Topside ionospheric Vary-Chap scale height retrieved from the COSMIC/FORMOSAT-3 data at midlatitudes

An α-Chapman function with a continuously varying scale height can be used to describe the topside F2 vertical electron density profile that seamlessly connects the ionosphere with the plasmasphere. The local time, seasonal, latitudinal and longitudinal variations of the Vary-Chap scale heights (VCSHs) in 2007 at midlatitudes are investigated using the electron density profiles retrieved from the COSMIC/FORMOSAT-3 ionospheric radio occultation observations. The results show that the VCSHs at 400–550 km have prominent altitudinal dependence and diurnal, seasonal variations, and are not tightly correlated with the neutral or plasma-scale heights. The values of the VCSHs in the daytime are larger than those in the nighttime in spring, autumn and winter, comparable and even larger nighttime ones can be detected in summer. The VCSHs change slowly from the midnight to predawn and change abruptly around the sunrise, and the daytime maximum are in winter and the minimum are in summer. In the midlatitudes, the daytime VCSHs decrease with latitude, while at nighttime, the latitudinal changes of VCSHs are not so distinct. The daytime VCSHs in northern hemisphere are larger than those in southern hemisphere. The VCSHs have complicated longitudinal variations and no obvious wavelike structures. What is more, the IRI-2012 model is capable of reproducing the trend of the diurnal, seasonal and latitudinal VCSH variations. But the values have large differences with those retrieved from the COSMIC/FORMOSAT-3 data.