Evolution of Reversed Electron Energy Spectra in Earth’s Radiation Belts Observed by CIRBE

Abstract Following an intense geomagnetic storm (Dst < −150 nT) on 5 November 2023, electron fluxes in the outer radiation belt were reduced by more than three orders of magnitude at energies below ∼700 keV while they were enhanced at ∼1–3 MeV, leading to reversed energy spectra. Using high‐energy resolution measurements from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat, we track the evolution of electron spectra across L ≈ 2.5–4.5 over ∼12 days, clearly presenting the spatial extents and structures of reversed energy spectra. We also perform one‐dimensional pitch angle diffusion simulations induced by hiss and magnetosonic (MS) waves to reproduce the key features of the observed reversed spectra. These results highlight the unique observations of CIRBE and the important roles of hiss and MS waves in controlling electron dynamics in radiation belts after geomagnetic storms.