J. D. Hill

The structure of X‐ray emissions from triggered lightning leaders measured by a pinhole‐type X‐ray camera

We investigate the structure of X-ray emissions from downward triggered lightning leaders using a pinhole-type X-ray camera (XCAM) located at the International Center for Lightning Research and Testing. 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, 5 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), and a dart-stepped leader and a chaotic dart leader exhibited compact emission (i.e., a narrow lateral distribution of strong X-ray emission). 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 and 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.

Rocket‐triggered lightning propagation paths relative to preceding natural lightning activity and inferred cloud charge

Lightning Mapping Array (LMA) data are used to compare the propagation paths of seven rocket-triggered lightning flashes to the inferred charge structure of the thunderstorms in which they were triggered. This is the first LMA study of Florida thunderstorm charge structure. Three sequentially (within 16u2009min) triggered lightning flashes, whose initial stages were the subject of Hill et al. (2013), are reexamined by comparing the complete flashes to the preceding natural lightning to demonstrate that the three rocket-triggered flashes propagated through an inferred negative charge region that decreased from about 6.8 to about 4.4u2009km altitude as the thunderstorm dissipated. Two other flashes were also sequentially triggered (within 9u2009min) in a thunderstorm that contained a convectively intense region ahead of a stratiform region, with similar observed results. Finally, two unique cases of triggered lightning flashes are presented. In the first case, the in-cloud portion of the triggered lightning flash, after ascending to and turning horizontal at 5.3u2009km altitude, just above the 0°C level, was observed to very clearly resemble the geometry of the in-cloud portion of the preceding natural lightning discharges. In the second case, a flash was triggered relatively early in the storms lifecycle that did not turn horizontal near the 0°C level, as is usually the case for triggered lightning in dissipating storms, but ascended to nearly 7.5u2009km altitude before exhibiting extensive horizontal branching.

Does the lightning current go to zero between ground strokes? Is there a current “cutoff”?

At the end of 120 prereturn stroke intervals in 27 lightning flashes triggered by rocket-and-wire in Florida, residual currents with an arithmetic mean of 5.3u2009mA (standard derivation 2.8u2009mA) were recorded. Average time constants of the current decay following return strokes were found to vary between 160u2009µs and 550u2009µs, increasing with decreasing current magnitude. These results represent the most sensitive measurements of interstroke lightning current to date, 2 to 3 orders of magnitude more sensitive than previously reported measurements, and contradict the common view found in the literature that there is a no current interval. Possible sources of the residual current are discussed.

Observations of corona in triggered dart‐stepped leaders

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 (1.16u2009µs exposure time, 380u2009ns dead time) at an altitude of 65u2009m 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 1.5u2009µs. In 21 exposures, corona streamers propagate, on average, 9u2009m below the bright leader tip.

The angular distribution of energetic electron and X-ray emissions from triggered lightning leaders

[1]xa0We 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.

The energy spectrum of X-rays from rocket-triggered lightning

Although the production of X-rays from natural and rocket-triggered lightning leaders have been studied in detail over the last 10xa0years, 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 pileup 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 3′′ NaI(Tl)/photomultiplier tube scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than 1′′ of lead placed over the detector (all the detectors are in a 1/8′′ 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 Relativistic Runaway Electron Avalanche. It has a power law dependence on the energies of the photons, and the power index, λ, is between 2.5 and 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.

Estimation of triggered‐lightning dart‐stepped‐leader currents from close multiple‐station dE/dt pulse measurements

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 (<400u2009m) and low altitude (<70u2009m). The model-predicted dE/dt pulses were compared with measured dE/dt pulses at nine locations ranging from 27 to 391u2009m from the channel base for four dE/dt pulses radiated from two triggered dart-stepped leaders. The dE/dt pulses at the closest station, 27u2009m, 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 560u2009ns, 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 1.5u2009×u2009108 m/s with exponential spatial current decay constants from 13 to 27u2009m. 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.

Electric field derivative waveforms from dart-stepped-leader steps in triggered lightning: DART-STEPPED-LEADER WAVEFORMS

Electric field derivative (dE/dt) pulse waveforms from dart-stepped-leaders in rocket-and-wire triggered lightning, recorded a distance of 226u2009m from the channel base, are characterized. A single dE/dt pulse associated with a leader step consists of a fast initial rise of the same polarity as the following return stroke followed by an opposite polarity overshoot of smaller amplitude and subsequent decay to background level, without superimposed secondary pulses. A “slow front” often precedes the fast initial rise. For 47 single dE/dt leader pulses occurring during the final 15u2009µs of 24 dart-stepped-leaders, the pulse mean half-peak width was 76u2009ns, mean 10-to-90% risetime 73u2009ns, and mean range-normalized peak amplitude 2.5u2009V/m/µs. For integrated dE/dt, the mean half-peak width was 214u2009ns and the mean range-normalized peak amplitude 0.21u2009V/m. Most dart-stepped-leader dE/dt pulses are more complex than a single pulse. Interpulse interval and average peak amplitude range normalized to 100u2009km were measured for both single and complex dE/dt pulses during the final 15u2009µs of 10 dart-stepped-leaders preceding triggered return strokes with peak currents ranging from 8.1 to 31.4 kA. The average range-normalized dE/dt and numerically integrated dE/dt (electric field) peak amplitude increased from 0.9 to 4.9u2009V/m/µs and 0.13 to 0.47u2009V/m, respectively, with increasing return stroke peak current while the interpulse interval remained relatively constant at about 2u2009µs. Strong positive linear correlations were found between both average range-normalized peak pulse amplitude and interstroke interval versus the following return stroke peak current.