First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle
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 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 -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.
Cite
@article{arxiv.2505.04625,
title = {First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle},
author = {Tim Linden and Jung-Tsung Li and Bei Zhou and Isabelle John and Milena Crnogorčević and Annika H. G. Peter and John F. Beacom},
journal= {arXiv preprint arXiv:2505.04625},
year = {2026}
}
Comments
17 pages, 11 figures. Appendix adds 3 pages and 3 figures. Matches journal version