Constraining source properties using TGF-associated radio emissions
Résumé
Terrestrial gamma ray flashes (TGFs) are sub-millisecond bursts of energetic photons produced by the bremsstrahlung from electrons accelerated in the electric fields in thunderstorms. In sufficiently high fields, high-energy electrons can "run away" and create what is known as relativistic runaway electron avalanches (RREAs). TGFs have been routinely observed from space, and occasionally from ground, since their discovery. Some observations have been found to be concurrent with radio emissions detected on ground. Different radio signals have been linked to TGFs, most notably EIPs [e.g., Lyu et al, GRL, 42, 6836-6843, 2015] and slow LF pulses [e.g., Cummer et al., GRL, 38, L14810, 2011]. Moreover, radio signals associated with TGFs may lead to very useful information about TGF production processes [Dwyer and Cummer, JGR, 118, 3769-3790, 2013]. In the present work we study the generation of electromagnetic radiation from TGF sources to constrain the context of TGF-producing electron beams. We use a Monte Carlo model that considers every individual collision of electrons with air molecules [Celestin and Pasko, JGR, 116, A03315, 2011]. This allows us to explicitly quantify the density of low-energy secondary electrons and ions produced during RREAs for various field configurations. We simulate the radio wave propagation using an FDTD model [Marshall et al., JGR, 117, A03316,2012], and compare the results with recent measurements [e.g., Pu et al., GRL, 46, 6990-6997, 2019]. The timescale of the field collapse in the acceleration region and the amplitude of the current moment are of special interest to constrain the geometry of TGF-producing RREAs.