Estimation of auroral emission altitude in the region of the ISS magnetic footprint during a relativistic electron precipitation event 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 auroral emissions near the ISS magnetic footprint during REP events are 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 produced by 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- and ground-based observations contribute to a better understanding of the spatial context of wave–particle interaction associated with the REP events.