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The high‐resolution Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM‐X) is used to study the impacts of gravity waves (GWs) on the thermospheric circulation and composition. The resolved GWs are found to propagate anisotropically with stronger eastward components at most altitudes. The dissipation of these waves in the thermosphere produces a net eastward forcing that reaches peak values between 200 and 250 km at mid‐high latitudes in both hemispheres. Consequently, the mean circulation is weakened in the winter hemisphere and enhanced in the summer, which in turn impacts the thermospheric composition. Most notably, the column integrated O/N2 in both hemispheres is reduced and agrees better with observations. The mean thermospheric GW forcing in the meridional direction has comparable amplitude and acts to modify the gradient‐wind relationship. Plain Language Summary Small‐scale waves originate from the lower atmosphere have been shown to propagate into the thermosphere. To study their effects a high‐resolution whole atmosphere model has been employed. Using this high‐resolution model, which can partially resolve the small‐scale waves, we can directly quantify the force exerted by these waves on the general circulation in the thermosphere. We found that such force is strong, and affects the thermospheric circulation in both winter and summer hemisphere. This consequently changes the distribution of important thermospheric species. One measure of the thermospheric composition is the ratio of atomic oxygen and molecular nitrogen, which is an indicator of the relative abundance of atomic and molecular species. This ratio has been grossly over‐estimated in previous modeling studies. It is reduced as a result of the circulation change, and is much better agreement with observations. Key Points Gravity waves (GWs) resolved by high‐resolution WACCM‐X displays anisotropic propagation GW forcing alters thermospheric circulation The circulation change leads to a much improved thermospheric O/N2

In the current work, temperature and wind data from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite during the years 2002–2007 were used to describe the seasonal variations of the westward propagating 6.5-day planetary wave in the mesosphere and lower thermosphere (MLT). Thermospheric composition data from the TIMED satellite and ionospheric total electron content (TEC) from the International Global Navigation Satellite System (GNSS) Service were then employed to carry out two case studies on the effect of this dissipating wave on the thermosphere/ionosphere. In both cases, there were westward anomalies of ~ 30–40 m s−1 in zonal wind in the MLT region that were caused by momentum deposition of the 6.5-day wave, which had peak activity during equinoxes. The westward zonal wind anomalies led to extra poleward meridional flows in both hemispheres. Meanwhile, there were evident overall reductions of thermospheric column density O / N2 ratio and ionospheric TEC with magnitudes of up to 16–24 % during these two strong 6.5-day wave events. Based on the temporal correlation between O / N2 and TEC reductions, as well as the extra poleward meridional circulations associated with the 6.5-day waves, we conclude that the dissipative 6.5-day wave in the lower thermosphere can cause changes in the thermosphere/ionosphere via the mixing effect, similar to the quasi-two-day wave (QTDW) as predicted by Yue and Wang (2014).

The high-frequency and small horizontal scale gravity waves may be reflected and ducted in non-hydrostatic simulations, but usually propagate vertically in hydrostatic models. To examine gravity wave propagation, a preliminary study has been conducted with a global ionosphere–thermosphere model (GITM), which is a non-hydrostatic general circulation model for the upper atmosphere. GITM has been run regionally with a horizontal resolution of 0.2° long × 0.2° lat to resolve the gravity wave with wavelength of 250 km. A cosine wave oscillation with amplitude of 30 m s−1 has been applied to the zonal wind at the low boundary, and both high-frequency and low-frequency waves have been tested. In the high-frequency case, the gravity wave stays below 200 km, which indicates that the wave is reflected or ducted in propagation. The results are consistent with the theoretical analysis from the dispersion relationship when the wavelength is larger than the cutoff wavelength for the non-hydrostatic situation. However, the low-frequency wave propagates to the high altitudes during the whole simulation period, and the amplitude increases with height. This study shows that the non-hydrostatic model successfully reproduces the high-frequency gravity wave dissipation.

