Dynamic Properties of Particle Injections Inside Geosynchronous Orbit: A Multisatellite Case Study

Abstract

Four closely located satellites at and inside geosynchronous orbit (GEO) provided a great opportunity to study the dynamical evolution and spatial scale of premidnight energetic particle injections inside GEO during a moderate substorm on 23 December 2016. Just following the substorm onset, the four spacecraft, a LANL satellite at GEO, the two Van Allen Probes (also called “RBSP”) at ~5.8 R _E, and a THEMIS satellite at ~5.3 R _E, observed substorm‐related particle injections and local dipolarizations near the central meridian (~22 MLT) of a 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 R _E 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) with a peak‐to‐peak amplitude of ~25 nT over ~25 s; RBSP‐A measured a dispersed/weaker injection with no corresponding DF. The spatiotemporally localized DF was accompanied by an impulsive, westward electric field (~20 mV m ⁻¹). The fast, impulsive E ×  B drift caused the radial transport of the electron and ion injection regions from GEO to ~5.8 R _E. 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.

Key Points

• Four spacecraft observations at and inside GEO reveal the dynamical nature of particle injections and their related local field changes

• The dispersionless injections inside GEO are highly localized in azimuth and accompanied by a sharp dipolarization front (DF)

• The DF‐related fields have a much greater impact on electron and light ion injections, but little affect heavy ion injections