Preliminary indirect measurement of cosmic-ray proton spectrum using Earth’s γ-ray data from Fermi Large Area Telescope

Abstract

Cosmic rays (CRs) are high-energy particles, mostly protons, propagating in space. The rigidity (momentum per charge) spectrum of CRs is well described by a power law for which the spectral index is approximately 2.8 around 30 - 1000 GV. Recent measurements by PAMELA and AMS-02 indicate an abrupt change of the CR proton spectral index at about 340 GV. When CRs interact with the Earth’s upper atmosphere, γ rays can be produced and detected by space-based detectors. Here we use the Earth’s γ-ray data collected by the Fermi Large Area Telescope along with a proton-air interaction model to indirectly determine the CR proton spectral index and compare against observations by other instruments.