Global-scale Observations and Modeling of Far-Ultraviolet Airglow During Twilight

Abstract: The NASA Global-scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph performs observations of upper-atmosphere airglow from the sunlit disk and limb of the Earth, in order to infer quantities such as the composition and temperature of the thermosphere. To interpret the measurements, the observational and solar illumination geometry must be considered. We use forward models of upper atmosphere density and composition, photoelectron impact, airglow emissions, radiative transfer, and line-of-sight integration, to describe the expected observations, and here test those calculations against observations near the terminator, and near the limb. On the night side of the terminator, broad regions of faint airglow are seen, particularly near the winter solstice. These are caused by photoelectrons that were transported along field lines from magnetically conjugate areas in the other hemisphere, where those areas are still illuminated. We perform model calculations to demonstrate that this process is the source of the emission, and obtain good agreement with its morphology and intensity. In some regions, the observed emissions are not as intense as the model simulations. Some of the reductions in electron flux are explained by changes in magnetic field strength; in other cases, particularly at high magnetic latitude, the cause is unknown, but must occur along extended field lines as they reach into the plasma sheet.