Role of Magnetic Arcades in Explaining the Puzzle of the Gamma-Ray Emission from the Solar Disk

Abstract

The interpretation of gamma-ray emission originating from the solar disk (0.5° in angular size) as due to the interaction of Galactic Cosmic Rays (GCRs) with the solar atmosphere has remained a central challenge in solar physics. After the seminal work by Seckel, Stanev, and Gaisser based on GCRs’ magnetic mirroring, discrepancies between models and observations persist, indicating the need for a novel approach. The present work focuses on exploring the impact of a closed magnetic field geometry in the low photosphere on the observed gamma-ray flux. We track numerically with the PLUTO code the trajectories of test-particle protons within a static ∼20 Mm scale height magnetic arcade adjacent to jets. By making use of numerical vertical density profiles, we inject particles at distinct chromospheric/photospheric altitudes, mimicking the migration of GCRs from neighboring flux tubes into closed arcades. Remarkably, our model reproduces a flat gamma-ray spectrum below ∼33 GeV, a nearly isotropic emission at ∼10 GeV, both consistent with Fermi-LAT observations, and a near-limb emission at ∼1 TeV. Our model can also reproduce the flux-drop detected by HAWC (∼1 TeV). Finally, we argue that the spectral dip observed at ∼40 GeV may result from the flux suppression at low energy due to the cross-field diffusion, which would produce a cutoff. These findings underscore the pivotal role of closed magnetic field structures in shaping the solar disk gamma-ray emission.

Keywords

• Solar atmosphere
• Solar magnetic fields
• Galactic cosmic rays
• Solar gamma-ray emission