Origin of the 30–50 GeV dip in the solar disk gamma-ray spectrum

Determine the physical origin of the 30–50 GeV dip feature in the solar disk gamma-ray spectrum observed by Fermi-LAT and reconcile this feature with existing simulations of cosmic-ray interactions with the solar atmosphere (e.g., FLUKA and Geant4 with PFSS/Bifrost magnetic field configurations), which currently do not reproduce or explain the dip.

Background

Fermi-LAT observations of the quiescent Sun have revealed a puzzling spectral dip in the 30–50 GeV energy range. This feature challenges standard models where gamma rays are produced by cosmic-ray interactions in the solar atmosphere and influenced by magnetic fields. Multiple simulation frameworks, including FLUKA and Geant4 implementations with detailed solar magnetic field models (PFSS and Bifrost), have not accounted for or reproduced this dip.

Clarifying the origin of this spectral dip is important both to understand high-energy processes on the solar disk and to resolve discrepancies between data and theory, particularly since the solar disk gamma-ray spectrum can exceed expected fluxes at the highest energies.