Optical Spectra and Emission Altitudes of Double-Layer STEVE: A Case Study

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.