Initial Observations by the GOLD Mission

Abstract

The NASA Global‐scale Observations of the Limb and Disk (GOLD) mission has flown an ultraviolet‐imaging spectrograph on SES‐14, a communications satellite in geostationary orbit at 47.5°W longitude. That instrument observes the Earth's far ultraviolet (FUV) airglow at ~134–162 nm using two identical channels. The observations performed include limb scans, stellar occultations, and images of the sunlit and nightside disk from 6:10 to 00:40 universal time each day. Initial analyses reveal interesting and unexpected results as well as the potential for further studies of the Earth's thermosphere‐ionosphere system and its responses to solar‐geomagnetic forcing and atmospheric dynamics. Thermospheric composition ratios for major constituents, O and N2, temperatures near 160 km, and exospheric temperatures are retrieved from the daytime observations. Molecular oxygen (O2) densities are measured using stellar occultations. At night, emission from radiative recombination in the ionospheric F region is used to quantify ionospheric density variations in the equatorial ionization anomaly (EIA). Regions of depleted F region electron density are frequently evident, even during the current solar minimum. These depletions are caused by the “plasma fountain effect” and are associated with the instabilities, scintillations, or “spread F” seen in other types of observations, and GOLD makes unique observations for their study.

Comparison of GOLD Nighttime Measurements With Total Electron Content: Preliminary Results

“…The satellite is located in a geostationary orbit over 47.5°W. The only scientific instrument onboard is a Far Ultraviolet (FUV) imager, which has two similar and independent channels (Channel A and Channel B) that perform required measurements in the Far-Ultraviolet (Eastes et al, 2017(Eastes et al, , 2020. As the instrument makes measurements from the geostationary orbit, GOLD is able to provide images of the same region during a similar universal time (UT) range every day.…”

Daily Variability in the Terrestrial UV Airglow

“…An ongoing debate over the effect of temperature on this relation is unresolved; where Zhang et al [23,24] recently argued that a solar flux related variation in O/N 2 is related to the change in temperature in the lower thermosphere, [25] argue that the emission ratio that provides the O/N 2 product is independent of the thermospheric temperature. The question of the extent to which the ratio of these emissions is related to temperature can be addressed by surveying the changes of the two emissions over time, under similar conditions of solar illumination, latitude and longitude, and viewing angle, and possibly including the temperature product that GOLD provides [2]. The results presented here offer insight into the behavior of these emissions at solar minimum, and potential variations in the O/N 2 product that originate in processes not yet considered.…”

“…Since the analysis uses data from the regions marked by white boxes (discussed shortly), areas in this region are excluded, though they are regularly used in deriving higher level products. While calculation of radiances [2] remove all these patterns, the effect of those corrections on the statistical uncertainties could complicate the analysis we perform, so we work only with raw instrument counts. All the photon events produced in the imaging period are represented here, and the number of data samples included in each pixel was consistent (for each latitude and longitude bin in the fixed grid) during the study period.…”

“…This ratio can be viewed as a proxy for the ratio of OI to N 2 in the thermospheric column, given that abundances in the photoelectron-excited region reflect the relative abundances of the constituents at lower and higher altitudes [19]. Further analysis is required to accurately retrieve that product [2], but the trends in this ratio allow for the interpretation of changes in the relative abundance of OI and N 2 over the study period. Here, the trend in the NH region toward lower OI vs. N 2 is evident, corresponding to the slow change in the thermosphere from wintertime high OI abundance to lower abundance generally observed in summertime.…”

“…The research effort described here focuses on variability in the relative abundance of the two major constituents of the upper atmosphere, atomic oxygen (OI) and molecular nitrogen (N 2 ). It addresses this topic using remote spectrographically-separated, photometric measurements of the photo-emissions of the upper atmosphere from the NASA GOLD mission [1,2] that provides repeatable, daily measurements, with nearly identical observing geometry. Questions on the source of changes in these emissions and how they are related to changes in the underlying composition can be addressed with these new observations of the Earth's upper-atmospheric airglow.…”