High‐latitude ion convection in the nighttime F region

A large data base of Atmosphere Explorer C measurements of the bulk ion velocity vector in the high‐latitude F region has been examined. The concept of shear and rotational flow reversals is expanded upon to determine the extent to which the boundary between antisunward and sunward convection (i.e., a polar cap boundary) is an electrostatic equipotential. A most important factor in characterizing the nature of the large‐scale geometrical configuration of the nighttime F region convection is the location and extent of quasi‐equipotential regions of the convection reversal boundary.