Identification of auroral emission altitudes associated with relativistic electron precipitation events observed by ISS-CALET

We report a multi-MeV relativistic electron precipitation (REP) event as observed by the CALorimetric Electron Telescope and the Monitor of All-sky X-ray Image aboard the International Space Station together with low-cost twin all-sky auroral imagers installed at Athabasca, Canada. We find that visible aurora associated with REP is not frequent—observed in only two of 10 conjugate REP events during our survey period from September 2024 to September 2025—and that one case, on 3 May 2025, featured diffuse aurora near the ISS magnetic footprint. Using two independent stereoscopic methods, we estimate the auroral emission height to be ~ 90 km, which is typically due to tens-of-keV electron precipitation. Combined with the MeV electron precipitation detected by CALET, such broadband electron precipitation from tens-of-keV to MeV is consistent with the hypothesis of chorus-driven REP events. The emission altitude shows no systematic latitudinal variation, which is not consistent with field-line curvature scattering type energy-dispersed precipitation. The combined space–ground conjunction observations contribute to better understanding of the spatial context of wave–particle interaction associated with the REP events.