Global-scale observations and modeling of far-ultraviolet airglow

Monday, December 16, 2019

The 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.

Image depicting measurement, model simulation, and normalized differences
Measurement, model simulation, and normalized differences for the 19 December case at 07:22 UT. Top row (a-c): OI 135.6 nm doublet. Bottom row (d-f): N2 LBH bands 137–154 nm. Left panels (a and d): observed slant column brightness. Center panels (b and e): Model with magnetic effects included. Right panels (c and f): Percentage difference (model – measurement) / measurement).

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 as the model calculations. Some of the reductions in are explained by changes in magnetic field strength that affect the motions of the electrons; in other cases, particularly at high magnetic latitude, the cause is unknown, but must occur along the long paths that they travel through the magnetosphere.

Publication Name: Journal of Geophysical Research Space Physics
First HAO Author's Name: Stan Solomon

Link to the manuscript at the arXiv preprint server: