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Abstract Leveraging observations by two NASA missions—GOLD (Global‐scale Observations of the Limb and Disk) and ICON (Ionospheric Connection Explorer), we investigate concurrent responses of thermospheric composition, temperatures, and neutral winds to the geomagnetic storm on 3–4 November 2021, as well as their interplay at low and middle latitudes. The synergetic observations reveal remarkable depletions up to 60%–70% in GOLD O/N2, along with large enhancements in GOLD temperatures poleward of 30° in the middle thermosphere. Meridional winds from ICON observations are altered by ∼100 m/s equatorward of 25°N latitude and at 250 km, characterized by a reversal of prevailing northward winds to geomagnetic storm‐driven southward winds. This study fills a need, after a decade‐long gap, for observing concurrent and co‐located responses of composition, temperatures, and neutral winds in the thermosphere to geomagnetic storms.

Abstract The ratio of O to N2 number densities (O/N2) at different altitudes is an important parameter in describing thermospheric neutral composition changes and their effects on the ionosphere during geomagnetic storms. However, storm‐induced vertical variations in O/N2 and its dependence on the O and N2 perturbations are still not fully understood. Here, the Thermosphere/Ionosphere Electrodynamics General Circulation Model simulations were used to investigate the responses of thermospheric composition at different pressure levels to the super geomagnetic storm occurred on November 20 and 21 in 2003. Our analysis shows that the behaviors of O/N2 perturbations on different pressure levels are similar above ∼180 km altitude. In the middle and low thermosphere of below ∼300 km, the storm‐time O/N2 decrease is mainly caused by a large reduction of O number density. However, N2 enhancement plays a vital role in O/N2 decreases in the upper thermosphere. The O/N2 enhancement is mainly attributed to the N2 decreases at all pressure levels. The changes of O and N2 number densities at a constant pressure level can be explained by the perturbations of their mass mixing ratio (mmr) and total mass density (ρ). The regions of the O/N2 decrease are characterized by the O mmr decrease and N2 mmr enhancement, whereas the regions of the O/N2 increase are characterized by the O mmr increase and N2 mmr decrease. The ρ value that shows the decrease globally at most pressure levels during the storm either enhance or reduce the O and N2 perturbations.

Abstract During the storm on 3 and 4 November 2021, the Global‐scale Observations of the Limb and Disk (GOLD) mission observed well separated equatorial ionization anomaly (EIA) crests post sunset on 3 November, but merged EIA on 4 November. We used the Whole Atmosphere Community Climate Model‐eXtended to simulate the EIA structures during the two nights. The simulations show two separated post sunset EIA crests on 3 November but merged post sunset EIA crests on 4 November, which are qualitatively consistent with the GOLD observations. Numerical simulations and Ionospheric Connection Explorer neutral wind observations illustrate that the formation of merged EIA crests was due to several hours of downward E × B drifts before and after sunset. Further diagnostic analysis revealed that it was mainly driven by westward electric fields caused by the disturbance dynamo electric field during the recovery phase of the storm.

Abstract We report a new ionosphere phenomenon: Equatorial ionization anomaly (EIA) discontinuity (EIAD), based on OI 135.6 nm radiance observations from the Global Observations of Limb and Disk (GOLD), ground‐based total electron content maps and in‐situ ion density data from Constellation Observing System for Meteorology, Ionosphere, and Climate‐2. The EIAD occurs when the OI radiance of the EIA crest has a local minimum, at a fixed UT, with the radiance in the local longitude region being weaker than that on the east and west sides. In the GOLD field‐of‐view, EIAD follows the seasonal variations of EIA. EIAD appears more often over the Atlantic Ocean and Africa than over South America. It occurs more in the southern crest during the December solstice, and more in the northern crest during both equinoxes. EIAD can occur under both quiet and disturbed times.

The Global-scale observation of limb and disk mission observed an extra electron density (Ne) peak after sunset at approximately 30°S near 40°W on 4 November 2019, which is poleward and immediately next to the southern equatorial ionization anomaly (EIA) crest. This Ne peak is different from previously reported mid-latitude peaks that occur at all local times. The Whole Atmosphere Community Climate Model-eXtended captures this phenomenon. Model diagnostic analysis reveals that the decrease of Ne and hmF2 between 15° and 25°S makes Ne near 30°S appear as an extra density peak relative to the southern EIA crest. Transport by poleward meridional wind decreases Ne between 15° and 25°S. Moreover, the upward E × B drifts due to pre-reversal enhancement lift the plasma between the dip equator and 15°S but do not affect Ne much between 15° and 25°S with a low drift speed. Comparison with days without the extra peak shows the importance of E × B drift latitudinal variations on the extra peak formation. This study provides new insights into the dynamic variability of the nighttime ionosphere. [ABSTRACT FROM AUTHOR]

