Evidence for a New Component of High-Energy Solar Gamma-Ray Production

TL;DR

This paper presents evidence for a new equatorial component in high-energy solar gamma-ray production, revealing distinct mechanisms and spectral features during different solar activity phases, challenging existing theories.

Contribution

It uncovers a previously unknown equatorial gamma-ray component and provides detailed imaging, advancing understanding of solar gamma-ray emission mechanisms.

Findings

Identification of a new equatorial gamma-ray component during solar minimum

Detection of a spectral dip at 30-50 GeV in the gamma-ray spectrum

Observation of 6 gamma rays above 100 GeV during solar minimum, none during solar maximum

Abstract

The observed multi-GeV gamma-ray emission from the solar disk --- sourced by hadronic cosmic rays interacting with gas, and affected by complex magnetic fields --- is not understood. Utilizing an improved analysis of the Fermi-LAT data that includes the first resolved imaging of the disk, we find strong evidence that this emission is produced by two separate mechanisms. Between 2010-2017 (the rise to and fall from solar maximum), the gamma-ray emission is dominated by a polar component. Between 2008-2009 (solar minimum) this component remains present, but the total emission is instead dominated by a new equatorial component with a brighter flux and harder spectrum. Most strikingly, although 6 gamma rays above 100 GeV are observed during the 1.4 years of solar minimum, none are observed during the next 7.8 years. These features, along with a 30-50 GeV spectral dip which will be discussed in a companion paper, were not anticipated by theory. To understand the underlying physics, Fermi and HAWC observations of the imminent Cycle 25 solar minimum are crucial.