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}.

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.

We propose a new method to derive the nightside thermsopheric density by extending GUVI dayside limb observations using empirical orthogonal function (EOF) analysis. First, we acquire the GUVI dayside total mass density during 2002‐2005 to construct a preliminary empirical model (EM). Simultaneously, we decompose the background thermospheric density from US Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Extended (NRLMSISE‐00) model into different empirical orthogonal functions (EOFs). The decomposed EOFs are then used to fit the continuous density from EM, to develop a new nightside extended model (NEM). The preliminary EM and developed NEM are further evaluated with CHAMP satellite observations. Higher correlation coefficients and smaller relative standard errors (RSE) between CHAMP observations and the NEM results are obtained than those between CHAMP observations and the EM results, and the NEM results are in good agreement with the CHAMP observations in time series during both daytime and nighttime, which all prove the NEM method is effective to the reproduction and extension of GUVI original dayside observations. Furthermore, the NEM reveals two typical seasonal variation features, the semiannual variation and equinoctial asymmetry of thermospheric density. The model provides an effective tool to derive the nightside thermospheric density and explore the thermospheric intrinsic structure, and needs the further development to achieve more widespread application of the thermosphere.

%B Space Weather %8 10/2019 %G eng %U https://onlinelibrary.wiley.com/doi/abs/10.1029/2019SW002304 %! Space Weather %R 10.1029/2019SW002304