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  • Open Access

First observations of solar halo gamma rays over a full solar cycle

Tim Linden1,2,*, Jung-Tsung Li3, Bei Zhou4,5, Isabelle John6,7, Milena Crnogorčević1, Annika H. G. Peter3,8,9, and John F. Beacom3,8,9

  • *Contact author: linden@fysik.su.se
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Phys. Rev. D 112, 103030 – Published 21 November, 2025

DOI: https://doi.org/10.1103/qm68-ng62

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 subdivide the data and provide the first γ-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 astrophysical processes near the solar surface.

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Phys. Rev. D 112, 103030– Published 21 November, 2025
  • Received 27 May 2025
  • Accepted 28 October 2025
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