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

Abstract

Terrestrial gamma-ray flashes (TGFs) are short intense flashes of gamma rays\nassociated with lightning activity in thunderstorms. Using Monte Carlo\nsimulations of the relativistic runaway electron avalanche (RREA) process,\ntheoretical predictions for the temporal and spectral evolution of TGFs are\ncompared to observations made with the Gamma-ray Burst Monitor (GBM) on board\nthe Fermi Gamma-ray Space Telescope. Assuming a single source altitude of 15\nkm, a comparison of simulations to data is performed for a range of empirically\nchosen source electron variation time scales. The data exhibit a clear\nsoftening with increased source distance, in qualitative agreement with\ntheoretical predictions. The simulated spectra follow this trend in the data,\nbut tend to underestimate the observed hardness. Such a discrepancy may imply\nthat the basic RREA model is not sufficient. Alternatively, a TGF beam that is\ntilted with respect to the zenith could produce an evolution with source\ndistance that is compatible with the data. Based on these results, we propose\nthat the source electron distributions of TGFs observed by GBM vary on time\nscales of at least tens of microseconds, with an upper limit of approx. 100\nmicroseconds.\n