the channel-base current, close electric field and electric field derivative, and high-speed video. The current reflections appear as dips in the measured channel-base current and in the electric fields, and as bipolar pulses in the electric field derivative waveforms. The current dips occur 2.7 to 13.9 μ s after the initiation of the return stroke current at ground. The observed phenomenon results from a portion of the upward propagating return stroke current wave being reflected from a channel impedance discontinuity apparently associated with an isolated section of unexploded triggering wire, as inferred from high-speed video records. A transmission line model is used to model the close electric field and electric field derivative of the postulated initial and reflected current waves, starting with the measured channel base current, and the results are compared favorably with measurements made at 92 to 326 m. From the measured time between the return stroke current initiation at the ground and the time the current reflection reaches the channel base and produces the current dip, along with the reflection height inferred from the video records, we find the average of the upward and downward reflected return stroke current speed for each of the ten strokes to be between 0.81 and 2.06×10 8  ms −1 .
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Luminosity in the initial breakdown stage of cloud-to-ground and intracloud lightning (2016)
    Wiley
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    Publication Date: 2016-01-12
    Description: We present simultaneously-measured luminosity and electric field data from the initial breakdown (IB) stage in seven cloud-to-ground (CG) and eight intracloud (IC) lightning discharges along with, in three cases, radar and Lightning Mapping Array (LMA) data. The data were taken in north-central Florida in 2013 and 2014. Seven CG discharges had an arithmetic mean {S.D.} IB stage luminosity pulse train duration of 3 ms {2 ms}, and within the seven CG discharges, thirty luminosity pulses had the following means {S.D.}: 10% to 90% rise time 25 µs {16 µs}, full width at half maximum 68 µs {21 µs}, and delay between onset of the electric field pulse and associated luminosity pulse 8 µs {8 µs}. Eight IC discharges had a mean {S.D.} IB stage luminosity pulse train duration of 11 ms {4 ms}, and within the eight IC discharges, thirty-seven luminosity pulses exhibited mean rise times, widths, and delays of: 59 µs {36 µs}, 176 µs {70 µs}, and 34 µs {20 µs}, all significantly greater than in the CG case. The roughly ten LMA sources associated with each of the three IB stages in 2014 are grouped horizontally within about 1 km 2 . The mean altitude {S.D.} of the LMA points during two CG IB stages is 5 km {600 m} and 4.3 km {250 m} and during one IC discharge is 6.2 km {550 m}. We discuss the role of optical scattering in delaying and distorting the observed luminosity waveforms.
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    Topics: Geosciences , Physics
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  • 4
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    First Images of Thunder: Acoustic Imaging of Triggered Lightning (2015)
    Wiley
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    Publication Date: 2015-06-20
    Description: An acoustic camera comprising a linear microphone array is used to image the thunder signature of triggered lightning. Measurements were taken at the International Center for Lightning Research and Testing (ICLRT) in Camp Blanding, FL during the summer of 2014. The array was positioned in an end-fired orientation thus enabling the peak acoustic reception pattern to be steered vertically with a frequency-dependent spatial resolution. On 14 July 2014, a lightning event with 9 return strokes was succesfully triggered. We present the first acoustic images of individual return strokes at high frequencies (〉1kHz) and compare the acoustically-inferred profile with optical images. We find: (i) a strong correlation between the return stroke peak current and the radiated acoustic pressure, (ii) an acoustic signature from an M-component with an unusual fast risetime. These results show that acoustic imaging enables clear identification and quantification of thunder sources as a function of lightning channel altitude.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Coordinated Lightning, Balloon-borne Electric Field, and Radar Observations of Triggered Lightning Flashes in North Florida (2015)
    Wiley
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    Publication Date: 2015-06-20
    Description: This study examines coordinated storm and triggered lightning observations made July – August 2013 at the International Center for Lightning Research and Testing to determine why triggered flashes in Florida typically transition from an upward vertical channel entering the cloud to horizontal structure near the storm's melting level. Data from a balloon-borne electric field meter, a mobile 5-cm wavelength radar, and a small-baseline VHF Lightning Mapping Array acquired during a period in which three flashes were triggered on 1 August confirmed the hypothesis that the transition to horizontal lightning structure just above the melting level occurred in a layer of negative charge. This experiment was the first to provide vertical profiles of the electric field in Florida storms, from which their vertical charge distribution could be inferred. Three dissipating storms observed on different days all had negative charge near the melting level, but a growing mature storm had positive charge there.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 6
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    The Energy Spectrum of X-Rays from Rocket-triggered Lightning (2015)
    Wiley
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    Publication Date: 2015-09-30
    Description: Although the production of X-rays from natural and rocket-triggered lightning leaders have been studied in detail over the last ten years, the energy spectrum of the X-rays has never been well measured because the X-rays are emitted in very short but intense bursts that result in pulse pile-up in the detectors. The energy spectrum is important because it provides information about the source mechanism for producing the energetic runaway electrons and about the electric fields that they traverse. We have recently developed and operated the first spectrometer for the energetic radiation from lightning. The instrument is part of the Atmospheric Radiation Imagery and Spectroscopy (ARIS) project and will be referred to as ARIS-S (ARIS Spectrometer). It consists of seven NaI(Tl)/photomultiplier tube (PMT) scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than of lead placed over the detector (all the detectors are in a thick aluminum box). Using X-ray pulses preceding 48 return strokes in 8 rocket-triggered lightnings, we found that the spectrum of X-rays from leaders is too soft to be consistent with RREA. It has a power-law dependence on the energies of the photons, and the power index, λ , is between 2.5 to 3.5. We present the details of the design of the instrument and the results of the analysis of the lightning data acquired during the summer of 2012.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Lightning current and luminosity at and above channel bottom for return strokes and M-components (2015)
