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 ionosphericFregion is used to quantify ionospheric density variations in the equatorial ionization anomaly (EIA). Regions of depletedFregion 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 “spreadF” seen in other types of observations, and GOLD makes unique observations for their study.

After days of intense solar activity, active region AR3664 launched seven CMEs toward Earth producing an extreme G5 geomagnetic storm commencing at 17:05 UT on 10 May 2024. The storm impacted power grids, disrupted precision navigational systems used by farming equipment, and generated aurora seen around the globe. The storm produced remarkable effects on composition, temperature, and dynamics in the Earth's thermosphere that were observed by NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission and are reported here for the first time. We use synoptic disk images of ΣO/N₂ and neutral temperature (at ∼160 km) measured by GOLD to directly link dynamics resulting from the storm with dramatic changes in thermospheric composition and temperature. We observe a heretofore unseen spatial morphology simultaneously in ΣO/N₂, neutral temperature, and total electron content. Equator‐to‐pole temperature differences reach 400 K with high latitude peak neutral temperatures near 160 km exceeding 1400 K.

Following the 2022 Tonga Volcano eruption, dramatic suppression and deformation of the equatorial ionization anomaly (EIA) crests occurred in the American sector ∼14,000 km away from the epicenter. The EIA crests variations and associated ionosphere‐thermosphere disturbances were investigated using Global Navigation Satellite System total electron content data, Global‐scale Observations of the Limb and Disk ultraviolet images, Ionospheric Connection Explorer wind data, and ionosonde observations. The main results are as follows: (a) Following the eastward passage of expected eruption‐induced atmospheric disturbances, daytime EIA crests, especially the southern one, showed severe suppression of more than 10 TEC Unit and collapsed equatorward over 10° latitudes, forming a single band of enhanced density near the geomagnetic equator around 14–17 UT, (b) Evening EIA crests experienced a drastic deformation around 22 UT, forming a unique X‐pattern in a limited longitudinal area between 20 and 40°W. (c) Thermospheric horizontal winds, especially the zonal winds, showed long‐lasting quasi‐periodic fluctuations between ±200 m/s for 7–8 hr after the passage of volcano‐induced Lamb waves. The EIA suppression and X‐pattern merging was consistent with a westward equatorial zonal dynamo electric field induced by the strong zonal wind oscillation with a westward reversal.

We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N₂ column density ratio (∑O/N₂) in part of the Northern Hemisphere measured by Global‐scale Observations of the Limb and Disk (GOLD) exhibited large and long‐lived depletions during weak geomagnetic activity in May and June 2019. The depletions reached 30% of quiet time values, extended equatorward to 10°N and lasted more than 10 hr. Furthermore, numerical simulation results are similar to these observations and indicate that the ∑O/N₂ depletions were pushed westward by zonal winds. The ∑O/N₂ evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N₂ depletions is similar to that during a geomagnetic storm. The effects of weak geomagnetic activity are often ignored but, in fact, are important for understanding thermosphere neutral composition variability and hence the state of the thermosphere‐ionosphere system.

The National Aeronautics and Space Administration Global‐scale Observations of the Limb and Disk (GOLD) mission observed a unique structure of thermospheric column density ratio of O to N₂ (∑O/N₂) during a geomagnetic storm on day of year (DOY) 130 (May 10) to DOY 132 in 2019. The percentage difference of ∑O/N₂ between the storm time (DOY 131) and the quiet time (DOY 128) had a relatively enhanced ∑O/N₂ region sandwiched by two depleted regions over North America and the Atlantic Ocean in the Northern Hemisphere. This enhanced ∑O/N₂ region is called the neutral tongue here. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model also predicted a similar ∑O/N₂ structure with the same spatial and temporal evolution as that seen in the GOLD observations on DOY 131. Model diagnostic analysis revealed that the neutral tongue was formed when the neutral wind in the region changed from mostly equatorward to poleward, which transported the ∑O/N₂ rich air from lower latitudes into this longitude section, forming the neutral tongue. The neutral tongue separated a large depletion region formed earlier into two smaller depleted regions, one on each side of the neutral tongue.

We conducted observational and modeling studies of thermospheric composition and ionospheric total electron content (TEC) variations during two geomagnetically quiet periods (maximum Kp = 1.7) at solar minimum. Daytime thermospheric O and N₂ column density ratio (∑O/N₂) observed by Global‐scale Observations of the Limb and Disk and TEC from a network of ground‐based Global Navigation Satellites System receivers both exhibited large (∼30% of reference values) and long‐lived (5–11 h) day‐to‐day variations in roughly the same mid‐latitude geographic regions. Numerical simulations replicated the observed variability, though not perfectly. Analysis of the simulations suggested that the variations were mainly generated in the high‐latitudes and were subsequently advected equatorward and westward. When high‐latitudes input was turned off in simulations, the variations were negligible. This suggested the potentially important role of high‐latitude geomagnetic forcing in thermospheric composition and ionospheric density variations at mid‐latitudes even during some “geomagnetically quiet” periods at solar‐minimum.