Recent Advancements on the Sources of Inner Radiation Belt Particles

Abstract:

Soon after the discovery of Earth’s Van Allen radiation belts almost six decades ago, it was recognized that the main source of inner belt (L<2) protons, with kinetic energies of ~tens to hundreds of MeV, is Cosmic Ray Albedo Neutron Decay (CRAND), during which protons retain most of the neutrons’ kinetic energy while the electrons have lower energies, mostly below 1 MeV. The viability of the electron source was, however, uncertain because measurements showed that electron intensity can vary greatly while the neutron decay rate should be almost constant. Measurements from the Relativistic Electron-Proton Telescope integrated little experiment (REPTile) on board Colorado Student Space Weather Experiment (CSSWE) CubeSat, in a highly inclined low Earth orbit, show that CRAND is the main electron source for the radiation belt near its inner edge, and also contributes to the inner belt (L<2) elsewhere. As for the protons, measurements from Relativistic Electron-Proton Telescope (REPT) on Van Allen Probes show that the equatorial proton intensity (with energy ≲36 MeV), near L = 2 has a steady increase that is consistent with inward diffusion of trapped solar protons, as shown by positive radial gradients in phase space density at fixed values of the first two adiabatic invariants. This is a direct evidence of another significant source for the inner radiation belt protons at its outer edge in addition to the CRAND source. These will be two highlights of this presentation regarding our recent advancements in understanding of the sources of inner radiation belt particles.