Polar ionospheric electric-potential patterns in the northern (left) and southern (right) hemispheres for 1992 January 27, 1825 UT, when the interplanetary magnetic field had components southward and dawnward (adapted from Lu et al., 1994). For both hemispheres the ionosphere is viewed from above the north magnetic pole; thus for the southern hemisphere the ionosphere is viewed as though one were looking through the Earth. The coordinates are magnetic local time, increasing counterclockwise from midnight at the bottom, and magnetic latitude, poleward of 50º. The contour interval is 10 kV. Dashed contours indicate regions of large uncertainty in the estimated potentials.