The cross-polar cap potential difference &PHgr; (KRM) is estimated from ground magnetic perturbation data through the magnetometer inversion method of Kamide, Richmond, and Matsuhita (KRM), combined with an ''empirical'' ionospheric conductance distribution estimated from the DMSP X ray image data. A significant correlation is found between &PHgr; (KRM) and the AE(12) index; &PHgr; (KRM in kilovolts)=36+0.0823 AE(12, in nanoteslas) with the correlation coefficient being 0.80. &PHgr; (KRM) is then compared with the potential difference estimated from a more direct method of the satellite electric field measurements [Weimer et al., 1990> and also with &PHgr; (IMF) based on solar wind parameters [Reiff and Luhmann, 1986>. &PHgr; (IMF) is found to be linearly correlated with &PHgr; (KRM), as &PHgr;(IMF(=29.8+0.999 &PHgr; (KRM), with the highest correlation obtained for a 40-min lag in the interplanetary magnetic field (IMF). Note that &PHgr; (IMF) is systematically larger than &PHgr; (KRM) by 30 kV, suggesting the possibility that the theoretical method overestimates the cross-polar cap potential difference. During steady southward IMF periods where steady &PHgr; (IMF) variations are expected, significant fluctuations in calculated &PHgr; (KRM) values are obtained. Since the decrease in &PHgr; (KRM) is closely associated with enhancements in auroral particle precipitation during these periods, a highly correlative relation between &PHgr; (IMF) and &PHgr; (KRM) cannot be deduced unless that phases of substorms are taken into account. The overall high correlation between them, however, supports the view expressed by Wolf et al. [1986> that directly driven processes are more important than unloading processes during disturbed periods. ¿ American Geophysical Union 1992