Where does the plasmasphere begin? Revisit to topside ionospheric profiles in comparison with plasmaspheric TEC from Jason-1

Topside ionospheric profiles have been measured by Alouette 1 and ISIS 1/2 in the periods of 1962–1972 and 1972–1979, respectively. The profiles cover from the orbital altitude of 1000 km to the F2 peak and show large variations over local time, latitude, and seasons. We here analyze these variations in comparison with plasmaspheric total electron contents (pTECs) that were measured by Jason-1 satellite from the altitude of 1336 km to 20,200 km (GPS orbit). The scale heights of the profiles are generally smaller in the daytime than nighttime but show large day-to-day variations, implying that the ionospheric profiles at 1000 km are changing dynamically, rather than being in diffusive equilibrium. We also derived transition heights between O+ and H+, which show a clear minimum at dawn for low-latitude profiles due to decreasing O+ density at night. To compare with pTEC, we compute topside ionospheric total electron content (tiTEC) by integrating over 800–1336 km using the slope of the profiles. The tiTEC varies in a clear diurnal pattern from ~0.3 to ~1 and ~3 total electron content unit (TECU, 1 TECU = 1016 el m−2) for low and high solar activity, respectively, whereas Jason-1 pTEC values are distributed over 2–6 TECU and 4–8 TECU for low and high solar activity, respectively, with no apparent diurnal modulation. Latitudinal variations of tiTEC show distinctive hemispheric asymmetry while that of Jason-1 pTEC is closely symmetric about the magnetic equator. The local time and latitudinal variations of tiTEC basically resemble those of the ionosphere but are characteristically different from those of Jason-1 pTEC. Based on the difference between tiTEC and pTEC variations, we propose that the region above ~1300 km should be considered as the plasmasphere. Lower altitudes for the base of “plasmaspheric TEC,” as used in some studies, would cause contamination of ionospheric influence.