Time dependent worldwide distribution of atmospheric neutrons and of their products: 3. Neutrons from solar protons

During solar particle events from 1968 to 1971 we observed increases in the fast neutron flux at high latitude and at 55- to 75-g/cm² atmospheric depth. The increases correlated with the variations in the solar proton fluxes; the neutron yield per incident proton, above threshold, increased by a factor of 100 with increasing hardness of the proton spectrum. Within a factor of 2 the neutron specific yield fell on a smooth curve versus the spectral parameter P0, where the values of P0 were based on the SPME (solar proton monitor experiment) data from Explorer 34 and 41. The neutron yield from solar particle events was calculated from a Monte Carlo simulation of neutron production and transport in the atmosphere. We compare the observed fast neutron flux with that calculated using the solar proton spectra reported at the times of the measurements; the causes for variation among the reported proton spectra and between the calculated and the observed fast neutron flux are discussed. The calculation reproduced the results of experiments by others with moderated slow neutron counters in and above the atmosphere. We calculate that the contribution of solar particle fluxes to the production rates of neutrons, to the production rates of radiocarbon, and to the leakage rates of neutrons from the top of the atmosphere are 2–3 orders of magnitude below the galactic cosmic ray contribution during solar cycle 20.