Thermospheric Temperature and ΣO/N₂ Variations as Observed by GOLD and Compared to MSIS and WACCM‐X Simulations During 2019–2020 at Deep Solar Minimum

Abstract

The ultraviolet‐imaging spectrograph that comprises Global‐scale Observations of the Limb and Disk (GOLD) mission in geostationary orbit at 47.5°W longitude has taken full disk images at high cadence throughout the deep solar minimum period of 2019–2020. Synoptic (i.e., concurrent and spatially unified and resolved) observations of thermospheric temperature and composition at ∼150 km altitude are made for the first time, allowing GOLD to disambiguate temporal and spatial variations. Here we analyze the daytime effective temperature and column integrated O and N2 density ratio (ΣO/N2) data simultaneously observed by GOLD over 120°W–20°E longitude and 60°S–60°N latitude from 13 October 2019 to 12 October 2020. Daily zonal mean values are calculated for each latitude and compared with NRLMSIS 2.0 and simulations from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X). On average, the GOLD observations show higher temperatures than Mass Spectrometer Incoherent Scatter radar (MSIS) and WACCM‐X by ∼20–60 K (5%–10%) and 80–120 K (12%–18%), respectively. The ΣO/N2 ratios observed by GOLD are larger than the MSIS results by ∼0.4 (40%) but smaller than the WACCM‐X simulations by ∼0.3 (30%). The observed and modeled results are correlated at most latitudes (r = 0.4–0.8), and GOLD, MSIS, and WACCM‐X all display a similar seasonal variation and change with latitude. WACCM‐X simulates a larger annual variation in ΣO/N2, suggesting that the thermospheric circulation is overestimated and atmospheric waves and turbulence transport are not properly represented in the model.

Impacts of Gravity Waves on the Thermospheric Circulation and Composition

“…The O/N 2 from the coarse resolution simulation is larger than observations at all latitudes, with increasing bias at higher latitudes in the winter hemisphere. The over‐estimation is also seen when compared with GOLD observation (G. Liu et al., 2023). This ratio in the high resolution simulation shows an overall decrease, bringing it to better agreement with observations.…”

“…In the thermosphere, photochemistry and diffusive separation are the dominant processes controlling the composition, but transport processes of different spatial and temporal scales can cause significant variability. For example, O/N 2 , a measure of the relative abundance of atomic and molecular species, is known to vary on seasonal and semi‐annual scales (e.g., Fuller‐Rowell, 1998; G. Liu et al., 2023; D. J. Strickland et al., 2004). This is thought to be controlled by the variation of thermospheric general circulation, which causes more rigorous “stirring” during solstices when the circulation is strong (Fuller‐Rowell, 1998; Jones et al., 2018).…”

“…However, it is found that although physics‐based models can qualitatively simulate the seasonal/semi‐annual variation global averaged O/N 2 , they often over‐estimate the ratio, especially in the winter hemisphere. For example, in comparison with GUVI and GOLD measurements, the column integrated O/N 2 from WACCM‐X can be over twice as large in the winter hemisphere at middle to high latitudes (G. Liu et al., 2023; D. J. Strickland et al., 2004). The over‐estimation is also evident in TIE‐GCM during solstice when eddy diffusion is not imposed (L. Y. Qian et al., 2009, Figure 3).…”

Disk Images of Neutral Temperature From the Global‐Scale Observations of the Limb and Disk (GOLD) Mission

“…TDISK data have also been used to quantify changes in thermospheric neutral density and dynamics in response to the SpaceX launch time geomagnetic storm of 3-4 February 2022 (e.g., He et al, 2023;Kakoti et al, 2023;Laskar et al, 2023). TDISK data are used for validation of model simulations (e.g., Laskar et al, 2023;Liu et al, 2023) and thermospheric observations (Aryal et al, 2022;Cantrall & Matsuo, 2021;Park et al, 2022). It has been reported that GOLD TDISK data are high compared to MSIS2 model simulations (e.g., Laskar et al, 2023;Liu et al, 2023).…”

“…TDISK data are used for validation of model simulations (e.g., Laskar et al., 2023; Liu et al., 2023) and thermospheric observations (Aryal et al., 2022; Cantrall & Matsuo, 2021; Park et al., 2022). It has been reported that GOLD TDISK data are high compared to MSIS2 model simulations (e.g., Laskar et al., 2023; Liu et al., 2023). GOLD TDISK have been used to show that there are hemispheric asymmetries in the duration of thermospheric summer, which varies with latitude and local time (e.g., Wang et al., 2023).…”