Solar disk gamma-rays emission via synthetic magnetic field from photosphere to low corona

Abstract

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 efficiently GCRs leading to a gamma-ray flux consistent with the Fermi-HAWC observations (from $\sim 0.1$ GeV to $\sim 1$ TeV). Here, we model a synthetic magnetic field with a static, laminar structure of open field lines in the chromosphere increasingly braiding near the solar surface, with a scale height of $\sim 10^{-2} R_\odot$. The height-dependent increase in magnetic field line braiding is modulated by an exponential scalar function, mimicking the bending of the photo- and chromo-spheric 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-rays 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-LAT/HAWC. A rebrightening between approximately $30$ and $100$ GeV (following a $\sim 30$ GeV spectral dip), suggests an enhanced confinement within the photo-/chromospheric layer by a stronger braiding.