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

TL;DR

This study uses observations and models to analyze Earth's upper-atmosphere ultraviolet airglow during twilight, revealing how magnetic field lines influence emission patterns and intensities.

Contribution

It introduces a comprehensive modeling approach that explains twilight airglow phenomena, including the impact of magnetic field transport and discrepancies in observed versus modeled emissions.

Findings

Faint airglow observed near the night side terminator is caused by transported photoelectrons.

Good agreement between model predictions and observed airglow morphology and intensity.

Discrepancies in emission intensity are partly explained by magnetic field variations and plasma sheet interactions.

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.