Global‐Scale Observations and Modeling of Far‐Ultraviolet Airglow During Twilight

The NASA Global‐scale Observations of the Limb and Disk 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 nightside of the terminator, broad regions of faint airglow are seen, particularly near the winter solstice. These are caused by photoelectrons generated in magnetically conjugate areas in the other hemisphere that are still illuminated, transported along field lines, and then precipitated back into the atmosphere. Model calculations 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 bright as the model simulations. Some of the reduction in electron flux is 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 that reach into the plasma sheet. The NASA Global‐scale Observations of the Limb and Disk instrument is an ultraviolet imager and spectrograph that observes light from the upper atmosphere of the Earth, in order to infer quantities such as the composition and temperature of the thermosphere. To interpret the measurements, the observing 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 sunrise and sunset. At night but near twilight, broad regions of faint emissions of airglow light are seen, particularly during winter. These are caused by electrons that are created by ionization on the dayside, and then transported along field lines from magnetically conjugate areas in the other hemisphere, where those areas are still illuminated. Model calculations demonstrate that this process is the source of the emiss