The Emission of Terrestrial Gamma Ray Flashes From Encountering Streamer Coronae Associated to the Breakdown of Lightning Leaders

C Köhn,M Heumesser,T Neubert,K Nishikawa,O Chanrion,V Reglero

doi:10.1029/2020gl089749

Abstract

AbstractTerrestrial gamma ray flashes (TGFs) are beams of high‐energy photons associated to lightning. These photons are the bremsstrahlung of energetic electrons whose origin is currently explained by two mechanisms: energizing electrons in weak, but large‐scale thundercloud fields or the acceleration of low‐energy electrons in strong, but localized fields of lightning leaders. Contemporary measurements by the Atmosphere‐Space Interactions Monitor suggest that the production of TGFs is related to the leader step and associated streamer coronae when upward moving intracloud lightning illuminates. Based on these observations, we apply a particle‐in‐cell Monte Carlo code tracing electrons in the superposed electric field of two encountering streamer coronae and modeling the subsequent photon emission. We also perform a parameter study by solving the deterministic equations of motion for one electron. We find that this mechanism can explain the occurrence of TGFs with photons energies of several MeV lasting for tens to hundreds of μs, in agreement with observations.

Highlights

Terrestrial gamma ray flashes (TGFs) are bursts of energetic gamma rays with single photon energies of up to 40 MeV lasting for tens to hundreds of microseconds and are associated to the occurrence of lightning (Briggs et al, 2010; Fishman et al, 1994)
They were first observed above thunderclouds by the Burst and Transient Source Experiment (BATSE) (Fishman et al, 1994), confirmed later by various other missions such as AGILE (Marisaldi et al, 2010; Tavani et al, 2011), Fermi (Briggs et al, 2010), RHESSI (Cummer et al, 2005; Smith et al, 2010), and GROWTH (Tsuchiya et al, 2011) and are subject to the current ASIM (Atmosphere-Space Interactions Monitor) (Neubert et al, 2019) and future TARANIS (Tool for the Analysis of Radiation from lightning and Sprites) (Blanc et al, 2007) missions with payloads dedicated to the measurement of lightning flashes and associated phenomena like transient luminous events (TLEs) and TGFs
Electrons with this particular spatial and energy distribution are subsequently initiated for the second simulation step where they are accelerated in the superposed electric field of a lighting leader and two encountering streamer coronae, some of them gaining relativistic energies and emitting energetic γ rays

Summary

Introduction

Terrestrial gamma ray flashes (TGFs) are bursts of energetic gamma rays with single photon energies of up to 40 MeV lasting for tens to hundreds of microseconds and are associated to the occurrence of lightning (Briggs et al, 2010; Fishman et al, 1994). Only if electrons reach energies larger than approximately 150–200 eV, where the friction force in air takes its maximum, does the further electron acceleration become systematic rather than stochastic leading to a significant multiplication of energetic electrons (Babich, 2005; Gurevich, 1961; Kunhardt et al, 1986; Wilson, 1925), named runaway electrons by Eddington (1926)

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Results

Conclusion