Estimation of the solar flare neutron worst-case fluxes and fluences for missions traveling close to the Sun

摘要

A method to estimate the total fluence of solar flare neutrons at a spacecraft traveling in the innermost part of the heliosphere (at heliocentric radial distances of <1 AU) is presented. The results of the neutron production and emissivity codes of Hua and Lingenfelter (1987a, 1987b) scaled to one of the largest solar neutron events ever observed at the Earth are used to derive a conservative estimate of the energy spectrum of neutrons emitted from the Sun after a large solar flare. By taking into account the survival probability of a neutron to reach a certain heliocentric distance, we evaluate the observed time-integrated spectrum of solar neutrons as a function of the heliocentric distance of the observer. By considering (1) a working relationship between the soft X-ray class of a flare and the flare's production of solar neutrons, and (2) the number and size of soft X-ray flares that may occur during a mission traveling close to the Sun, we compute an upper limit for the total fluence of solar neutrons at energies >1 MeV, >10 MeV, >100 MeV and >1000 MeV to which such a mission may be exposed. We apply this method to the Solar Probe Plus mission. Although our method gives a conservative estimate of neutron fluxes, the predicted mission-integrated fluence of solar neutrons at Solar Probe Plus is orders of magnitude below that of solar energetic protons.