Remote sensing of gamma-ray emission from solar energetic proton interactions with the solar wind

The properties of solar energetic particles (SEPs) in solar flares are studied through remote imaging in the radio, hard X-ray, and gamma-ray energy ranges. However, the heliospheric SEP populations are observed only in situ by satellite measurements, which drastically limits our understanding of their spatial and temporal variations. Can those SEP populations be remotely imaged, as are the solar SEPs? We consider two possibilities for detecting faint gamma-ray emission from SEP interactions with solar wind (SW) ions. First, the 6.13 and 4.44 MeV gamma-ray lines of O-16 and C-12, respectively, produced by the interactions of the SEPs from a large low-energy (E < 30 MeV) gradual event are calculated and found to be far below a detectable level. Then the expected pi(0)-decay gamma-ray emission is calculated for the intense ground-level event (GLE) of 2005 January 20 and compared with (1) the observed Galactic and extragalactic background and (2) the expected near-solar emission from inverse-Compton scattering of solar photons by cosmic-ray electrons and from Galactic cosmic-ray collisions with the solar atmosphere. It appears feasible to detect the pi(0)-decay emission from that event with a detector of the size of the Large Area Telescope on GLAST. Earlier 1982 and 1991 flare observations of long-duration (hours) pi(0)-decays were attributed to E > 300 MeV protons captured in strong coronal loops, but we suggest that the observed emission was due to SEP-SW collisions following shock acceleration on open field lines.