Takefumi MITANI

Abstract The May 2024 geomagnetic superstorm provided the opportunity to explore how strong wave‐particle interactions affect energetic electron precipitation under intense driving. Using coordinated measurements from a balloon‐borne Timepix‐based X‐ray detector, ground‐based riometers and magnetometers, and Arase satellite observations, we identified quasi‐periodic bursts of energetic electron precipitation coincident with Pc5 ultra low frequency (ULF) wave oscillations. Arase satellite data revealed energy‐dispersed trapped energetic electron flux modulations in the “seed” energy range, indicating that trapped electron flux was likely modulated by ULF waves. This letter reveals that these flux enhancements surpassed the Kennel‐Petschek (K‐P) limit, creating intense chorus waves and driving periodic electron precipitation. Drift‐dispersion analysis traced these modulations back to a source in the post‐noon magnetospheric sector, matching balloon and ground‐based measurements. Here, we propose a novel indirect ULF wave‐driven mechanism for modulated energetic electron precipitation, whereby periodic modulations of “seed” electron fluxes enhance electron losses.

Abstract Energetic electron precipitation plays a pivotal role in shaping Earth's radiation belt dynamics and drives significant physical and chemical changes in the upper atmosphere. However, the detailed mechanisms governing the loss of relativistic electrons have remained unclear, largely due to the limited energy coverage and coarse resolution of previous measurements. Here we report high‐resolution observations of bursty electron precipitation across a broad energy range (0.3–2.3 MeV), obtained by the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. REPTile‐2 employs a novel instrument design that minimizes background to enable clean spectral measurements with the highest energy resolution achieved to date in low‐Earth orbit for this energy range. During the conjunction events when CIRBE was close to the same field line with Arase satellite at higher altitudes, our analysis shows that pitch angle diffusion driven by chorus waves can fully account for the observed three bursty precipitation events over the entire energy range. These results provide the definitive evidence for a unified chorus‐driven electron loss process acting across a wide energy range and underscore the critical importance of high‐resolution measurements in resolving long‐standing uncertainties in radiation belt dynamics. Furthermore, they offer new insight into the energy‐dependent atmospheric impacts of electron precipitation, with broad implications for space weather forecasting and upper atmospheric chemistry.