Three-step Acceleration of the Radiation Belt Relativistic Electrons by Interplanetary Shocks

Abstract Inward radial diffusion driven by ultra-low-frequency (ULF) waves is one of the dominant acceleration mechanisms of relativistic (>1.0 MeV) electrons in the Earth’s outer radiation belt. However, the role of interplanetary (IP) shocks in the ULF-wave-driven acceleration remains inadequately understood. Here we report the rapid acceleration events caused by IP shocks on 2013 January 17 and 18. Based on simultaneous observations, we propose a three-step physical process for the acceleration of relativistic electrons. Our results indicate that IP shocks not only excite ULF waves and provide source populations for the ULF-wave-driven acceleration, but also contribute to establishing a significant phase-space density radial gradient of relativistic electrons in the outer boundary. These can lead to rapid enhancements of relativistic electrons in the outer radiation belt.