The Importance of Solar Neutrons in Understanding Solar Flares
Abstract
Solar flares can be prodigious producers of energetic particles, and are thus rich laboratories for studying particle acceleration physics. However, few of the particles accelerated in flares reach spacecraft to be measured. Instead, we typically know of these particles through the secondary neutral radiation they emit when they interact with the solar atmosphere. This radiation takes various forms in the gamma channel, such as lines, pion-related emission and bremsstrahlung. Neutrons are also produced and are most readily seen when they combine with hydrogen producing deuterium and a 2.223 MeV gamma ray. Gamma-ray and neutron observations, in particular, sample the energy spectrum of solar flare protons and ions in specific and not necessarily overlapping energy ranges. The combination of gamma-ray and neutron time-dependent spectra provides a strong constraint on the spectral shape and the temporal evolution of flare particles, since the production cross sections are quite different and depend on the accelerated proton/ion energy. Neutrons bridge an important gap in ion energies at the solar flare between gamma production from nuclear excitations (1-10 MeV) and gamma production from pion decay (> 300 MeV). The production of neutrons that survive to reach Earth also essentially follows the accelerated ion power-law behavior at high energies but more importantly covers a very broad range in ion energy starting from 50 MeV/nucleon with no upper limit, overlapping with the pion channel above 300 MeV/nuc. The spectral shape and composition of solar flare energetic particles have been determined almost exclusively by gamma-ray observations. Solar neutron observations are rare and have been relegated primarily to non-spectroscopic observations. A solar neutron measurement from 20-200 MeV is key when combined with high- and low-energy gamma-ray measurements. The Solar Neutron TRACking (SONTRAC) Concept, based on scintillating-fiber bundles with silicon photomultiplier readout, would provide high-resolution imaging of fast neutrons at energies where the bulk of solar neutrons resides. We discuss the importance of solar neutron observations and highlight the recent performance of the SONTRAC instrument.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSH31C3307D
- Keywords:
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- 7534 Radio emissions;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY;
- 7549 Ultraviolet emissions;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY;
- 7554 X-rays;
- gamma rays;
- and neutrinos;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY;
- 7594 Instruments and techniques;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY