An evaluation of the statistical significance of the association between northward turnings of the interplanetary magnetic field and substorm expansion onsets

[1] An outstanding problem in magnetospheric physics is deciding whether substorms are always triggered by external changes in the interplanetary magnetic field (IMF) or solar wind plasma, or whether they sometimes occur spontaneously. Over the past decade, arguments have been made on both sides of this issue. In fact, there is considerable evidence that some substorms are triggered. However, equally persuasive examples of substorms with no obvious trigger have been found. Because of conflicting views on this subject, further work is required to determine whether there is a physical relation between IMF triggers and substorm onset. In the work reported here a list of substorm onsets was created using two independent substorm signatures: sudden changes in the slope of the AL index and the start of a Pi 2 pulsation burst. Possible IMF triggers were determined from ISEE-2 observations. With the ISEE spacecraft near local noon immediately upstream of the bow shock, there can be little question about propagation delay to the magnetopause or whether a particular IMF feature hits the subsolar magnetopause. Thus it eliminates the objections that the calculated arrival time is subject to a large error or that the solar wind monitor missed a potential trigger incident at the subsolar point. Using a less familiar technique, statistics of point process, we find that the time delay between substorm onsets and the propagated arrival time of IMF triggers are clustered around zero. We estimate for independent processes that the probability of this clustering by chance alone is about 10(-11). If we take into account the requirement that the IMF must have been southward prior to the onset, then the probability of clustering is higher, similar to10(-5), but still extremely unlikely. Thus it is not possible to ascribe the apparent relation between IMF northward turnings and substorm onset to coincidence.