First Analysis of Earth's Stratospheric γ-Ray Emission in Geographical Coordinates With Fermi LAT

Abstract

©2020. American Geophysical Union. All Rights Reserved. From low-Earth orbit, the Earth is an extremely bright γ-ray source compared to distant astrophysical objects. The high-energy γ-ray emission, from the Earth's upper atmosphere, is produced by the interactions between cosmic rays (CRs) and air molecules. As CRs are predominantly charged particles, the geomagnetic field acts as a spectrometer, forbidding CRs below a certain energy to arrive and interact with the atmosphere at a given geomagnetic location from a given direction. We use photon data between 0.2 and 20 GeV as observed by the Fermi Large Area Telescope (LAT) to create, for the first time, stratospheric γ-ray emission maps of the Earth in the geographical coordinate system, for different viewing directions from LAT altitude. These maps confirm a strong association between measured γ-ray fluxes and geomagnetic cutoffs for CRs. We also report the spectrum of the Earth's stratospheric γ-ray emission in cardinal directions and use the geomagnetic spectrometer effect to infer the atmosphere's yield function of γ rays per incoming CR proton as a function of CR rigidity at the altitude of the Fermi spacecraft. This work is relevant to the study of near-Earth radiation exposure for astronauts and spacecraft, modeling the geomagnetic field, and CR-air interactions.