The Sun as a Gamma-Ray Source

Abstract

The first measurements of high-energy particles following solar flares in space and on the ground were performed during the 1940s, and by the 1960s the experiments were accurate enough to deduce spectral parameters. It was recognized that γ-ray continuum and, most importantly, γ-ray lines should be observable at high altitudes if a sufficiently large fraction of these particles interacted in the solar atmosphere. Early suggestions were made by Biermann et al. (1951) and Morrison (1958). Later, more detailed calculations (Chupp 1964; Dolan and Fazio 1965; Lingenfelter and Ramaty 1967) predicted line emission at 0.511 MeV from positron annihilation, at 2.2 MeV from neutron capture on protons, at 4.4 MeV and 6.1 MeV from deexcitation of carbon and oxygen nuclei, respectively, and a continuum at higher energies from pion decay. The discovery of high-energy bremsstrahlung photons was ruade in 1958 during the International Geophysical Year by a balloon experiment (Peterson and Winckler 1958), and the unambiguous discovery of nuclear lines from two large flares in August 1972 by the OSO-7 satellite (Chupp et al. 1973) followed. A more detailed description of historical issues can be found in the review by Chupp (1996).