The vertical geoelectric ®eld measured at Vostok, Antarctica (78.58S, 1078E, L=75.0) over the 13 month interval May 1979±May 1980 is correlated with the interplanetary magnetic ®eld (IMF) components B y and B z at times when Vostok is connected to the dayside magnetosphere. No signi®cant association with IMF B x is found. The interaction of the solar wind and the Earth's magnetic ®eld generally results in anti-sunward plasma ¯ow in the high-latitude, polar ionosphere driven by a dawn-to-dusk, cross polar cap potential dierence pattern. Using the IZMEM model to infer the contribution of the cross polar cap potential dierence to the potential dierence between the ionosphere and the ground at Vostok for the measured IMF conditions, we show that this provides a viable mechanism for the IMF associations found. We demonstrate that the direct association of the geoelectric ®eld with the cross polar cap potential dierence is independent of a result (Park, 1976. Solar magnetic sector eects on the vertical atmospheric electric ®eld at Vostok, Antartica. Geophysical Research Letters 3(8), 475±478) showing an H15% decrease in the vertical geoelectric ®eld measured at Vostok, 1±3 days after the passage of IMF sector boundaries. Evidence is also presented supporting the Park result, for which a mechanism is yet to be con®rmed.