Modeling of X‐ray Images and Energy Spectra Produced by Stepping Lightning Leaders

Abstract

Recent ground‐based measurements at the International Center for Lightning Research and Testing (ICLRT) have greatly improved our knowledge of the energetics, fluence, and evolution of X‐ray emissions during natural cloud‐to‐ground (CG) and rocket‐triggered lightning flashes. In this paper, using Monte Carlo simulations and the response matrix of unshielded detectors in the Thunderstorm Energetic Radiation Array (TERA), we calculate the energy spectra of X‐rays as would be detected by TERA and directly compare with the observational data during event MSE 10‐01. The good agreement obtained between TERA measurements and theoretical calculations supports the mechanism of X‐ray production by thermal runaway electrons during the negative corona flash stage of stepping lightning leaders. Modeling results also suggest that measurements of X‐ray bursts can be used to estimate the approximate range of potential drop of lightning leaders. Moreover, the X‐ray images produced during the leader stepping process in natural negative CG discharges, including both the evolution and morphological features, are theoretically quantified. We show that the compact emission pattern as recently observed in X‐ray images is likely produced by X‐rays originating from the source region, and the diffuse emission pattern can be explained by the Compton scattering effects.

Spectral Hardness of X‐ and Gamma‐Ray Emissions From Lightning Stepped and Dart Leaders

“…Our results show that X-ray emissions associated with both stepped and dart leaders follow an inverse power-law distribution, and contain no significant evidence of RREA contribution. This in agreement with previous work by Arabshahi et al (2015) and Xu et al (2017). We also determined that the energy spectra of stepped leaders seem to be harder than dart leaders, that is, containing more gamma (>1 MeV) photons, and displaying a longer tail.…”

“…(2015) and Xu et al. (2017). We also determined that the energy spectra of stepped leaders seem to be harder than dart leaders, that is, containing more gamma (>1 MeV) photons, and displaying a longer tail.…”

“…Xu et al. (2017) used this data set as ground truth for their Monte Carlo simulations, and showed that the measured X‐ray spectrum of Schaal et al.’s flash is fully consistent with the thermal runaway acceleration mechanism, for a leader with 10 MV potential difference between its tip and the surrounding environment.…”

“…Since LaBr 3 (Ce) is not as subject to photon pile-up issues, this allowed the authors to estimate the X-ray single-photon energy spectrum. Xu et al (2017) used this data set as ground truth for their Monte Carlo simulations, and showed that the measured X-ray spectrum of Schaal et al's flash is fully consistent with the thermal runaway acceleration mechanism, for a leader with 10 MV potential difference between its tip and the surrounding environment. Mallick et al (2012) reported X-ray emissions associated with leaders of natural cloud-to-ground lightning and showed that, in some cases, subsequent strokes can emit more X-rays than the leaders preceding the first return stroke in the same flash.…”

Catalog of 2017 Thunderstorm Ground Enhancement (TGE) events observed on Aragats

“…Recently the RB/RREA mechanism was supplemented by a new source of the electrons, accelerated in the streamer tips of a developing lightning leader 24 . Electrons are accelerated up to energies of ~70 keV and then runaway 25 . In any case, the lower dipole between the main negative charge and its mirror on surface or/and lower positive charge layer (LPCR) in the bottom of cloud supports the downward electron acceleration; the electric field strength in the lower part of the thundercloud is also crucial for the lightning flash development.…”