Effects of solar wind density on the auroral electrojets and global auroras during geomagnetic storms

It was shown statistically, some thirty years ago, that substorms occurring during the initial phase of geomagnetic storms and those during the main phase are different in character. Substorms during geomagnetic storms, regardless of the phases in which they occur, were also shown to be different from normal isolated substorms in terms of the relative strength between the eastward and westward auroral electrojets. Based on the intensities of the auroral electrojets estimated from ground magnetometer data and on the distribution of large-scale auroras seen in Polar auroral images, the present study shows that the solar wind density does in fact control the intensity of the auroral electrojets and the associated auroral activity, 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 (or density) of the solar wind, while southward IMF dominates the main phase.