High energy cosmic-ray interactions with particles from the Sun

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Cosmic-ray protons with energies above 10¹⁶ eV passing near the Sun may interact with photons emitted by the Sun and be excited to a Δ+ resonance. When the Δ+ 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 π⁰ 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∘ around the Sun the flux of photon pairs is about 1.3e-3 particles/(km²), while the flux of muons is about 0.33e-3 particles/(km²). 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.