Modeling of High-Energy Photon Bursts From Lightning Leader

Abstract:

Terrestrial gamma-ray flashes (TGFs) are bursts of high-energy photons originating from the Earth's atmosphere in association with thunderstorm activity [e.g., Briggs et al., JGR, 118, 3805, 2013]. Additionally, X-ray bursts observed from the ground have been discovered to be produced by negative cloud-to-ground (-CG) lightning leaders in association with stepping processes [Dwyer et al., GRL, 32, L01803, 2005]. Using numerical modeling, it has been shown that the production of thermal runaway electrons by stepping lightning leaders and their further acceleration could explain the TGF spectrum for intracloud (IC) lightning potentials above ~100 MV [Xu et al., GRL, 39, L08801, 2012] and X-ray burst spectrum for -CG lightning potentials of ~5 MV [Xu et al., GRL, 41, 7406, 2014].

In this work, we address the physical processes leading to X-ray bursts from -CG discharges and TGFs produced by IC discharges in a unified fashion. We show how the leader-produced photon spectrum becomes harder with increasing lightning leader potential and how it progressively converges to typical photon spectrum associated with relativistic runaway electron avalanches (RREAs) in large-scale ambient electric fields for potentials greater than ~150 MV. We also demonstrate that the photon fluence in a burst is a very sharp function of the potential. This implies that only lightning leaders forming the strongest potentials can lead to the production of observable TGFs from space. We specifically study the effects of source altitudes on the results and the production of the required high potentials in lightning leaders in realistic thunderstorm charge configurations.