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.

The strongest Southern Hemisphere minor sudden stratospheric warming (SSW) in the last 40 years occurred in September 2019 and resulted in unprecedented weakening of the stratospheric polar vortex. Ionospheric total electron content (TEC) observations are used to provide an overview of statistically significant anomalies in the low‐latitude ionosphere during this event. Quasi‐semidiurnal perturbations of TEC are observed in response to the SSW, similar to those seen during Northern Hemisphere SSWs. Analysis indicates the existence of quasi‐periodic oscillations in TEC in the crests of the equatorial ionization anomaly, with strong 5‐ to 6‐day and 2‐ to 3‐day periodicities. Ionospheric anomalies from the combined effects of multiple mechanisms exceed a factor of 2, comparable to the strongest anomalies associated with Northern Hemisphere SSWs. These results also indicate a remarkable longitudinal variation in the character and magnitude of variations that could be related to a modulation of the non‐migrating diurnal tide.

[1] Synoptic scale disturbances in the Arctic wintertime middle atmosphere are investigated using Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) satellite temperature observations and U.K. Meteorological Office (MetO) assimilated analyses. A representative data set on 13 February 2002 is used as a case study. The observed strong vertical and horizontal thermal gradients in the Arctic winter upper stratosphere and mesosphere are evocative to front-like behavior in the troposphere. The observations from the SABER instrument and the MetO data of temperature and geopotential suggest that baroclinic instability is a process involved in enhancing the disturbance. Contrary to tropospheric baroclinic development, the ageostrophic circulation associated with the middle atmosphere disturbance is indirect and is driven by eddy momentum flux convergence due to vertically propagating planetary waves. Investigating perturbation geopotential through the Arctic middle atmosphere prior to and during the event, a dominant zonal wave number 1 planetary wave propagating into the upper stratosphere is observed. The observations suggest that, through this planetary wave action, the middle atmosphere becomes highly baroclinic and can promote baroclinic instability that can enhance temperatures to the extreme levels characteristic of these disturbances near the stratopause and in the lower mesosphere.Citation: Thayer, J. P., K. Greer, and V. L. Harvey (2010), Front-like behavior in the Arctic wintertime upper stratosphere and lower mesosphere,