Species-Dependent and Azimuthally-Directional Flux Changes of Dispersionless Ion Injections Inside Geosynchronous Orbit

Abstract

Dispersionless injection, involving sudden, simultaneous flux enhancements of energetic particles over a broad range of energy, is a characteristic signature of the particles that are experiencing a significant acceleration and/or rapid inward transport process. To provide clues to the physical processes that lead to the acceleration and transport of energetic ions in the dispersionless injection region, we conduct superposed epoch analyses of 75 dispersionless injection events identified by Van Allen Probes with focus on the species- and azimuthal angle- (φ) dependent signatures of ∼50–600 keV ions inside geosynchronous orbit. Our analysis shows that, on average, the light (hydrogen and helium) ion fluxes undergo a rapid, transient enhancement, while the heavy (oxygen) ion fluxes exhibit a more gradual, persisting enhancement. Such a species-dependent behavior could be explained in terms of different gyro-radius of the ion species. For events where the proton injection onset is 30–60 s earlier than the electron one, proton fluxes initially increase at small φ values (i.e., tailward guiding centers) and then at larger φ values (earthward ones). The initial signatures suggest a result of the earthward transport of injected protons, as seen at the explosive growth phase. For events where both electron and proton fluxes increase simultaneously, on the other hand, proton fluxes isotropically increase with no significant φ dependence. Such an isotropic proton flux enhancement may imply a local process in which charged protons are rapidly accelerated to higher energies at the spacecraft location.