GEANT4 Studies of Electromagnetic Shower Propagation in the Earth's Atmosphere

Terrestrial Gamma-ray Flashes (TGFs), first observed by Earth-orbiting satellites in the 1990s, are now a well-known component of lightning emission. They have since been observed numerous times on the ground, in association with both natural and anthropogenic lightning. While the term TGF was applied to these phenomena due to their discovery in spaceborne gamma-ray detectors, a better description of their nature might be "Electromagnetic Showers of Terrestrial Origin". Independent of whether the primary particle is an energetic (MeV or higher) electron or a gamma ray, after passing through several radiation lengths of material (air) the result will be a shower of secondary particles composed of a mixture of photons, electrons and positrons. This has important consequences for the detection and interpretation of TGF events. In this presentation, we describe simulations of TGF-like electromagnetic showers carried out using the GEANT4 toolkit for the simulation of the passage of particles through matter. We determine the particle makeup of these showers and their energies, and how these evolve with the number of radiation lengths traversed. We consider possible effects due to interactions of the charged particles with the Earth's magnetic field. Then, we apply these calculations to recent space- and ground-based TGF observations including those of the European Space Agency's ASIM detector and the Telescope Array cosmic ray observatory in western Utah, U.S.A.