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The interpretation of gamma-ray emission originating from the solar disk (0.5 deg 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 (SSG91) 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.
Credit: Puzzoni et al. 2024
Gamma-ray emission in the GeV–TeV range from the solar disk is likely to arise from collisions of galactic cosmic rays (GCRs) with solar atmospheric plasma. In a previous study, we demonstrated that closed turbulent magnetic arcades trap GCRs efficiently, leading to a gamma-ray flux consistent with the Fermi-HAWC observations (from ∼0.1 GeV to ∼1 TeV). Here, we model a synthetic magnetic field with a static, laminar structure of open field lines in the chromosphere, increasingly braided near the solar surface, with a scale height of ∼10‑2 R⊙. The height-dependent increase in magnetic field line braiding is modulated by an exponential scalar function, mimicking the bending of the photo- and chromospheric magnetic field revealed by polarimetric observations and reproduced by MHD simulations. Employing 3D test-particle numerical simulations, we investigate how distorted magnetic field lines affect the gamma-ray production by injecting GeV–TeV protons into both magnetically laminar and braided regions. We find that with the chosen spatial resolution, this synthetic magnetic field can account for the >10 GeV gamma-ray spectrum observed by Fermi/HAWC. A rebrightening between approximately 30 and 100 GeV (following a ∼30 GeV spectral dip) suggests an enhanced confinement within the photo-/chromospheric layer by stronger braiding.
Credit: Puzzoni et al. 2025
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