Dynamics of Ultra-relativistic Electrons on 19 December 2015: Combinations of Adiabatic and Non-adiabatic Effects

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 variations of ultra-relativistic electron fluxes within one day 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 angle distributions 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 plays a dominant role at lower L* after the second shock. Additionally, pitch angle scattering loss driven by concurrent EMIC waves result in the dropouts at low pitch angles (< ~45°) after each shock. Furthermore, the precipitations in a close magnetic conjugation after the first shock provide sufficient evidence for EMIC-induced loss. Our results also show that the dropouts throughout the outer belt in the second orbit are attributed to a combination of magnetopause shadowing effect at L* > 4.5 and EMIC-driven pitch angle scattering 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.