Energetic Solar Particle Access to the Near-Equatorial Inner Magnetosphere

Solar proton events are comprised of energetic protons of solar and interplanetary origin. Such energetic particles are able to access the magnetosphere at various locations according to their cutoff rigidity. The specific properties of solar proton access are of great interest for space weather prediction purposes. Using Van Allen Probes/Relativistic Electron-Proton Telescope (REPT) 20-200 MeV proton data we examine four of the strongest solar proton events over the lifetime of the mission. We present evidence of the direct magnetospheric access of these energetic solar protons and find strong flux increases at L<4. Results indicate that small and sudden flux changes measured by ACE spacecraft sensors upstream of Earth are also seen in the near-equatorial inner magnetosphere. Using the East-West asymmetry of solar protons as a proxy for cutoffs we examine the highly dynamic cutoff rigidity. We find there is evidence for: (1) cutoff rigidity dependence on MLT; (2) suppressed cutoffs with rapid Dst changes; and (3) rapid evolution of cutoffs even during quiet magnetospheric conditions.