Dynamics of Ultra‐Relativistic Electrons on 19 December 2015: Combinations of Adiabatic and Non‐Adiabatic Effects

Abstract Due to solar wind‐magnetosphere coupling, energetic electron fluxes in the outer radiation belt are profoundly influenced by enhanced solar activities. Utilizing observations from Van Allen Probes (VAPs) and low Earth orbit MetOp‐02, here we report a case study of dramatic pitch‐angle dependent dynamics of ultra‐relativistic electrons from 19 to 20 December 2015. We focus on two orbits of VAPs, which contains two successive interplanetary shocks in the first orbit and then storm main phase in the second orbit. Consequently, the ultra‐relativistic electron fluxes exhibit around 90°‐peaked distributions at L * > 5 in dayside magnetosphere right after each shock, followed by dropouts at almost all pitch angles throughout the outer radiation belt. Electron phase space density (PSD) profiles show that adiabatic effects contribute to the accelerations at high pitch angles (>∼45°) and L * > 5 for both shocks while inward radial diffusion driven by ULF waves plays a dominant role at lower L * after the second shock. Additionally, interactions between concurrent EMIC waves and electrons result in the dropouts at low pitch angles (<∼45°) after each shock. Furthermore, conjugate precipitation after the first shock provides sufficient evidence for EMIC‐induced loss. Our results also show that the dropouts in the second orbit are attributed to a combination of magnetopause shadowing effect at L * > 5 and EMIC‐driven loss at L * < 4. Our study provides direct observational evidence that combinations of multi‐mechanisms, including adiabatic and non‐adiabatic effects, result in the dramatic dynamics of ultra‐relativistic electrons within one day.