Characterizing the source properties of terrestrial gamma ray flashes

Abstract

Monte Carlo simulations are used to determine source properties of terrestrial gamma ray flashes (TGFs) as a function of atmospheric column depth and beaming geometry. The total mass per unit area traversed by all the runaway electrons (i.e., the total grammage) during a TGF, Ξ, is introduced, defined to be the total distance traveled by all the runaway electrons along the electric field lines multiplied by the local air mass density along their paths. It is shown that key properties of TGFs may be directly calculated from Ξ and its time derivative, including the gamma ray emission rate, the current moment, and the optical power of the TGF. For the calculations presented in this paper, a standard TGF gamma ray fluence, F0 = 0.1 cm−2 above 100 keV for a spacecraft altitude of 500 km, and a standard total grammage, Ξ0 = 1018 g/cm2, are introduced, and results are presented in terms of these values. In particular, the current moments caused by the runaway electrons and their accompanying ionization are found for a standard TGF fluence, as a function of source altitude and beaming geometry, allowing a direct comparison between the gamma rays measured in low‐Earth orbit and the VLF‐LF radio frequency emissions recorded on the ground. Such comparisons should help test and constrain TGF models and help identify the roles of lightning leaders and streamers in the production of TGFs.

Analysis of Individual Terrestrial Gamma‐Ray Flashes With Lightning Leader Models and Fermi Gamma‐Ray Burst Monitor Data

“…In the present paper, using satellite-measured photon fluence and corresponding best fit leader model, we have estimated this number in three steps. If we take the avalanche multiplication into account, most estimated numbers of seeds are consistent with previously reported estimations using measurements of radio signals and gamma-ray fluxes (e.g., Cummer et al, 2015;Dwyer et al, 2017). Second, we calculate the number of source bremsstrahlung photons that is required at the production altitude in order to reproduce the gamma-ray flux obtained in the first step.…”

A Terrestrial Gamma‐Ray Flash inside the Eyewall of Hurricane Patricia

“…The goal of these observations was to measure the brightness and energy spectra of nearby TGFs to compare to models and satellite data. TGFs observed by satellites are consistent with simulations of an RREA process producing 10 16 –10 19 gamma rays (Cummer et al, ; Dwyer et al, ; Mailyan et al, ). Airborne observations provide an opportunity to discover if there exist TGFs too faint or low in the atmosphere to be seen by satellites, as well as a means to observe TGFs from angles and positions impossible from space.…”

“…A terrestrial gamma-ray flash inside the eyewall of Hurricane Patricia. Journal of Geophysical Research: Atmospheres, 123, 4977-4987. https://doi.org/10.1029/ 2017JD027771 producing 10 16 -10 19 gamma rays (Cummer et al, 2005;Dwyer et al, 2017;Mailyan et al, 2016). Airborne observations provide an opportunity to discover if there exist TGFs too faint or low in the atmosphere to be seen by satellites, as well as a means to observe TGFs from angles and positions impossible from space.…”

Elves Accompanying Terrestrial Gamma Ray Flashes

“…The modeling results of relativistic feedback discharges show that the peak photon emission rate can be a few times larger for shorter TGFs than longer ones (Dwyer, ; Liu & Dwyer, ). On the other hand, the total fluences of TGFs of different durations are on the same order of magnitude (Dwyer et al, ). TGFs with shorter durations are expected to produce larger peak current moments, because the TGF discharge current is spread out over a shorter time (Dwyer, ; Dwyer & Cummer, ).…”