A comprehensive model is developed to compute the volume emission rate of O+(2P) 732.0 nm dayglow emission. The modeled volume emission rates are compared with the measurements as provided by Atmosphere Explorer C satellite and Dynamics Explorer 2 spacecraft. It is found that the model explains quite well the measured emission profiles. The present model is used to study the effect of atomic oxygen abundance on the volume emission rate of 732.0 nm dayglow emission at the equator for equinox and solstice. To study the effect of atomic oxygen abundance on 732.0 nm dayglow emission, the atomic oxygen number densities obtained from the NRLMSISE‐00 model are increased (or decreased) in an increment (or decrement) of 20% and are incorporated into the model to compute volume emission rate profiles. The present study shows that the peak emission rate (PER) varies linearly below the reference level of atomic oxygen number density and does not vary linearly above the reference level of atomic oxygen number density. The atomic oxygen number density at reference level corresponds to that value which is obtained from the NRLMSISE‐00 model. It is found that the altitude of peak emission rate moves upward as the F10.7 solar index increases. On average the upward movement of altitude of PER is about 9 km for both the equinox and solstice cases. The upward movement of the altitude of peak emission rate is due to the enhancement in atomic oxygen number density with increase in F10.7 solar index. Key Points A study of 732.0 nm dayglow emission VER profiles at equator for different atomic oxygen density conditions Variation of altitude of PER under varying solar activity

The Global Ultraviolet Imager (GUVI) onboard the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet emissions from O and N2 in the thermosphere. Transformation of far ultraviolet radiances measured on the Earth limb into O, N2, and O2 number densities and temperature quantifies these responses and demonstrates the value of simultaneous altitude and geographic information. Composition and temperature variations are available from 2002 to 2007. This paper documents the extraction of these data products from the limb emission rates. We present the characteristics of the GUVI limb observations, retrievals of thermospheric neutral composition and temperature from the forward model, and the dramatic changes of the thermosphere with the solar cycle and geomagnetic activity. We examine the solar extreme ultraviolet (EUV) irradiance magnitude and trends through comparison with simultaneous Solar Extreme EUV (SEE) measurements on TIMED and find the EUV irradiance inferred from GUVI averaged (2002–2007) 30% lower magnitude than SEE version 11 and varied less with solar activity. The smaller GUVI variability is not consistent with the view that lower solar EUV radiation during the past solar minimum is the cause of historically low thermospheric mass densities. Thermospheric O and N2 densities are lower than the NRLMSISE‐00 model, but O2 is consistent. We list some lessons learned from the GUVI program along with several unresolved issues. Key Points A new thermospheric composition and temperature database from 2002 to 2007 Simultaneous altitudinal and geographic variations from limb remote sensing Solar, geomagnetic, seasonal, and long‐term trends detected and quantified

Long-term variability has previously been observed in the relative magnitude of annual and semi-annual variations in the critical frequency (related to the peak electron concentration) of the ionospheric F2 layer (foF2). In this paper we investigate the global patterns in such variability by calculating the time varying power ratio of semi-annual to annual components seen in ionospheric foF2 data sequences from 77 ionospheric monitoring stations around the world. The temporal variation in power ratios observed at each station was then correlated with the same parameter calculated from similar epochs for the Slough/Chilton data set (for which there exists the longest continuous sequence of ionospheric data). This technique reveals strong regional variation in the data, which bears a striking similarity to the regional variation observed in long-term changes to the height of the ionospheric F2 layer. We argue that since both the height and peak density of the ionospheric F2 region are influenced by changes to thermospheric circulation and composition, the observed long-term and regional variability can be explained by such changes. In the absence of long-term measurements of thermospheric composition, detailed modelling work is required to investigate these processes.

