The response of thermospheric composition to geomagnetic storms has been studied for several decades. The first such study was carried out by Seaton (1956), who proposed that an increase in molecular oxygen (O 2 ) number density might account for the decrease of electron density during a major storm that took place on January 25, 194925, . Jacchia (1959 first presented evidence of thermospheric density increases during storms using the measured changes in satellite orbital elements, which were explained in terms of atmospheric heating during geomagnetically disturbed conditions. Prölss and von Zahn (1974aZahn ( , 1974b and Prölss (1980Prölss ( , 1987Prölss ( , 2011 did further studies on the storm-time perturbations in composition, mainly by using the in situ multi-satellite observations near 300 km (e.g., Dynamics Explorer [DE]-2 and ESRO 4 satellites). Based on these studies, a picture of the main thermospheric compositional perturbations during geomagnetic storms was established: an increase of heavier constituents (molecular nitrogen [N 2 ] and oxygen [O 2 ]), and a height-dependent change of atomic oxygen (O) at high-latitudes and middle-latitudes (decreases at lower heights (<∼300 km) and increases higher up); an increase of all constituents at low latitudes over all heights in the thermosphere. In addition, the basic evolution of these perturbations can be described as follows. When a storm begins, strong temperature enhancements occur at high latitudes due to Joule heating, which lead to upward winds (Burns et al., 1995). The nitrogen-rich air is brought up from lower altitudes into the upper thermosphere. The horizontal pressure gradient from temperature differences, in conjunction with ion drag, drives strong horizontal wind changes within a few hours (Ponthieu et al., 1988;Prölss, 2011). Horizontal winds transport the nitrogen-rich air toward the middle-low latitudes at night (

The Global Ultraviolet Imager (GUVI) aboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet airglow emissions in the thermosphere. The retrieved altitude profiles of thermospheric neutral density from GUVI daytime limb scans are significant for ionosphere-thermosphere study. Here, we use the profiles of the main neutral density to derive the total mass density during the period 2002-2007 under geomagnetic quiet conditions (ap < =12). We attempt to compare the obtained total mass density with the Challenging Minisatellite Payload (CHAMP) observations, making use of an empirical model (GUVI model hereafter). This GUVI model is aimed to solve the difficulty of the direct comparison of GUVI and CHAMP observations due to their different local times at a given location in a given day. The GUVI model is in good agreement with CHAMP observations with the small standard deviations of their ratios (less than 10%) except at low solar flux levels. The correlation coefficients are greater than 0.9, and the relative standard errors are less than 20%. Comparison between the GUVI model and CHAMP observations during solar minimum shows a large bias (~30%). The large bias at low solar flux levels might be due to the limitation of F 10.7 as an extreme ultraviolet radiation flux proxy and the fitting method. Our results demonstrate the validity and accuracy of our model based on GUVI data against the density data from the CHAMP satellite.

TIMED/Global Ultraviolet Imager (GUVI) limb measurements of far‐ultraviolet airglow emission have been used to investigate middle‐low latitude thermospheric composition and neutral temperature responses to the 20 and 21 November 2003 (day of year [DOY] 324 and 325) superstorm. Altitude profiles of O, N2 number densities and temperature, as well as O/N2 column density ratio (∑O/N2), on the storm days along the GUVI limb tracks are compared with those on DOY 323 (quiet time). The storm‐time composition and temperature responses were global and evolved continuously as the storm progressed. Specially, N2 and temperature increased almost globally at all altitudes during the storm and their perturbation structures were similar. The magnitudes of their enhancements both increased with altitude and latitude. The storm‐induced O perturbations decreased in the lower thermosphere but increased in the upper thermosphere. Transition heights of O perturbations from decrease to increase changed with latitude and time. During the storm main and recovery phases, the storm‐induced ∑O/N2 decreases were mostly related to the O depletion in the low‐middle thermosphere, whereas ∑O/N2 increases during the storm were primarily caused by N2 depletion. There was a remarkable hemispheric asymmetry in composition responses as they have different morphologies and lifetime, especially during the storm recovery phase.

The ratio of O number density to N2 number density (O/N2) is an important parameter to describe thermospheric composition changes and its effects on the ionosphere. Based on Global Ultraviolet Imager (GUVI) limb measurements, we investigate the seasonal behaviors of O/N2 volume density ratio on different constant pressure levels during geomagnetically quiet periods. The global O/N2 shows the prominent annual and semiannual variations with solar activity dependence. An empirical model considering the solar activity and annual/semiannual variations can reasonably reproduce the original O/N2. The modeled O/N2 captures the hemispheric asymmetry of the annual variations in both length and magnitude. Global maps of the seasonal harmonic components of the modeled O/N2 indicate the latitudinal and altitudinal dependence of O/N2 seasonal variations. The annual component dominates over the semiannual component at mid‐latitudes, but it is smaller than the semiannual component at low latitudes. In the Northern Hemisphere, and at low geomagnetic latitudes of the Southern Hemisphere, the annual component peaks around December solstice at all altitudes, whereas at middle geomagnetic latitudes of the Southern Hemisphere, it peaks around June solstice. The semiannual component peaks at the equinoxes in almost all regions over the globe at all altitudes. The annual and semiannual amplitudes both increase with altitude. In addition, O/N2 annual variations and solar activity dependence are more influenced by the thermal expansion and contraction.

