Optical emissions associated with terrestrial gamma ray flashes

International audienceTerrestrial gamma ray flashes (TGFs) are high-energy photon bursts produced by high-energyelectrons originating in the Earth’s atmosphere through bremsstrahlung processes. In this paper, we presentmodeling studies on optical emissions resulting from the excitation of air molecules produced by thelarge population of electrons involved in TGF events based on two possible production mechanisms:relativistic runaway electron avalanches (RREAs) and acceleration of thermal runaway electrons producedby high-potential intracloud lightning leaders. Numerical models developed in this study are first validatedthrough the calculation of fluorescence emissions from air excited by energetic electrons and comparisonwith available laboratory observations. Detailed discussion of the role of excitation and ionization collisionson the formation of the electron energy distribution is presented. Moreover, using Monte Carlo simulations,we show that electron energy distributions established from the two TGF production mechanismsconsidered here are inherently different over the full energy range. The strong energy dependence of thecapability of electrons to generate excited states responsible for optical emissions from neutral and ionizednitrogen molecules leads to intrinsic differences in optical emissions produced by different mechanismsof TGF production. We also show that TGFs are most likely accompanied by detectable levels of opticalemissions and that the distinct optical features are of significant interest for constraining and validatingcurrent TGF production models