Quantifying Radial Diffusion Rate Through Multi‐MeV Electron Drift Oscillations Driven by Broadband ULF Waves: A Case Study of the September 2019 Geomagnetic Storm

Abstract During the September 2019 geomagnetic storm, long‐lasting, drift‐periodic flux oscillations of multi‐MeV electrons were observed by the REPT instrument on the Van Allen Probes–A. These flux oscillations occurred across the outer belt during the storm main phase, coinciding with enhanced Pc5 ULF wave activity and elevated electron fluxes. During the recovery phase, the oscillations gradually decayed at the center of the outer belt but persisted for days at its inner edge. Using 2D test particle simulations driven by constructed broadband ULF wave fields, we simulated multi‐MeV electron fluxes during two satellite passes and successfully reproduced observed drift‐periodic flux oscillations. The close agreement between simulation and observation confirms a causal relationship between drift‐periodic flux oscillations and resonant interactions of electrons and broadband ULF waves. We further derived the radial diffusion coefficient from the simulation and compared it with empirical models. The magnitude of the resultant radial diffusion coefficient aligns closely with the model by Liu et al. (2016, https://doi.org/10.1002/2015gl067398 ), though it is lower than those by Brautigam and Albert (2000, https://doi.org/10.1029/1999ja900344 ) and Ozeke et al. (2014, https://doi.org/10.1002/2013ja019204 ). The energy‐ and L‐dependence of the diffusion coefficient is also consistent with the model by Liu et al. (2016, https://doi.org/10.1002/2015gl067398 ). We estimated the uncertainty in the derived radial diffusion coefficient to be approximately half an order of magnitude, primarily limited by the instrument's energy resolution. These results demonstrate the potential of inferring radial diffusion rates from electron flux measurements alone and underscore the importance of high‐energy‐resolution electron measurements for accurately quantifying radiation belt dynamics.