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Thunderstorms are known to create terrestrial gamma ray flashes (TGFs) which are microsecond-long bursts created by runaway of thermal electrons from propagating lightning leaders, as well as gamma ray glows that possibly are created by relativistic runaway electron avalanches (RREA) that can last for minutes or more and are sometimes terminated by a discharge. In this work we predict a new intermediate thunderstorm radiation mechanism, which we call TGF afterglow, as it is caused by the capture of photonuclear neutrons produced by a TGF. TGF afterglows are milliseconds to seconds long; this duration is caused by the thermalization time of the intermediate neutrons. TGF afterglows indicate that the primary TGF has produced photons in the energy range of 10-30 MeV; they are nondirectional in contrast to the primary TGF. Gurevich et al. might have reported TGF afterglows in 2011.

Plain Language Summary Thunderstorms are known to create high-energy radiation, such as short flashes of gamma rays called terrestrial gamma ray flashes (TGFs) and long duration gamma ray glows. Whereas TGFs are the result of the developing lightning discharge, lightning is observed to terminate the usual gamma ray glows. In this work we predict a new intermediate thunderstorm radiation mechanism, which we call TGF afterglow. It is causally related to a TGF as it is formed by intermediate neutrons generated in the TGF through the photonuclear interaction. It is of intermediate duration, longer than the microsecond fast TGF and faster than the seconds to minutes long gamma ray glow. We show, by means of Monte Carlo simulations, that TGF afterglows produce detectable signals above the cosmic ray background. TGF afterglows might have been observed by Gurevich et al. in 2011.