Takefumi MITANI

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.

Abstract On 15 February 2018 a co‐rotating interaction region (CIR) from an equatorial coronal hole reached the Earth. The CIR initiated a moderate and slowly intensifying geomagnetic storm, which began with a large and strong substorm injection. The substorm injection was exceptionally well‐observed by an array of spacecraft including LANL‐GEO satellites, Van Allen Probes (RBSP), Arase (ERG), and MetOp/POES, as well as ground‐based instruments. These observations enable the unambiguous identification of several important features that have been impossible to measure directly in other events. The substorm injection extended well inside the geosynchronous orbit. A fortuitous conjunction of RBSP‐A (moving inbound) and Arase (simultaneously moving outbound at the same magnetic local time) allows us to establish, very precisely, the location of the inner edge of the injection region at L  = 3.8−3.9. In supporting observations, North American riometers saw precipitation extending down to L  ≈ 4 but not lower. Arase and RBSP‐A also observed whistler‐mode hiss waves inside the plasmasphere. Analysis of the resonance conditions shows, conclusively, and for the first time, that they were produced by the drifting injected electrons. RBSP‐A observations also show the injection (or transport) of electrons into or through the slot region within hours of the substorm injection onset. Previous studies were not able to clearly connect or separate substorm injections and slot‐filling processes. These new observations clearly identify slot‐filling as a spatially and temporally separate process that is not a direct result of substorm injection.