Unique latitudinal shape of ion upper transition height (H_T) surface during deep solar minimum (2008–2009)

Abstract

The ionospheric upper transition height (H T ) is found to increase dramatically by~100 km from 2008-2009 to 2010 only for a marginal increase in solar activity (F 10.7 ) by 11.76 solar flux units. The latitudinal variation of H T surface during 2008-2009 period exhibits a local minimum at equatorial latitudes and increase at low latitudes. Further, the H T at equatorial latitudes exhibits slower rate of increase than at low latitudes. These interesting features are new and different from those reported in literature. A quick loss of O + and increase in H + ions are observed around~550 to 650 km indicating that the charge exchange reaction is responsible for the slower rate of increase and lowered H T at equatorial latitudes. These new aspects of H T are more conspicuously observed during this deep solar minimum period where the resonant charge exchange reaction is taking place at altitudes as low as~550 km.

Empirical Model of the Lowest Cutoff Altitude of Global Hiss Near Magnetic Equator

“…Different from the light ions (H + ), the gyrofrequency of the heavy ions (O + ) is lower than the collision frequency ($${f}_{{\mathrm{O}}^{+}}< \upsilon $$) at the altitude of ∼360–760 km, indicating that the lowest cutoff altitude of the dayside equatorial hiss is located in the topside ionosphere ( L ca ∼ 1.1 R E ∼ 637.1 km). The topside ionosphere is a transition region between the ionosphere (the O + ions) and the plasmasphere (the H + ions) (Heelis et al., 2009; Li et al., 2015; Li, Yu, et al., 2022; Li, Zhou, et al., 2022; Tulasi et al., 2015). The hiss waves observed in this region are also called “ionospheric hiss” (Xia et al., 2019; Zhima et al., 2017).…”