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We report an event study of STEVE on 17 July 2018, with focus on the optical spectra and emission altitudes of STEVE. We find that the STEVE comprises two traces, one at a higher elevation angle and the other at a lower elevation angle. The two traces merge into one when viewed near the zenith. Spectrograph measurements show that both STEVE traces are characterized by enhancements over broadband wavelengths, that is, an airglow continuum, but they differ in their red-line (630 nm) component: The higher-elevation STEVE contains substantial red-line enhancement over background, while the lower-elevation STEVE does not. Based upon triangulation analyses using multiple optical instruments, we evaluate that the two STEVE traces are likely emitted from distinctly different altitudes: The higher-elevation STEVE comes from 250-km altitude, while the lower-elevation one is from <= 150-km altitude. Our results impose implications and constraints on the possible underlying mechanisms of STEVE.

Plain Language Summary The recently discovered STEVE nightglow is active at times of auroral activity but does not seem to be an aurora. It is located at lower latitudes than the usual aurora, and as photographed on the citizen science cameras which played a large role in drawing attention to it, it has a different color. In this study, using a combination of scientific optical instruments, especially the spectrograph recently deployed at Lucky Lake, Canada, by the University of Calgary, we investigate the optical spectral properties of STEVE and their emission altitudes. Two major spectral (color) components of STEVE are identified. One is characterized by a broadband enhancement over the entire visible wavelength range, contributing to the apparently "whitish" color of the STEVE, and the other is contributed by oxygen airglow concentrated at 630 nm, adding a "reddish" tint to the STEVE. In the event occurring on 17 July 2018, the STEVE is found to comprise two emission structures originating from different altitudes, one at 250 km and the other at =150-km altitude. Both STEVE structures contain the white component but only the higher-altitude one shows a substantial red component. That red color is the same red as seen in auroras that come from high altitudes in the atmosphere, while the mechanism of the white component remains to be explored.