Solar wind triggering of substorm expansion onset

A substantial fraction of all substorms with sudden onsets, as identified by minima in the AL index, are found to be triggered by the IMF. Analysis of data from the first six months of 1978 shows auroral zone magnetic records to be disturbed most of the time, with distinct substorms identified in 23 percent of these disturbed hours. For the subset of substorms with sharp onsets and corresponding solar wind data, 44 percent are found to be correlated with northward turnings of the IMF, while 28 percent occur during steady southward IMF. The results suggest that sharp substorm expansion onset is caused by a process internal to the magnetosphere which can be influenced by changes in the solar wind stress applied to the magnetosphere.