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.