Physics of Solar Neutron Production: Questionable Detection of Neutrons from the 2007 December 31 Flare

Abstract

Spacecraft observations in the inner heliosphere offer the first opportunity to measure 1-10 MeV solar neutrons. We discuss the physics of low-energy neutron production in solar flares and show that, even at interacting-particle energies of 2 MeV/nucleon, neutrons with energies >10 MeV are produced. On the other hand, a significant fraction of 1-10 MeV neutrons result from interactions of >10 MeV/nucleon ions in typical flare spectra. We calculate the escaping neutron spectra for mono-energetic and power-law particle spectra at the Sun for the location and observation angle of MESSENGER at the time of its reported detection of low-energy neutrons associated with the 2007 December 31 solar flare. We detail concerns about this questionable observation of solar neutrons: 1. the inferred number of accelerated protons at the Sun for this modest M2-class flare was 10X larger than any flare observed to date, 2. the onset and duration of the 'solar' neutron count rate was similar to that of the solar energetic particles (SEPs), and 3. the authors' argument that the SEPs were dominated by electrons and so could not have produced the neutron counts locally in the spacecraft. In contrast we argue that solar energetic protons and alpha particles, through local neutron production and accidental coincidences, were the source of most of the reported 'solar-neutron' counts.