Polar cap convection for B_z northward

The Digisonde at Qanaq, Greenland (87°Λ) measures the ionospheric convection velocity in the central polar cap. The average local time variation of the velocity for Bz northward under various By conditions is presented. The nightside drifts for Bz northward tend to be antisunward, and resemble those which occur for southward IMF, as pointed out by Heppner and Maynard (1987). However, it is difficult to reconcile the observed diurnal drift variation with the Heppner-Maynard distorted two-cell model. Modified forms of the Potemra et al. (1984) four cell model offer a more plausible framework in which to understand the Qanaq data. The polar cap cells are relatively small, and are displaced towards the dayside.