The impact of Solar magnetic field configurations on the production of gamma rays at the Solar disk

TL;DR

This work investigates how solar magnetic-field configurations shape the steady gamma-ray emission from Galactic cosmic-ray interactions with the solar atmosphere. By propagating protons with CRPropa 3.2 through both a baseline DQCS field and PFSS extrapolations, and by including realistic hadronic interactions and chromospheric densities, the authors quantify energy- and location-dependent gamma-ray production on the solar disk and produce maps of production sites across the solar cycle. They find that magnetic mirroring and global field topology noticeably alter both the spectral shape and spatial distribution, with stronger effects at certain energies and solar activity levels; while baseline fluxes fall short of FERMI/HAWC data, they outline plausible enhancements from heavier nuclei and deeper atmospheric interactions and provide a neutrino flux estimate near current IceCube limits. The study underscores the potential of gamma-ray observations as probes of solar magnetic structure and highlights the need for multi-scale, more sophisticated magnetic-field models to fully explain the observations and to predict future detections at higher energies or with next-generation detectors.

Abstract

The Sun produces a steady signal of high-energy gamma rays through interactions of Galactic cosmic rays (GCRs) with its atmosphere. Observations with Fermi-LAT and HAWC have revealed a gamma-ray flux significantly higher than early theoretical predictions, with unexpected temporal and spectral features that suggest a crucial role of the solar magnetic field. In this work, we model GCR-induced gamma-ray emission at the solar disk using the CRPropa framework with realistic hadronic interactions, chromospheric density profiles, and several magnetic field configurations over the solar cycle. This allows us to quantify the gamma-ray emission of the entire solar disk for different phases of the solar activity cycle and we present, for the first time, maps of the production locations of gamma rays on the solar surface. We consider both mono-energetic and realistic power-law injection spectra in a simplified dipole-quadrupole current sheet model and potential-field source surface (PFSS) extrapolations for Carrington rotations during solar maximum and minimum. Our results show that magnetic mirroring and large-scale field topology strongly affect the spectral shape and spatial distribution of the emission, with slightly enhanced fluxes predicted at solar minimum. While our simulated baseline fluxes remain below observations, additional effects, such as heavier nuclei, Parker-field mirroring, and deeper atmospheric interactions, could result in further enhancements of fluxes closer to observational values. Hadronic interactions do not only produce gamma rays but also neutrinos. We estimate the expected neutrino flux from the Sun based on our gamma-ray predictions. We find that the expected flux is slightly below current upper limits from IceCube.

Figures (14)

• Figure 1: Schematic illustration of the simulation setup.
• Figure 2: Magnetic field configuration in the DQCS and the PFSS model for three different Carrington Rotations.
• Figure 3: Density profile for the chromosphere and the corona. The dashed grey line indicates the starting position of the GCRs.
• Figure 4: Parametrisation of the local interstellar spectrum with the data measured by AMS-02 AMS, CALET Calet, CREAM Cream, Pamela Pamela, Voyager Voyager13Voyager16Voyager19 and DAMPE Dampe.
• Figure 5: Production height of the gamma rays over the surface in different magnetic field configurations. The column denotes the magnetic field and the row the initial particle energy. The dotted line shows the minimal resolvable gamma ray density with 1 photon per spatial bin.