High-energy solar neutrons: 1. Production in flares

Abstract

The intensity and energy spectrum of neutrons above 1 Mev produced in the solar photosphere by flare-accelerated protons and α particles is calculated. The solar neutron yield from accelerated particles on helium is found to exceed the yield from evaporation and spallation in heavier elements for incident energies above about 30 Mev and continues to dominate until about 1 bev. At higher bombarding energies, the p, π+n reaction in hydrogen is the main neutron source. More than 90% of the total solar neutron production is from knock-on reactions by protons in hydrogen and helium. Owing to this relatively high ratio of knock-on to evaporation neutrons, the solar neutron spectrum is much higher in energy than the spectrum of secondary neutrons produced in the earth's atmosphere by galactic cosmic rays. A Monte Carlo calculation is performed to determine the solar neutron flux escaping into space. The absolute intensity of the solar neutron flux at the earth and its geophysical significance will be treated in the accompanying paper, part 2.