Self-Consistent Modeling of Relativistic Electron Beams Giving Rise to Terrestrial Gammaray Flashes

Terrestrial gamma ray flashes (TGFs), which occur during thunderstorms, are very short bursts of gamma rays produced by bremsstrahlung emission from energetic electrons created in Relativistic Runaway Electron Avalanches (RREAs). This phenomenon can only occur under a relatively high electric field, which allows the electrons to accelerate continuously and create even more secondary high-energy runaway electrons while partially ionizing the air. How such phenomenon establishes itself within the environment of thunderclouds remains up for debate. In recent years, radio emissions over a wide range of frequencies have been observed in association with TGFs. For example, Energetic In-cloud Pulses (EIPs) [e.g., Lyu et al, GRL, 42, 68366843, 2015], slow LF pulses [e.g., Cummer et al., GRL, 38, L14810, 2011], and VHF emissions [e.g., Lyu et al, GRL, 45, 2097-2105, 2018]. In this work, we seek to estimate radio emissions associated with TGFs on a wider range than in previous theoretical work. For that purpose, we developed a collisional Monte Carlo code coupled with an electromagnetic Particle-In-Cell (PIC) model to simulate relativistic electron avalanches and the associated electromagnetic field produced by high- and low-energy particles. In so doing, we obtain self-consistent simulations of RREAs, hence constraining deeper the production context of TGFs. A better understanding of these radio waves can provide critical information about the mechanisms underlying TGFs.