The Sun at TeV energies: gammas, neutrons, neutrinos and a cosmic ray shadow

TL;DR

This paper models how cosmic rays interact with the Sun, producing observable gamma rays, neutrons, and neutrinos, and uses the solar shadow effect to infer properties of the solar interior and magnetic field.

Contribution

It introduces a new approach using Liouville's theorem to connect the solar cosmic ray shadow with secondary particle fluxes, providing predictions consistent with observations.

Findings

Gamma flux matches Fermi-LAT data

Neutron flux estimated at 100-300 per year per m^2

Neutrino flux exceeds atmospheric background at >0.1 TeV

Abstract

High energy cosmic rays reach the surface of the Sun and start showers with thousands of secondary particles. Most of them will be absorbed by the Sun, but a fraction of the neutral ones will escape and reach the Earth. Here we incorporate a new ingredient that is essential to understand the flux of these solar particles: the cosmic ray shadow of the Sun. We use Liouville's theorem to argue that the only effect of the solar magnetic field on the isotropic cosmic ray flux is to interrupt some of the trajectories that were aiming to the Earth and create a shadow. This shadow reveals the average solar depth crossed by cosmic rays of a given rigidity. The absorbed cosmic ray flux is then processed in the thin Solar surface and, assuming that the emission of neutral particles by low-energy charged particles is isotropic, we obtain (i) a flux of gammas that is consistent with Fermi-LAT observations, (ii) a flux of 100-300 neutrons/(year m^2) produced basically in the spallation of primary He nuclei, and (iii) a neutrino flux that is above the atmospheric neutrino background at energies above 0.1-0.6 TeV (depending on the solar phase and the zenith inclination). More precise measurements of the cosmic ray shadow and of the solar gamma flux, together with the possible discovery of the neutron and neutrino signals, would provide valuable information about the magnetic field, the cycle, and the interior of the Sun.