The production of low-energy neutrons in solar flares and the importance of their detection in the inner heliosphere

Abstract

Neutron detectors on spacecraft in the inner heliosphere can observe the low-energy (<30MeV) solar-flare neutrons that are not easily observable at Earth because they are lost to decay during transit. We present calculations of low-energy neutron production using a computer code incorporating updated neutron-production cross sections for the proton and α-particle reactions with heavier elements at all ion energies, especially at low energies (E ion < 10MeVnucleon -1) most important for producing low-energy neutrons from these reactions. We calculate escaping-neutron spectra and neutron-capture line yields from ions propagating in a magnetic loop with various kinetic-energy spectra. This study provides the basis for planning inner-heliospheric missions having a low-energy neutron detector. The MESSENGER spacecraft orbiting Mercury has such a detector. We conclude that a full understanding of ion acceleration, transport, and interaction at the Sun requires observation of both neutrons and gamma rays with detectors of comparable sensitivity. We find that the neutron-capture line fluence at 1AU is comparable to the 1-10MeV neutron fluence at 0.5AU, and therefore as effective for revealing low-energy ion acceleration. However, as the distance from the Sun to the neutron detector decreases, the tremendous increase of the low-energy neutron flux allows exploration of ion acceleration in weak flares not previously observable and may reveal acceleration at other sites not previously detected where low-energy neutrons could be the only high-energy signature of ion acceleration. Also, a measurement of the low-energy neutron spectrum will provide important information about the accelerated-ion spectrum that is not available from the capture line fluence measurement alone. © 2012. The American Astronomical Society. All rights reserved.