Compton scattering in terrestrial gamma-ray flashes detected with the Fermi gamma-ray burst monitor

Max-Planck-Institut fur extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany(Received 30 June 2014; published 20 August 2014)Terrestrial gamma-ray flashes (TGFs) are short intense flashes of gamma rays associated with lightningactivity in thunderstorms. Using Monte Carlo simulations of the relativistic runaway electron avalanche(RREA) process, theoretical predictions for the temporal and spectral evolution of TGFs are compared toobservations made with the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray SpaceTelescope. Assuming a single source altitude of 15 km, a comparison of simulations to data is performedfor a range of empirically chosen source electron variation time scales. The data exhibit a clear softeningwith increased source distance, in qualitativeagreement with theoretical predictions. The simulated spectrafollow this trend in the data, but tend to underestimate the observed hardness. Such a discrepancy mayimply that the basic RREA model is not sufficient. Alternatively, a TGF beam that is tilted with respect tothe zenith could produce an evolution with source distance that is compatiblewith the data. Based on theseresults, we propose that the source electron distributions of TGFs observed by GBM vary on time scales ofat least tens of microseconds, with an upper limit of ∼100 μs.