Statistical investigation for shock-speed-dependent “surfing” acceleration of relativistic electrons at geostationary orbit with FengYun-4 measurement

Impulsive electric fields (IEFs) triggered by interplanetary shocks (IPSs) are effective drivers of prompt relativistic electron acceleration and injection in the Earth’s outer radiation belt. Electrons whose drift speeds are comparable to the propagation speed of the IEF can gain energy the most efficiently. Such a process has been observed by Zhang et al. (2024, 2025) with Van Allen Probe measurements and has been referred to as “surfing” acceleration. In this study, we utilized data from the FengYun-4A (FY-4A) satellite at geostationary orbit (GEO) from 2016 to 2022 to investigate electron flux variations during IPS arrivals. Through a statistical analysis of 38 selected typical IPS events, we calculated the electron drift speeds corresponding to the energy channels exhibiting the maximum variation of residual flux and compared them with the IPS propagation speeds. At GEO, this energization may be observed as a local flux dropout rather than an enhancement when the background radial phase-space-density gradient is negative. The statistical results reveal a positive correlation between the resonant electron drift speed and the IPS propagation speed. This observational evidence supports the “surfing” acceleration mechanism, suggesting that the resonant acceleration energy is determined by the IPS propagation speed, rather than the traditionally assumed local fast-mode speed. By extending the observational evidence of this mechanism to GEO altitudes, this study not only validates the applicability of this mechanism but also further deepens our understanding of the coupling mechanism between solar wind driving and radiation belt dynamics.