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.