Topside ionospheric scale heights retrieved from Constellation Observing System for Meteorology, Ionosphere, and Climate radio occultation measurements

The vertical scale height (VSH) in the topside ionosphere can be derived from electron density profiles. In this study, the electron density profiles retrieved from the COSMIC/FORMOSAT-3 (a Constellation Observing System for Meteorology, Ionosphere, and Climate mission; C/F3 for short) ionospheric radio occultation (IRO) observations have been collected to investigate the local time, seasonal, latitudinal, and longitudinal variations of the VSH. With the postprocessed C/F3 IRO electron density profiles during the interval from day of year (DOY) 194 in 2006 to DOY 60 in 2008, we conduct an analysis on the behaviors of VSH at an altitude of 400 km. There are appreciable latitudinal variations in VSH. A new finding is a significant peak around dip equator during daytime in four seasons. Away from the equatorial peak, it is obvious that the VSH generally increases at higher latitudes. The equatorial VSH undergoes a significant diurnal variation with a local noon maximum. The peak shifts to sunrise time with increasing dip latitude, and the values of daytime VSH become comparable with those at nighttime at low latitude in both hemispheres, which is somewhat different from the feature revealed from Arecibo incoherent scatter radar observations. One of the crucial findings in our results is the most outstanding feature of VSH, that is, the presence of a substantial longitudinal structure in equatorial regions. A wave-like longitudinal feature is found in equatorial VSH during the daytime in four seasons, while it becomes weaker or absent at other local time intervals and at higher latitudes. This investigation also confirms that the behaviors of VSH are not strongly consistent with those of the neutral or plasma-scale heights.