First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle

TL;DR

This study analyzes 15 years of Fermi-LAT data to detect and model solar halo gamma-ray emission caused by cosmic-ray interactions, revealing time-dependent solar modulation effects and spatial properties of the solar environment.

Contribution

Developed a novel analysis method for moving sources, enabling the first detailed detection and modeling of solar halo gamma rays over a full solar cycle.

Findings

Detected solar halo gamma rays with high significance between 31.6 MeV and 100 GeV.

First to observe time-dependent changes in solar modulation potential.

Provided new insights into cosmic-ray interactions near the Sun.

Abstract

We analyze 15 years of Fermi-LAT data and produce a detailed model of the Sun's inverse-Compton scattering emission (solar halo), which is powered by interactions between ambient cosmic-ray electrons and positrons with sunlight. By developing a novel analysis method to analyze moving sources, we robustly detect the solar halo at energies between 31.6 MeV and 100 GeV, and angular extensions up to 45$^\circ$ from the Sun, providing new insight into spatial regions where there are no direct measurements of the galactic cosmic-ray flux. The large statistical significance of our signal allows us to sub-divide the data and provide the first $\gamma$-ray probes into the time-variation and azimuthal asymmetry of the solar modulation potential, finding time-dependent changes in solar modulation both parallel and perpendicular to the ecliptic plane. Our results are consistent with (but with independent uncertainties from) local cosmic-ray measurements, unlocking new probes into both astrophysical and beyond-standard-model processes near the solar surface.