Optical emissions associated with terrestrial gamma-ray flashes

In this paper, we present modeling studies on optical emissions resulting from the excitation of air molecules produced by the large population of electrons involved in TGF events based on two production mechanisms: relativistic runaway electron avalanches (RREA) and production of thermal runaway electrons by high-potential +IC lightning leaders. Numerical models used in this study are first validated through comparison with available laboratory observations. Using Monte Carlo simulations, we show that electron energy distributions established from the two TGF production mechanisms are inherently different over the full energy range, mainly because of the difference in the driving electric fields. Furthermore, we show that TGFs are most likely accompanied with detectable levels of optical emissions. We also demonstrate that, due to the fundamental difference in the acceleration and avalanche multiplication processes undergone by runaway electrons, optical emissions generated by the two viable TGF production mechanisms are intrinsically different. These distinct optical features are of significant interests for constraining and validating current TGF production models.