Modeling Low‐Frequency Radio Emissions From Terrestrial Gamma Ray Flash Sources

Abstract : Relativistic runaway electron avalanches (RREAs) occur when electrons in electric fields in air reach energies above which they gain more energy from the electric field than they lose to collisions with the surrounding atmosphere. RREAs are known to happen in the electric fields in thunderstorms, and are considered to be the mechanism responsible for producing Terrestrial Gamma-ray Flashes (TGFs). As RREAs propagate, they leave a trail of low-energy electrons and positive and negative ions behind. These populations of charged particles will carry currents as they move in the thunderstorm electric field. In the present work, we model the charged species left behind by the propagating RREA, and the resulting radio emissions in the context of injection of thermal runaway seed electrons by a leader. We find that for certain initial conditions, these radio emissions match the slow low-frequency (LF) pulses that have previously been observed concurrently with TGFs. This confirms that the slow LF pulses are likely generated directly by the TGF source itself, as has been previously suggested using a different TGF production model. Slow LF pulses may therefore potentially be used to infer characteristic properties of TGF sources.