Fermi-LAT Observations of Two Gamma-Ray Emission Components from the Quiescent Sun

TL;DR

This paper reports the first clear separation and measurement of gamma-ray emissions from the quiescent Sun, identifying both disk and extended inverse Compton components with high significance using Fermi-LAT data.

Contribution

It provides the first high-statistics detection and separation of solar disk and extended gamma-ray emissions, confirming inverse Compton predictions and challenging existing models of solar gamma-ray flux.

Findings

Solar disk gamma-ray flux is about 7 times higher than models predicted.

Extended inverse Compton emission matches theoretical predictions.

Distinct separation of disk and extended emission components achieved.

Abstract

We report the detection of high-energy gamma rays from the quiescent Sun with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope (Fermi) during the first 18 months of the mission. These observations correspond to the recent period of low solar activity when the emission induced by cosmic rays is brightest. For the first time, the high statistical significance of the observations allows clear separation of the two components: the point-like emission from the solar disk due to cosmic ray cascades in the solar atmosphere, and extended emission from the inverse Compton scattering of cosmic ray electrons on solar photons in the heliosphere. The observed integral flux (>100 MeV) from the solar disk is (4.6 +/- 0.2 [statistical error] +1.0/-0.8 [systematic error]) x10^{-7} cm^{-2} s^{-1}, which is ~7 times higher than predicted by the "nominal" model of Seckel et al. (1991). In contrast, the observed integral flux (>100 MeV) of the extended emission from a region of 20 deg radius centered on the Sun, but excluding the disk itself, (6.8 +/-0.7 [stat.] +0.5/-0.4 [syst.]) x10^{-7} cm^{-2} s^{-1}, along with the observed spectrum and the angular profile, are in good agreement with the theoretical predictions for the inverse Compton emission.