Modeling the Internal Redistribution of Earth's Proton Radiation Belt by Interplanetary Shocks

A large proton belt enhancement occurred on 24 March 1991 following an interplanetary shockthat impacted the dayside magnetopause at ∼03:40 UT. Its formation was measured by the proton telescopeaboard CRRES and attributed to the injection and inward transport of solar energetic particles (SEPs) by anazimuthally propagating electric field pulse induced by the shock's compression of the magnetosphere. This ledto an increase in the flux of high energy (>25 MeV) protons by several orders of magnitude at L ≈ 2.5 which hasbeen well‐studied. However, a flux enhancement by up to one order of magnitude was also seen in 1–20 MeVprotons at L ≈ 2. Protons in this energy range pose a hazard to orbiting spacecraft as a major contributor to solarcell nonionizing dose. The 1–20 MeV enhancement cannot be explained by the inward transport of a solarproton source, because a newly injected source population at the required energy would have a drift velocity toolow to interact with the pulse. Instead, we hypothesize that the 1–20 MeV enhancement was caused by theredistribution of radiation belt protons to different drift shells by the pulse. To test this hypothesis, we apply anovel method to predict the change in phase space density during a shock event which utilizes reverse‐timeparticle tracing simulations. Our results show that the 1–20 MeV enhancement can be accounted for by internalredistribution as hypothesized. We thus identify a new mechanism for proton belt enhancements that does notdepend on a SEP source and present a way to model it.