High energy cosmic-ray interactions with particles from the Sun

Abstract

Cosmic-ray protons with energies above ${10}^{16}\text{ }\text{ }\mathrm{eV}$ passing near the Sun may interact with photons emitted by the Sun and be excited to a ${\ensuremath{\Delta}}^{+}$ resonance. When the ${\ensuremath{\Delta}}^{+}$ 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 ${\ensuremath{\pi}}^{0}$ 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\ifmmode^\circ\else\textdegree\fi{} around the Sun, the flux of photon pairs is about $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }\mathrm{\text{particles}}/({\mathrm{km}}^{2}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{yr})$, while the flux of muons is about $0.33\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }\mathrm{\text{particles}}/({\mathrm{km}}^{2}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{yr})$. 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 occurring close to the Sun are also briefly discussed.