CROSS-POLAR CAP POTENTIAL DIFFERENCE, AURORAL ELECTROJET INDEXES, AND SOLAR-WIND PARAMETERS

The cross-polar cap potential difference PHI (KRM) is estimated from ground magnetic perturbation data through the magnetometer inversion method of Kamide, Richmond, and Matsushita (KRM), combined with an "empirical" ionospheric conductance distribution estimated from the DMSP X ray image data. A significant correlation is found between PHI (KRM) and the AE(12) index; PHI (KRM, in kilovolts) = 36 + 0.082 AE(12, in nanoteslas) with the correlation coefficient being 0.80, PHI (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 PHI (IMF) based on solar wind parameters [Reiff and Luhmann, 1986]. PHI(IMF) is found to be linearly correlated with PHI (KRM), as PHI (IMF) = 29.8 + 0.999 PHI (KRM), with the highest correlation obtained for a 40-min lag in the interplanetary magnetic field (IMF). Note that PHI (IMF) is systematically larger than PHI (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 PHI (IMF) variations are expected, significant fluctuations in calculated PHI (KRM) values are obtained. Since the decrease in PHI (KRM) is closely associated with enhancements in auroral particle precipitation during these periods, a highly correlative relation between PHI (IMF) and PHI (KRM) cannot be deduced unless the phases of substoms 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.