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

Abstract

Fishman et al. (1994) discovered bright, short bursts of gamma-rays emerging from thunderclouds on Earth in the data from the Burst and Transient Source Experiment (BATSE) instrument aboard the Compton Gamma-Ray Observatory. These sub-millisecond bursts were dubbed Terrestrial Gamma-ray Flashes (TGFs), and have since been observed by several spacecraft, notably RHESSI (

Conditions for Inception of Relativistic Runaway Discharges in Air

“…The 10–50 μs are broadly consistent with times required for runaway electrons producing streamers to traverse the above mentioned spatial scales. The quantitative modeling of related processes indicates that slow low‐frequency (LF) pulses are likely generated directly by the TGF sources (Berge et al., 2022). It can be seen from Figure 3 that as soon as the streamer zone size reaches the threshold dδ ≃ 100 m for negative leader tip and dδ ≃ 1,000 m for positive leader tip the inception conditions for the relativistic runaway discharge will be satisfied.…”

“…(2014), Figure 6, and this can be attributed to these authors employing the simulation domain with 10 km vertical extent filled with air at sea‐level pressure leading to reduction of range scales and therefore overestimation of frequencies of all physical interactions by up to a factor of 3. Additionally, for the 10 km, the speed of light round trip time is 66/ δ μs leading to modeled TGF pulse durations on the order of 1 ms (Dwyer, 2012; Liu & Dwyer, 2013) significantly exceeding the observed ∼10 μs and ∼50 μs durations of NBEs and EIPs, respectively (Tilles et al., 2020), and slow LF pulses (Berge et al., 2022).…”

Self Consistent Modeling of Relativistic Runaway Electron Beams Giving Rise to Terrestrial Gamma‐Rays Flashes

“…As the medium is highly collisional, the dynamics of electron and ion densities is considered to evolve according to drift‐diffusion equations, as usual in discharge physics (e.g., Bourdon et al., 2007) and also used in the context of RREAs to simulate the effects of low‐energy electrons and ions (e.g., Berge et al., 2022; Liu & Dwyer, 2013): $$$\frac{\partial {n}_{e}}{\partial t}+\nabla \cdot {n}_{e}\vec{{v}_{e}}=\left({\nu }_{i}-{\nu }_{a}\right){n}_{e}$$$ $$$\frac{\partial {n}_{p}}{\partial t}+\nabla \cdot {n}_{p}\vec{{v}_{p}}={\nu }_{i}{n}_{e}$$$ $$$\frac{\partial {n}_{n}}{\partial t}+\nabla \cdot {n}_{n}\vec{{v}_{n}}={\nu }_{a}{n}_{e}$$$ where n e , n n , and n p are the electron, negative ion, and positive ion densities, respectively; ν i is the electron‐impact ionization frequency, and ν a is the electron attachment frequency considering dissociative attachment producing a negative oxygen atom $${\mathrm{O}}^{-}$$ (two‐body attachment) and a three‐body attachment producing a negative oxygen molecular ion $${\mathrm{O}}_{2}^{-}$$; and v e , v n , and v p are the electron, negative ion, and positive ion drift velocities, respectively. Because of the magnitude of the densities and the timescales of interest in the present work, we neglect ion‐ion recombination, electron‐ion recombination, and the diffusion of ions (see Berge et al., 2022, Section 2.1). Given the electron densities obtained in the present study, diffusive fluxes of electrons are much lower than drift fluxes over the smallest length scale consi...…”

On the Self‐Quenching of Relativistic Runaway Electron Avalanches Producing Terrestrial Gamma Ray Flashes

“…Even though the electron density does not vary any longer, the flux of runaway electrons is still present after this time and so does the associated production of positive ions through ionization and negative ions through attachment. As a result of the flux of runaway electrons, the ion density increases linearly in time because ion recombination processes occur over a much longer timescale (see discussion in Berge et al (2022), Section 2.1). Gourbin and Celestin (2023b) showed that the electron saturation density caused by self-consistent effects occurs when the density is high enough so that the associated relaxation time is equaling the RREA characteristic growth time:…”