Ion drift measurements from DMSP F13 provide the first opportunity to study the evolution of high latitude potentials in both hemispheres as a magnetic cloud passed Earth. Early in the event with IMF BZ strongly southward, the polar cap expanded and the potential measured across it reached 174 kV, then decreased as the IMF rotated toward BY<0. During the initial northward IMF phase convection in the polar cap was dominated by large lobe cells, clockwise in the southern and counterclockwise in the northern hemisphere. With purely northward IMF, convection patterns had four cells and were irregular on the day and night sides of the magnetic dawn-dusk meridian. Potential distributions measured with a southward or westward IMF agree with predictions of the Weimer [1996> model, suggesting that some global MHD simulations overestimate dayside merging rates. ¿ 1998 American Geophysical Union