Sources of Plasma Troughs in the Midlatitude and Subauroral Topside Ionosphere

The data of observations of plasma density in the large-scale ionospheric trough by the ion trap onboard the Kosmos-900 satellite on April 15, 1977, are presented. They enable one to understand both the general configuration and the fine details of the trough. There are sharp gradients of density (especially, in the subtroughs) superimposed on the less prominent variations of density in the main trough, which are characteristic of the average pattern of the trough. It was demonstrated that one of the possible mechanisms of forming the troughs(i.e., plasma cavities) is the electrostatic instability of plasma in the topside ionosphere.