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

Abstract

1. We analyzed measurements by the Global-scale Observations of the Limb and Disk mission of far-ultraviolet emissions near the terminator.2. Airglow excitated by photoelectrons originating in the magnetically conjugate hemisphere is observed and simulated.3. The conjugate photoelectron source explains much of the twilight airglow, but losses occur in the plasmasphere and magnetosphere. AbstractThe 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. Plain Language SummaryThe NASA Global-scale Observations of the Limb and Disk (GOLD) 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 are then 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 the observed shape and intensity. In some regions, the observed emissions are not as intense...

First Global‐Scale Synoptic Imaging of Solar Eclipse Effects in the Thermosphere

“…Effects of photoelectrons from the sunlit conjugate hemisphere increasing the nighttime airglow have been reported recently (Solomon et al, 2020). For the 2017 eclipse, SAMI3 simulations predicted a reduction in electron density (including photoelectrons) at eclipse's conjugate hemisphere (Huba & Drob, 2017).…”

Initial Observations by the GOLD Mission

“…Some examples and analysis of those data have been presented by Laskar et al (2020). A special mode that could also provide additional insights is the conjugate photoelectron excitation sometimes seen near the terminator on the nightside (Solomon et al, 2020). Alternate modes of observation continue to evolve and some may become part of the standard operations.…”

Conjugate Photoelectron Energy Spectra Derived From Coincident FUV and Radio Measurements

“…More recently, Solomon et al. (2020) and Kil et al. (2020) have reported pre‐dawn airglow enhancements observed by GOLD (Eastes et al., 2008, 2017) and SSUSI (Paxton et al., 2017, and references therein), respectively.…”

“…Nevertheless, the high fidelity achieved by multi‐stream electron transport comes with a considerable computational price, and it is sensitive to the selected pitch angle resolution. Other faster method such as the two‐stream electron transport implemented in GLOW has demonstrated to be good at reproducing twilight airglow emissions observed by GOLD at latitudes lower than 60° (Solomon et al., 2020), although some discrepancies observed at mid and high latitudes were associated with the reduced number of pitch angles used, and other neglected terms such as scattering losses or meridional transport.…”