High energy cosmic-ray interactions with particles from the Sun

TL;DR

This paper explores the potential detection of high-energy cosmic-ray interactions with solar photons, predicting observable muons and photons near the Sun, and discusses the feasibility with future detectors.

Contribution

It introduces a novel method to measure cosmic-ray flux via interactions near the Sun and evaluates the detectability with upcoming observatories.

Findings

Photon pair flux near the Sun is about 1.3e-3 particles/(km^2·yr).

Muon flux near the Sun is about 0.33e-3 particles/(km^2·yr).

Current detectors are insufficient, but future arrays may detect these signals.

Abstract

Cosmic-ray protons with energies above $10^{16}$ eV passing near the Sun may interact with photons emitted by the Sun and be excited to a $\Delta^+$ resonance. When the $\Delta^+$ decays, it produces pions which further decay to muons and photons which may be detected with terrestrial detectors. A flux of muons, photon pairs (from $\pi^0$ decay), or individual high-energy photons coming from near the Sun would be a rather striking signature, and the flux of these particles is a fairly direct measure of the flux of cosmic-ray nucleons, independent of the cosmic-ray composition. In a solid angle within $15^\circ$ around the Sun the flux of photon pairs is about $\SI{1.3e-3}{}$ particles/(km$^2\cdot$yr), while the flux of muons is about $\SI{0.33e-3}{}$ particles/(km$^2\cdot$yr). This is beyond the reach of current detectors like the Telescope Array, Auger, KASCADE-Grande or IceCube. However, the muon flux might be detectable by next-generation air shower arrays or neutrino detectors such as ARIANNA or ARA. We discuss the experimental prospects in some detail. Other cosmic-ray interactions occuring close to the Sun are also briefly discussed.