Solar Wind Triggering of Substorm Expansion Onset

Abstract

Recent reports claim that the onset of the expansion phase of magnetospheric substorms is triggered by a sudden northward turning of the interplanetary magnetic field vector (IMF). We investigate this claim by studying the relation between sudden onsets of magnetic activity (AL index) and changes in the IMF (rectified electric field). Specific examples of the relation between the solar wind and magnetic activity demonstrate that sharp onsets in AL occasionally appear to be independent of changes in the solar wind, but more frequently they appear to be triggered by northward turnings. A statistical analysis of this relation for the first six months of 1978 shows that auroral zone magnetic disturbance is present during approximately 73% of all hours. Distinct substorms defined by minima in the AL index can be identified in 23% of these disturbed hours. Sharp onsets in AL occur in about 45% of the identified substorms. For the subset of substorms with sharp onsets and corresponding solar wind data, 28% occur during steady southward IMF. However, a larger fraction of the sharp onsets (44%) appears to be correlated with northward turnings of the IMF.
Because a significant number of sharp substorm expansion onsets occur during steady solar wind conditions, we conclude that the onset is caused by a process internal to the magnetosphere. However, since many other sharp onsets appear to be correlated with changes in either the IMF orientation or the solar wind dynamic pressure, we conclude this internal process can be influenced by changes in solar wind stress applied to the magnetosphere.