A study of ionospheric data recorded at Slough/Chilton, UK, from 1935 to 2012, has revealed long-term changes in the relative strength of the annual and semi-annual variability in the ionospheric F2 layer critical frequencies. Comparing these results with data from the southern hemisphere station at Stanley in the Falkland Islands between 1945 and 2012 reveals a trend that appears to be anti-correlated with that at Chilton. The behaviour of foF2 is a function of thermospheric composition and so we argue that the observed long-term changes are driven by composition change. The ionospheric trends share some of their larger features with the trend in the variability of the aa geomagnetic index. Changes to the semi-annual/annual ratio in the Slough/Chilton and Stanley data may therefore be attributable to the variability in geomagnetic activity which controls the average latitudinal extent of the auroral ovals and subsequent thermospheric circulation patterns. Changes in ionospheric composition or thermospheric wind patterns are known to influence the height of the F2 layer at a given location. Long-term changes to the height of the F2 layer have been used to infer an ionospheric response to greenhouse warming. We suggest that our observations may influence such measurements and since the results appear to be dependent on geomagnetic longitude, this could explain why the long-term drifts observed in F2 layer height differ between locations.

Changes in the thermospheric composition and temperature influence satellite drag coefficients through functional dependencies in the closed‐form solutions, and gas‐surface interactions via accommodation coefficients. This study investigates drag coefficient variations for the Gravity Recovery And Climate Experiment (GRACE) and Communications/Navigation Outage Forecasting System (C/NOFS) satellites under varying atmospheric conditions and satellite orientations. The closed‐form solutions of Diffuse Reflection and Incomplete Accommodation (DRIA) and Cercignani‐Lampis‐Lord (CLL) gas‐surface interaction models have been used to calculate the drag coefficients. The momentum and energy accommodation coefficients, derived using empirical models, are used as input variables in the closed‐form solutions to specify the nature of the gas‐surface interactions. The results provide a realistic view of drag coefficient variations for the atmospheric changes observed for low‐Earth orbit satellites. The analysis reveals that increasing the atomic oxygen mole fraction leads to significant decreases in the drag coefficients, with CLL showing greater variability than DRIA. The variability of the drag coefficients with neutral temperature demonstrates a strong dependence on satellite shapes, with GRACE drag coefficients increasing with temperature while C/NOFS drag coefficients decrease. Analysis of the C/NOFS orbits demonstrates drastic changes in the gas‐surface interactions, transitioning from oxygen‐dominated diffuse scattering at lower altitudes to helium‐dominated quasi‐specular interactions at higher altitudes. These variations persist during the September 2011 geomagnetic storm, with slightly reduced drag coefficients during storm‐time conditions compared to quiet periods. The GRACE drag coefficients are highly sensitive to pitch and yaw angle variations, while the C/NOFS drag coefficients show minimal sensitivity due to its more symmetrical geometry. Plain Language Summary The drag coefficients of satellites, obtained through analytic solutions, have an explicit functional dependence on the neutral atmospheric composition and temperature. Atmospheric conditions can cause further variation in drag coefficients through the momentum or energy accommodation coefficients of incoming particles, which shift the interaction between incoming atmospheric particles and satellite surfaces. In this work, we explore the variability of the drag coefficients of Gravity Recovery And Climate Experiment (GRACE) and Communications/Navigation Outage Forecasting System (C/NOFS) satellites due to different atmospheric conditions and satellite shape and orientation. The calculation of drag coefficients uses analytic solutions of different gas‐surface interaction models, which define the nature of interaction between particles and satellite surfaces. The accommodation of incoming particle energy/momentum is derived using recently developed empirical models. Therefore, the results provide a realistic view of drag coefficient variation for Low‐Earth Orbit (LEO) satellites. The comparison of drag coefficients derived from two different models assesses the degree of change in the assumed gas‐surface interactions resulting from identical atmospheric variation. The difference in the satellite shapes represents different aerodynamic flows around their surfaces, which is reflected in the drag coefficient variation with temperature as well as pitch and yaw angle. The comparison of drag coefficients of C/NOFS orbits during quiet and active geomagnetic periods reveals the effect of storm‐time conditions on drag coefficients. Key Points We explore the variation in the drag coefficients of GRACE and C/NOFS satellites under varying atmospheric conditions A comparison of DRIA and CLL drag coefficients shows the effect of assuming different gas‐surface interaction on drag coefficients

This brief note introduces several analytical approaches to OH* layer parameters. The number density and height of the OH* layer peak are determined by the distributions of atomic oxygen and temperature, and by corresponding vertical gradients. The theory can be applied to satellite-borne and ground-based airglow measurements, as well as to model results.