Effects of a localized dipolarization front on energetic particle injections inside geosynchronous orbit

Abstract:

We study the dynamical evolution and spatial scale of pre-midnight energetic particle injections inside geosynchronous orbit (GEO) during a substorm on 23 December 2016, using four closely-located satellites at and inside GEO. Just following the substorm onset, the four spacecraft, a LANL satellite at GEO, the two Van Allen Probes (also called "RBSP") at ~5.8 RE, and a THEMIS satellite at ~5.3 RE, observed substorm-related particle injections and local dipolarizations near the central meridian of a global wedge-like current system. The large-scale evolution of the electron and ion (H, He, and O) injections was almost identical at the two RBSP spacecraft with ~0.5 RE apart. However, the initial short-timescale particle injections exhibited a striking difference between RBSP-A and -B: RBSP-B observed an energy dispersionless injection which occurred concurrently with a transient, strong dipolarization front (DF); RBSP-A measured a dispersed/weaker injection with no corresponding DF. The spatiotemporally-localized DF was accompanied by an impulsive, westward electric field. The fast, impulsive ExB drift caused the radial transport of the electron and ion injection regions from GEO to ~5.8 RE. The penetrating DF fields significantly altered the rapid energy- and pitch angle-dependent flux changes of the electrons and the H and He ions inside GEO. Such flux distributions could reflect the transient DF-related particle acceleration and/or transport processes occurring inside GEO. In contrast, O ions were little affected by the DF fields.