The last solar minimum period was anomalously extended and low in EUV irradiance compared with previous solar minima. It can readily be expected that the thermosphere and the ionosphere must be correspondingly affected by this low solar activity. While there have been unanimous reports on the thermospheric changes, being cooler and lower in its density as expected, the ionospheric responses to low solar activity in previous studies were not consistent with each other, probably due to the limited ionospheric observations used for them. In this study, we utilized the measurements of total electron content (TEC) from TOPEX and JASON-1 satellites during the periods of 1992 to 2010, which includes both the last two solar minimum periods, in order to investigate how the ionosphere responded to the extremely low solar activity during the last solar minimum compared with previous solar minimum. Although the global daily mean TECs show negligible differences between the two solar minimum periods, the global TEC maps reveal that there are significant systematic differences ranging from about À30% to +50% depending on local time, latitude, and season. The systematic variations of the ionospheric responses seem to mainly result from the relative effects of reduced solar EUV production and reduced recombination rate due to thermospheric changes during the last solar minimum period.
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.
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