    Wiley
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    Publication Date: 2015-09-30
    Description: We measured current and luminosity at the channel bottom of 12 triggered lightning discharges including 44 return strokes, 23 M-components, and 1 ICC pulse. Combined current and luminosity data for impulse currents span a 10-90% risetime range from 0.15 to 192 microseconds. Current risetime and luminosity risetime at the channel bottom are roughly linearly correlated ( τ r , I  = 0.71 τ r , L 1.08 ). We observed a time delay between current and the resultant luminosity at the channel bottom, both measured at 20% of peak amplitude, that is approximately linearly related to both the luminosity 10-90% risetime ( Δτ 20, b  = 0.24 τ r , L 1.12 ) and the current 10-90% risetime ( Δτ 20, b  = 0.35 τ r , I 1.03 ). At the channel bottom, the peak current is roughly proportional to the square root of the peak luminosity ( I P  = 21.89 L p 0.57 ) over the full range of current and luminosity risetimes. For two return strokes we provide measurements of stroke luminosity vs. time for 11 increasing heights to 115 m altitude. We assume that measurements above the channel bottom behave similarly to those at the bottom and find that (1) one return stroke current peak decayed at 115 m to about 47% of its peak value at channel bottom, while the luminosity peak at 115 m decayed to about 20%, and for the second stroke 38% and 12%, respectively; and (2) measured upward return stroke luminosity speeds of the two strokes of 1.10×10 8 and 9.7×10 7 ms -1 correspond to current speeds about 30% faster. These results represent the first determination of return stroke current speed and current peak value above ground derived from measured return stroke luminosity data.
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    Topics: Geosciences , Physics
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  • 8
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    Initial breakdown and fast leaders in lightning discharges producing long lasting disturbances of the lower ionosphere (2016)
    Wiley
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    Publication Date: 2016-05-31
    Description: The recent discovery of long recovery, early VLF scattering events (LOREs) indicates that the electric field changes from lightning discharges are capable of producing long lasting disturbances (up to tens of minutes) in the upper mesosphere and lower ionosphere. Comparison of lightning mapping array, broadband (up to 10 MHz) electric field, and VLF (∼300 Hz to 42 kHz) magnetic field measurements shows that the field changes produced by initial breakdown (IB) processes and the following leaders in natural, cloud-to-ground lightning discharges are detectable in VLF magnetic field measurements at long distances. IB radiation has been detected in VLF for lightning discharges occurring up to 2630 km away from the VLF observing station. Radio atmospherics associated with 52 LOREs, 51 regular recovery events, and 3,098 flashes detected by NLDN and/or GLD360 were examined for IB radiation occurring up to 15 msec before the return stroke. Our analysis reveals that, in contrast to regular recovery early VLF events, LOREs are strongly associated with lightning discharges which exhibit an intense IB process and a fast first leader (typical duration 〈4 msec). These experimental results demonstrate that initial breakdown and leader processes are indicators of discharge properties highly relevant to the total energy transfer between lightning discharges and the middle/upper atmosphere.
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  • 9
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    Estimation of triggered-lightning dart-stepped-leader currents from close multiple-station dE/dt pulse measurements (2015)
    Wiley
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    Publication Date: 2015-01-21
    Description: The modified transmission line model is used to derive the vertically-propagating leader-step currents necessary to radiate measured dart-stepped-leader dE/dt pulses from triggered lightning at close range (〈400 m) and low altitude (〈70 m). The model-predicted dE/dt pulses were compared with measured dE/dt pulses at nine locations ranging from 27 to 391 m from the channel base for four dE/dt pulses radiated from two triggered dart-stepped leaders. The dE/dt pulses at the closest station, 27 m, were unipolar, dominated by electrostatic and induction components of the radiated dE/dt, and of opposite polarity to the more distant initial dE/dt peaks. The other, more distant, eight stations exhibited bipolar dE/dt pulses, being more or less dominated by the dE/dt radiation component. The derived leader-step current has a slow front that precedes a fast transition to peak amplitude followed by a slow decay to zero after several microseconds. For the four modeled dE/dt pulses, the estimated causative leader-step current peak amplitudes varied from 0.9 to 1.8 kA, the half-peak widths ranged from 370 to 560 ns, the charge transfers were about 1 mC, and the peak current derivatives were about 10 kA/µs. The upward propagation speeds of the leader-step current were from 1.1 to 1.5x10 8 m/s with exponential spatial current decay constants from 13 to 27 m. One dE/dt pulse is analyzed in more detail by studying changes in model-predicted waveforms versus current initiation altitude and by examining the effect of varying model input parameters.
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    Topics: Geosciences , Physics
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  • 10
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    Observations of corona in triggered dart-stepped leaders (2015)
    Wiley
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    Publication Date: 2015-02-21
    Description: Corona streamers are a critical component of lightning leader step formation, and are postulated to produce the very high electric fields at their tips that produce runaway electrons resulting in the observed x-ray bursts associated with leader stepping. Corona emanating from the vicinity of the leader tip between leader steps was analyzed using three sequential high-speed video sequences of dart-stepped leaders in three different triggered lightning flashes during the summers of 2013 and 2014 in northeast Florida. Images were recorded at 648 kiloframes per second (1.16 µs exposure time, 380 ns dead time) at an altitude of 65 m or less. In each image sequence, the leader propagates downward in consecutive frames, with corona streamers observed to fan outward from the bright leader tip in less than the image frame time of about 1.5 µs. In 21 exposures, corona streamers propagate, on average, 9 m below the bright leader tip.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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