Recent Advances in Understanding Earth’s Inner Radiation Belt

Abstract:

Soon after the discovery of Earth’s Van Allen radiation belts six decades ago it was recognized that the decay of knock-on neutrons is the main source of trapped energetic (above 10s of MeV) inner belt protons. Only recently has it been recognized [e.g., Li et al., 2017] that the decay of low energy neutrons contributes to energetic electrons (up to 782 keV) and is the dominant source of energetic electrons at the inner edge of the inner belt. A long-term study of sub-MeV electrons in the inner belt showed that at L≤1.14, where only quasi-trapped electrons exist, the electron flux is anti-correlated with sunspot number but proportional to the cosmic ray intensity, which further confirms that these electrons are produced by Cosmic Ray Albedo Neutron Decay (CRAND). At L≥1.2, however, both quasi-trapped and trapped electrons can be greatly enhanced during geomagnetic storms, indicating a different source, such as inward injection, is dominant. As for inner belt protons, detailed analysis of Van Allen Probes data showed that the solar proton source dominates at L>1.5 and energies <100 MeV and the CRAND source dominates otherwise. Inward diffusion of 10s of MeV solar protons to L<2 adds to and merges with the pre-existing trapped protons in the inner belt. Long-term measurements of 10s of MeV protons at low-Earth orbit (LEO) show a clear solar cycle variation which anti-correlates with sunspot number. However, the magnitude of the variation is much greater than the solar cycle variation of galactic cosmic rays (>GeV) that are regarded as a source of these trapped protons. With respect to protons (>36 MeV) mirroring near the magnetic equator, both measurements and simulations show no clear solar cycle variation at L>1.2, but there is clear solar cycle variation and a strong spatial proton flux gradient at L≤1.2. A comparison between measurements and simulations shows that energy loss of trapped protons due to collisions with free and bound electrons in the ionosphere and atmosphere is the dominant mechanism for the strong spatial gradient and solar cycle variation at the inner edge of the inner belt protons. This fact is also of key importance for spacecraft and instrument design and operation in near-Earth space.