The Source Brightness Distribution of Terrestrial Gamma‐Ray Flashes From the ALOFT Flight Campaign

Abstract Terrestrial Gamma‐ray Flashes (TGFs) are naturally occurring phenomena that consist of bursts of gamma radiation associated with thunderclouds. Measurements of TGFs have been predominantly based on satellite instruments due to the significantly larger observation area compared to ground measurements. Due to atmospheric attenuation of the gamma rays between the TGF source deep in the atmosphere, there is a minimum brightness threshold TGFs must reach to be detectable from space. How many TGFs occur below this threshold and the minimum possible source brightness of a TGF is still not known. In this paper we show 43 TGFs with reliable locations detected by the Airborne Lightning Observatory for Fly's Eye Geostationary Lightning Mapper (GLM) Simulator (FEGS) and TGFs (ALOFT) flight campaign, which recorded observations using the NASA ER‐2 aircraft at an altitude of 20 km, targeting gamma‐ray glowing thunderclouds and TGFs. Monte Carlo simulations were used to estimate the source photon brightness of the TGFs. Based on the results for 37 of the 43 TGFs, it was found that the TGF source brightness distribution follows a power‐law with index with min max which hardens below , >100 keV photons at source. Additionally, it was found that at least 97% of the ALOFT TGFs cannot be detected from space, and TGFs can have source brightnesses down to five orders of magnitude lower than what is detectable from space.