Solar activity dependence of the topside ionosphere at low latitudes

Abstract

We investigated the solar activity dependence of the topside ionosphere with ROCSAT‐1 observations. The distribution of the plasma density at 600 km altitude shows features with considerable local time, season, and solar activity differences. In the daytime, plasma density peaks around the dip equator. This peak is more distinct in equinoxes and weaker in May–July, and it enhances with solar activity in all seasons. The seasonal behavior of this peak is primarily controlled by the seasonal variations of neutral density and E × B vertical drift. The enhancement of the peak with solar activity is related to the effect of E × B vertical drift. Around sunset, double peaks are found in the latitudinal distribution of plasma density in solar maximum equinoxes and December solstice, which are mainly attributed to the effects of strong prereversal enhancement (PRE) vertical drift. Moreover, the plasma density at 600 km altitude strongly depends on the solar proxy P = (F107 + F107A)/2. At higher altitudes, e.g., 800 km, the amplification trend prevails in the solar activity variations of plasma density. In contrast, the plasma density at 600 km altitude presents three kinds of patterns (linear, amplification, and saturation), which has not been reported. Saturation effect is found at equinox sunset around the dip equator. This saturation effect is attributed to the increase in the PRE vertical drift with solar activity. Solar activity effects of ROCSAT‐1 plasma density are argued to be the combined effects induced by the changes in the peak height, the scale height, and the peak electron density, respectively. Among these factors, the rise of the F2 peak is more important for the equatorial plasma density at 600 km altitude.