Modelling of the Cosmic-Ray Induced Gamma-Ray Emission of the Earth's Atmosphere

Interactions of cosmic rays (CRs) with the Earth's atmosphere produce X-rays and γ-rays, and other secondary particles (secondary nuclei, neutrons/anti-neutrons, protons/anti-protons, and electrons/positrons) that are a strong back- ground for balloon-borne and space-based instruments. Indirect searches for particle dark matter in CRs, measurement of CRs and their modulation over the solar cycle, etc., crucially rely on understanding the secondary particle fluxes generated by the primary CR interactions in the atmosphere. Modelling the CR-induced γ-ray emission of the atmosphere provides the most useful method to understand these particle backgrounds, because a direct test can be made with observations without additional modelling and assumptions for the transport of charged CRs in the geomagnetic field. In this paper, we describe our modelling of the CR-induced γ-ray emission from the Earth's atmosphere using the GEANT4 Monte Carlo code. We compare our model with the γ-ray data from the Fermi-LAT and other instruments.