Global-scale observations and modeling of far-ultraviolet airglow during twilight

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.