OBSERVED AND PREDICTED POTENTIAL DISTRIBUTIONS DURING THE OCTOBER 1995 MAGNETIC CLOUD PASSAGE

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 B-Z strong ly southward, the polar cap expanded and the potential measured across it reached 174 kV, then decreased as the IMF rotated toward B-Y < 0. During the initial northward IMF phase convection in the polar cap was dominated by large lobe cells, clockwise in the southern and counterc lockwise in the northern hemisphere. With purely northward IMF, convec tion patterns had four cells and were irregular on the day and night s ides of the magnetic dawn-dusk meridian. Potential distributions measu red with a southward or westward IMF agree with predictions of the Wei mer [1996] model, suggesting that some global MHD simulations overesti mate dayside merging rates.