Gamma-ray emission in planetary atmospheres due to relativistic runaway electron avalanches

Introduction: On Earth, relativistic electrons can accelerate inside thunderstorms due to large scale electric fields, producing x-ray and gamma ray radiation as they propagate [1]. Figure 1 shows the effective drag force acting on an electron as it moves through Earth’s atmosphere at STP [2]. The solid curve is due to inelastic collisions with air molecules and the dashed curve includes the effects of bremsstrahlung emission. In-situ measurements have found maximum electric fields near the breakeven field (Eb), which suggests that this process is common inside thunderstorms [3]. For a specific value of the electric field, > Eb, runaway electrons exist between energies εth and εmax. Above the critical value of the electric field (Ec), all thermal electrons can run away.