Isolated Peak of Oxygen Ion Fraction in the Post-Noon Equatorial F-Region: ICON and SAMI3/WACCM-X

In the equatorial region, the fraction of oxygen ions (O⁺) in the topside ionosphere contains information on the source altitude of the plasma, which is controlled, in part, by the vertical plasma motion in the F-region. Previous studies on this topic are restricted by limited coverage of local time, latitude, and season, leaving a significant knowledge gap in the distribution of the topside ionospheric composition. In this study, we statistically investigate the O⁺ fraction measured by ICON/IVM over all the local time sectors and seasons at low/midlatitudes. For the first time, we have found that an isolated peak in the O⁺ fraction emerges in the post-noon equatorial region. The peak is most prominent during equinoxes, while during solstices it is connected to the O⁺ fraction bulges in the local summer midlatitudes. Simulations with SAMI3 coupled with thermospheric parameters from WACCM-X reproduce the peak of the O⁺ fraction. The post-noon equatorial peak can be explained by the net vertical motion of plasma consisting of transports either parallel or perpendicular to geomagnetic field lines.