Response of dayside net downward field-aligned current to changes in the interplanetary magnetic field and to substorm perturbations
We have studied the response of the auroral eastward and westward electrojets and of the downward field-aligned currents across the noon sector which feed them to southward turnings of the interplanetary magnetic field (IMF) and to substorm onsets in the nightside magnetosphere. We have found that while southward turning of the IMF lead to electrojet intensifications and low-level substorm activity, northward turnings of the IMF appear cabable of triggering large substorm disturbances. Both northward and southward turnings of the IMF cause the downward field-aligned current across the noon sector, as well as the auroral electrojets, to strengthen within a few minutes of the impact of the changed IMF at the magnetopause. Since large substorms involve the sudden release of energy stored in the magnetotail, we conclude that the tail current system links directly to the overall magnetosphere-ionosphere current system involving the auroral electrojets. In this context a substorm associated with a northward turning of the IMF reflects a reconfiguration of the magnetotail in response to a change in the level of the solar-terrestrial interaction involving a decrease in the rate of which solar wind energy penetrates into the magnetosphere.