Solar wind density and auroral electrojets during geomagnetic storms

It was shown statistically, some 30 years ago, that substorms occurring during the initial phase of geomagnetic storms and those during the main phase are different in character. Based on the intensities of the auroral electrojets estimated from ground magnetometer data, we show that the solar wind density controls the intensity of the auroral electrojets, but the efficiency of the control depends strongly on the polarity of interplanetary magnetic field (IMF). The earlier statistical results can consistently be accounted for by considering that the initial phase of geomagnetic storms is caused by the high dynamic pressure, i.e., density, of the solar wind while southward IMF dominates the main phase.