How Can Propagation Speed of Interplanetary Shocks Affect the Radiation Belts Dynamic: A Comparative Observational and Statistical Investigation

Relativistic electron dynamics in Earth's radiation belts are significantly influenced by interplanetary shocks (IPSs) associated with coronal mass ejections. We show that the propagation speed of an IPS controls the motion of the induced impulsive electric field. This field then drives the resonant “drift-in-phase” acceleration of relativistic electrons. As a consequence, the relativistic electron energy with the maximum variation of residual flux observed is determined by the IPS propagation speed, which contrasts with the traditional assumption that fast magnetosonic wave speed controls the resonance. To validate this hypothesis, we compare two IPS-driven radiation belt enhancement events with distinct shock speeds and perform a statistical analysis of 39 IPS events observed from 2013 to 2018. The case studies and statistical results consistently show that the drift velocity of electrons at the energy of peak flux enhancement correlates much more strongly with the IPS shock speed than with the localized fast magnetosonic wave speed in the inner magnetosphere. These findings provide the first comprehensive observational confirmation that IPS propagation speed is the key parameter governing “drift-in-phase” acceleration of radiation belt electrons. This insight emphasizes the importance of IPS kinematics in radiation belt modeling and could improve the forecasting of shock-induced energetic electron enhancements.