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 (SO/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. Plain Language Summary This study presents two cases when geomagnetic forcing can be a plausible source of mid-latitude thermospheric composition and ionosphere density variations even during what is typically considered as geomagnetically quiet times (magnetic activity index Kp < 2). The column density ratio of thermospheric O and N2 (SO/N2) plays a major role in the daytime ionospheric F-region plasma density at mid-latitudes. In this study, thermospheric and ionospheric variations during geomagnetically quiet times are investigated with the two-dimensional images of SO/N2 provided by a satellite located in geostationary orbit and the ground-based total electron content (TEC) maps. Both SO/N2 and TEC displayed similar strong, long-lived and localized depletions and enhancements at mid-latitudes. Numerical simulations driven by an empirical model of geomagnetic activity, but with a climatological tide in the lower boundary, qualitatively produced the patterns of observed variations. Analysis of simulations revealed that SO/N2 variations were initially formed at high-latitudes and then transported equatorward and westward. When geomagnetic forcing was turned off in simulations, the modeled SO/N2 and TEC variations were negligible. This study suggests the potentially important roles of high-latitude forcing in thermosphere and ionosphere variations at mid-latitudes even during some "geomagnetically quiet" periods at solar-minimum. [ABSTRACT FROM AUTHOR]

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 (ΣO/N2) during a geomagnetic storm on day of year (DOY) 130 (May 10) to DOY 132 in 2019. The percentage difference of ΣO/N2 between the storm time (DOY 131) and the quiet time (DOY 128) had a relatively enhanced ΣO/N2 region sandwiched by two depleted regions over North America and the Atlantic Ocean in the Northern Hemisphere. This enhanced ΣO/N2 region is called the neutral tongue here. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model also predicted a similar ΣO/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 ΣO/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. [ABSTRACT FROM AUTHOR]

The Global-scale Observation of Limb and Disk (GOLD) mission, for the first time, provides synoptic two-dimensional (2D) maps of OI 135.6 nm observations. These maps describe the unambiguous and dynamic evolution of nighttime ionospheric F2-peak electron densities (NmF2) as the 135.6 nm airglow emission radiance correlates well with NmF2 at night. On November 19, 2018, the 135.6 nm radiance measured by GOLD, NmF2 measured by a digisonde, and GPS total electron content (TEC) measurements at Cachoeira Paulista (CP) all showed a postsunset enhancement, with an increase near 22:30 universal time. The 135.6 nm radiance map showed that this enhancement was due to the southward movement of the southern equatorial ionization anomaly (EIA) crest. Therefore, the GOLD observation showed the linkage between postsunset enhancement of NmF2 and EIA movement. Furthermore, unlike the southward movement of the southern crest, the corresponding EIA northern crest, however, did not show northward motion. This is the first time that the EIA hemispheric asymmetry, which included both different densities and movement of two crests in a short time period (<2-h), was captured. The cause of this asymmetric movement of the two crests is not clear and requires further investigation. [ABSTRACT FROM AUTHOR]

The National Aeronautics and Space Administration (NASA) Global-scale Observations of the Limb and Disk (GOLD) has been imaging the thermosphere and ionosphere since October 2018. It provides continuous measurements over a large area from its geostationary orbit. The unambiguous two-dimensional (2-D) maps of OI 135.6 nm radiance retrieved from GOLD after sunset are compared with the total electron content (TEC) maps measured by GPS receivers in the American sector. The OI 135.6 nm radiance observed by GOLD is an indicator of the peak electron density of the ionosphere F2 region, while the TEC is the total electron density in the column. Our comparisons show that the two data sets match each other very well in the equatorial ionization anomaly (EIA) morphology and its seasonal variability. Equatorial plasma bubbles (EPBs) are evident in GOLD nighttime OI 135.6 radiance. Corresponding depletions are shown in TEC maps, but without GOLD data as a reference, it is difficult to discern that the depletions are EPBs. In addition, both GOLD 135.6 radiance and TEC maps observed third peaks of electron density poleward of the southern EIA crests. Furthermore, both show that the ionosphere after sunset is quite dynamic and has strong day-to-day variability. In all, the GOLD and TEC have valuable synergy to allow us to gain a better understanding of the equatorial ionosphere. [ABSTRACT FROM AUTHOR]

To investigate gravity wave (GW) perturbations in the midlatitude mesopause region during boreal equinox, 433 h of continuous Na lidar full diurnal cycle temperature measurements in September between 2011 and 2015 are utilized to derive the monthly profiles of GW-induced temperature variance, T′², and the potential energy density (PED). Operating at Utah State University (42° N, 112° W), these lidar measurements reveal severe GW dissipation near 90 km, where both parameters drop to their minima (~20 K² and ~50m² s-2, respectively). The study also shows that GWs with periods of 3-5 h dominate the midlatitude mesopause region during the summer-winter transition. To derive the precise temperature perturbations a new tide removal algorithm suitable for all ground-based observations is developed to de-trend the lidar temperature measurements and to isolate GW-induced perturbations. It removes the tidal perturbations completely and provides the most accurate GW perturbations for the ground-based observations. This algorithm is validated by comparing the true GW perturbations in the latest mesoscale-resolving Whole Atmosphere Community Climate Model (WACCM) with those derived from the WACCM local outputs by applying this newly developed tidal removal algorithm. [ABSTRACT FROM AUTHOR]