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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. Plain Language Summary: The NASA Global‐scale Observations of the Limb and Disk (GOLD) mission has flown a dual‐channel, 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. The observations performed include images of the Earth's sunlit and nightside disk, limb scans, and stellar occultations, 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 temperatures and composition ratios for major constituents, O and N2, near 160‐km altitude 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 in the EIA, even during the current solar minimum. Key Points: GOLD makes global‐scale, synoptic measurements of the temperature, composition, and densities in the thermosphere‐ionosphere systemMost measurements by the GOLD instrument are made in one of four modesThe observations are providing new and surprising insights into the characteristics and behavior of the thermosphere and ionosphere [ABSTRACT FROM AUTHOR]

We present the first global images of the daytime ionosphere equatorial arcs as manifested in the 83.4‐nm airglow. These images were collected by the Limb‐Imaging Ionospheric and Thermospheric Extreme‐Ultraviolet Spectrograph that commenced operations on the International Space Station in early 2017. We compare these to simultaneous images of the ionospheric radiative recombination airglow at 135.6 nm measured between 250‐ and 350‐km tangent altitudes, where the emission is generated primarily by radiative recombination of ionospheric plasma. We find that these signatures of the dense crests of the Equatorial Ionization Anomaly, their symmetry, and daily variability at 1300–1600 LT over 1–6 April 2017 do not show any strong periodicity during this time. These results are also important to the joint interpretation of these two correlated extreme and far ultraviolet emission features measured under solar minimum conditions and the evaluation of absorption and radiative transfer effects that affect these emissions differently. Plain Language Summary: We have accumulated a set of new images of ultraviolet light that naturally emanates from the upper atmosphere and its embedded plasma (the ionosphere). In particular, we have imaged the pair of pervasive, dense, bands of plasma that surround Earth to the north and south of the magnetic equator. Our images were collected during the afternoon local time periods, which can expose how presunset conditions set up these bands of high plasma density to wax or wane through the postsunset hours. Changes in the density and symmetry of these bands can also be indicators of upper atmospheric winds, electric fields, and even perturbations from the lower atmosphere that propagate up through to these outer reaches of Earth's space environment. Key Points: Global maps of the daytime equatorial ionization anomaly (EIA) are made using 83.4 and 135.6 nm in absorption and emission, respectivelyThe EIA from 1300 to 1600 LT in April 2017 does not show evidence of longitudinal variability indicative of a wave 4 structureObserved north‐south asymmetry suggests meridional winds, with 135.6 nm more sensitive to changes than 83.4 nm from the low‐altitude vantage [ABSTRACT FROM AUTHOR]

The 135.6‐nm emission of atomic oxygen has become a workhorse for satellite remote sensing of the nighttime ionosphere. Previous work on interpreting the 135.6‐nm emission established the importance of properly modeling the mutual neutralization and radiative recombination contributions to the volume emission rate and emphasized the importance of modeling and interpreting the transfer and transport of the radiation. Recently, we developed and published a new inversion approach called Volume Emission Rate Tomography, which handles the radiation transfer as part of the tomography process. We compare electron density distributions derived using the Volume Emission Rate Tomography technique to interpret the 135.6‐nm measurements made by the Special Sensor Ultraviolet Limb Imager instruments aboard the Defense Meteorological Satellite Program satellites in 2010. These retrievals are validated against electron densities measured during overflights of the Advanced Research Project Agency Long‐range Tracking and Identification Radar incoherent scatter radar. The mean percentage difference between the Special Sensor Ultraviolet Limb Imager‐derived electron densities and those measured by Advanced Research Project Agency Long‐range Tracking and Identification Radar was 14%, when radiative recombination was the only source considered in the inversion process. When the mutual neutralization and radiation transport sources of emission are included in the inversions, the mean percentage decreased to 4%, thereby confirming that these sources are necessary to accurately derive the electron density from the 135.6‐nm emission. Key Points: Electron densities produced by tomographic inversions of 135.6‐nm measurements were compared to incoherent scatter radar measurementsInversions including all of the relevant physics are more accurate than those assuming optically thin emission and radiative recombinationThe tomographic retrievals differed from the electron density measurements by ~4%, validating the tomographic approach [ABSTRACT FROM AUTHOR]