Equatorial ionospheric electron density below the F-2 peak

True height profile analysis below the F-2 peak was employed in this study. Diurnal, seasonal, and solar cycle variations in electron density were exa mined from 100 km up the F-2 peak. Some results obtained for the equatorial region in the Indian and American sectors were confirmed in this study. Th e departure of the daytime electron density from that of a simple Chapman v ariation begins from an altitude of about 190 km at low solar activity and from 210 km at high solar activity. The midday minimum is observable from a bout 100 km below the F-2 peak at low solar activity. It is noticeable from about 40 to 90 km below the F-2 peak at high solar activity, except in Apr il, when a postnoon peak is not developed. Most of the features observed fi nd their explanation in the vertical E x B plasma drift variation.