Possible Observations of Plasmaspheric O ⁺

Abstract is a minor species in the plasmasphere, but its presence significantly alters the dynamics of the magnetosphere system. Many questions remain about the morphology and temporal evolution of plasmaspheric , in part because it has rarely been observed optically. Previous studies suggest that these questions could be answered by observing the dim extreme ultraviolet (EUV) signature at 83.4 nm from high Earth orbit. The Ionospheric Connection Explorer (ICON) spacecraft carried an EUV spectrograph to measure profiles in the daytime ionosphere. On 16 May 2022, during a zenith‐pointing calibration maneuver on the night side, the ICON EUV instrument detected an unusual 83.4 nm signal across the full spatial extent of the detector lasting several minutes. In this study, we present the data from this period and establish that the signal most likely originates from solar‐illuminated high‐altitude , one of the first optical observations of plasmaspheric . Through modeling and comparison to data from similar calibration maneuvers, we establish that it is most likely that ICON did not observe the quiescent plasmasphere but rather a transient dense torus.