Comparison of topside ionosphere scale height modeled by the Topside Sounder Model and incoherent scatter radar ionospheric model

The topside ionosphere scale height extracted from two empirical models are compared in the paper. The Topside Sounder Model (TSM) provides directly the scale height (HT), while the incoherent scatter radar ionospheric model (ISRIM) provides electron density profiles and its scale height (HR) is determined by the lowest gradient in the topside part of the profile. HT and HR are presented for 7 ISR locations along with their dependences on season, local time, solar flux F10.7, and geomagnetic index ap. Comparison reveals that HT values are systematically lower than respective HR values as the average offset for all 7 stations is 55 km. For the midlatitude stations Arecibo, Shigaraki, and Millstone Hill this difference is reduced to 43 km. The range of variations of HR is much larger than that of HT, as the HT range overlaps the lower part of the HR range. Dependences on ap, DoY and LT are much stronger in the ISRIM than in TSM. This results in much larger values of HR at higher ap. Diurnal amplitude of HR is much larger than that of HT, with large maximum of HR at night. The present comparison yields the conclusion that the ISR measurements provide steeper topside Ne profiles than that provided by the topside sounders.