Fermi Large Area Telescope Observations of the Cosmic-Ray Induced gamma-ray Emission of the Earth's Atmosphere

TL;DR

This paper presents detailed measurements of Earth's atmospheric gamma-ray emission induced by cosmic rays using the Fermi LAT, revealing a power-law spectrum up to 500 GeV with high precision.

Contribution

First detailed spectral and spatial analysis of Earth's atmospheric gamma-ray emission using Fermi LAT data during commissioning.

Findings

Spectrum follows a power-law up to 500 GeV

Spectral index measured as 2.79+-0.06

High-statistics dataset with 6.4 million photons

Abstract

We report on measurements of the cosmic-ray induced gamma-ray emission of Earth's atmosphere by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The LAT has observed the Earth during its commissioning phase and with a dedicated Earth-limb following observation in September 2008. These measurements yielded 6.4 x 10^6 photons with energies >100MeV and ~250hours total livetime for the highest quality data selection. This allows the study of the spatial and spectral distributions of these photons with unprecedented detail. The spectrum of the emission - often referred to as Earth albedo gamma-ray emission - has a power-law shape up to 500 GeV with spectral index Gamma = 2.79+-0.06.