The Retrieving of Ionospheric Upper Transition Height Over Low Latitude From C/NOFS Ion Composition Measurements

The upper transition height (UTH), defined as the altitude where the dominant ion changes from O + to light-ions, is a key parameter in the topside ionosphere. In this study, we propose an innovative approach to retrieve ionospheric UTH over low latitudes by integrating the ion composition measurements from the C/NOFS satellite with the deep learning ionosphere model (DLIM). Compared to the direct UTH observations when the O + composition is 0.5, this approach can retrieve UTH when the O + composition ranges from 0.3 to 0.7. Our approach increases the number of available UTH measurements from approximately 92,100 to 1,075,000, which allows more comprehensive local time (LT) and magnetic latitude coverage. The proposed approach was validated using observations from the incoherent scatter radar (ISR) located in Jicamarca. The UTH between retrieved and measured from ISR shows good agreement, with the average value and standard deviation of discrepancy being 0.32 and 5.81 km, respectively. The retrieved UTH exhibits typical local time, seasonal and magnetic latitudinal dependence. Comparison results show that during low solar flux conditions, the retrieved UTH showed excellent consistency with the measured UTH with a relative error of less than 3%. Further analysis revealed a positive correlation between the UTH and the F10.7 index, which is stronger than results from directly measured UTH and ISR observations. The rate of change of retrieved UTH with respect to the F10.7 index varies with local time and can reach up 9.15 km/sfu at noontime (12:00 LT), which could be linked to the enhancement of E × B drifts. Plain Language Summary This study focuses on the upper transition height (UTH)-the altitude where light-ions begin to dominate over O + in the upper ionosphere. It is a key parameter in ionospheric physics and an important measurement for studying the ionosphere and how it connects to the plasmasphere. The C/NOFS satellite, with its unique elliptical orbit, has become an important means for observing UTH. However, due to the continuous orbital decay of the satellite, it was unable to effectively measure UTH during periods of high solar activity, and thus the satellite could only provide UTH observations from 2008 to 2010. To better study the climatological characteristics of UTH and its influencing factors, this paper proposes a method that combines an empirical model with satellite observation to retrieve UTH using ion observation data from the C/NOFS satellite. This algorithm increases the number of UTH observations from the approximately 92,100 to more than 1,075,000-an expansion by tenfold. Based on this extended data set, not only can the distribution characteristics of UTH with respect to latitude, local time, and season be obtained, but it is also possible to analyze its global variations with solar activity-an aspect that previous studies relying solely on direct observations were unable to achieve.