Given that the ionosphere is strongly determined by the thermosphere and its state depends on thermospheric parameters, we propose a new method to extract exospheric temperature (Tex) from electron density (Ne) profiles based on the relationship between the variations in Tex and Ne profiles established through simulation. Ne profiles and corresponding Tex from the Millstone Hill incoherent scatter radar (ISR) observations are used to test the method. ISR Ne profiles are used for Tex retrieval and ISR Tex is used to make a comparison with Tex calculated by model and retrieved Tex. The results show that the retrieved Tex effectively captures diurnal, anomalous and short‐period variations. The relative deviation between the retrieved–observed Tex is approximately 2%, which is significantly improved compared with the Mass Spectrometer Incoherent Scatter model, especially under disturbed conditions. This result confirms that thermospheric temperature variation can be deduced from ionospheric profiles and our method can be considered a useful tool to obtain Tex from Ne profiles.

A long‐lived (>10 hr) O/N2 column density ratio (∑O/N2) depletion at middle latitudes was observed by the Global‐scale Observations of the Limb and Disk (GOLD) during the 20 April 2020 geomagnetic storm. The observed ∑O/N2 depletion tilts latitudinally at equatorward boundary with the lowest latitude of ∼20°N at ∼75°W, and changes orientation near 75°W, with a north‐westward tilt to the west of ∼75° ${}^{\circ}$W and a north‐eastward tilt to the east of ∼75° ${}^{\circ}$W. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model shows the similar ∑O/N2 depletion patterns and magnitudes, but the longitudes with the most equatorward were ∼100°W and moved westward from ∼100°W to ∼140°W during the storm recovery period. Horizontal winds play major role in forming this middle‐latitude ∑O/N2 depletion structure. The strongest equatorward winds in the longitude sector near the magnetic pole resulted in the most equatorward expansion of all longitudes, generating the depletion with latitudinally tilted equatorward boundary. In addition, the high‐latitude westward zonal wind and the middle‐latitude eastward zonal wind maintained the long existence of this depletion in the GOLD Field of View. The further analysis on neutral winds shows that storm‐time meridional winds at middle latitudes in the mid‐low thermosphere are mainly dominated by pressure gradient force, while zonal winds are determined primarily by horizontal momentum advection.

The thermospheric and ionospheric responses to geomagnetic disturbances have been studied for several decades. The first type of studies is mostly focusing on one single strong storm (Kp ≥ 5, based on the criteria from NOAA: https://www.swpc.noaa.gov/noaa-scales-explanation) that is mostly generated due to coronal mass ejection. During geomagnetic storms, a tremendous amount of magnetospheric energy is deposited into the ionosphere and thermosphere, which leads to enhancement of Joule heating. Resultingly, enlarged neutral temperatures and vertical winds generate the thermospheric composition disturbance: larger increases of the molecular species relative to the atomic ones (A. G. Burns et al., 1995;Prölss, 1980) near the local midnight sector. Then the strengthened equatorward meridional wind transports the composition disturbance equatorward to mid and low latitudes, and corotated toward later local time (A.

We define a new thermospheric concept, the reference heights of O/N 2 , referring to a series of thermospheric heights corresponding to the fixed ratios of O to N 2 number density. Here, based on Global Ultraviolet Imager (GUVI) limb measurement, we compare O/N 2 column density ratio (∑O/N 2 ) and the reference heights of O/N 2 . We choose the transition height of O and N 2 (transition height hereafter), a special reference height at which O number density is equal to N 2 number density, to verify the connection with ∑O/N 2 during geomagnetically quiet periods. It is found that transition height and ∑O/N 2 have noticeable negative correlation with correlation coefficient of -0.887. An empirical model of transition height (O/N 2 model hereafter) is established based on nonlinear least-squares-fitting method. The considerable correlation (greater than 0.96), insignificant errors (less than 4%) and the great influencing weight of ∑O/N 2 to reference heights indicate the validity of O/N 2 model and the existence of quantitative relation between ∑O/N 2 and transition height. Besides, it is verified that the similar quantitative relation also exists between ∑O/N 2 and reference heights of other O/N 2 values. Namely, using the O/N 2 model coefficients, we can roughly get the whole altitude profiles of O/N 2 within 6% precision for any given ∑O/N 2 .