Climatology of ionospheric upper transition height derived from COSMIC satellites during the solar minimum of 2008

The electron density profiles retrieved from the Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) satellite Radio Occultation (RO) observations during 2008 are used to derive ionospheric upper transition height, where the density of O⁺ is equal to that of light ions (mainly H⁺ and He⁺). It is found that the ionosphere upper transition height is very low, with significant local time, latitude and seasonal variations, during the extremely low solar minimum of 2008. The transition height is higher in the daytime than at night, except over middle latitude region of winter hemisphere, where the transition height has minimum in the morning. There is a pronounced peak over equator for all seasons. The transition height is higher in summer than in winter hemisphere. Our results have comparability with C/NOFS satellite observations around the equatorial region during June-August of 2008. However, the IRI model gives much higher transition height than those from COSMIC and cannot reproduce its latitude and season variations well during 2008.