Statistical evaluations of the association between IMF triggering and substorm expansion phase onset

The primary goal of this dissertation is to examine the statement “all substorms are triggered by northward turning of the IMF”. Several questions relating to the triggering of substorms are considered. The thesis outline includes the following topics: (1) uncertainty of substorm onset timing, (2) typical behavior of the tail magnetic and plasma in relation to substorm onsets, (3) the frequency of substorm triggering, (4) differences between triggered and non-triggered substorms, (5) possible dependence of the detection of IMF triggers on the distance from the satellite to the earth-sun line, (6) statistical test of the significance of the association of IMF triggers with onsets, (7) substorm triggering in a MHD simulation.; We have developed a substorm onset timing procedure using the AL index and Pi 2 pulsations that appears to be successful in determining the main substorm onset as it clearly organize the tail signature. Using ISEE-2 tail observations, we found that more than 70% of all substorms exhibit plasma sheet thinning and change in the tail magnetic field. The time sequence and form of variations in the tail can be used to distinguish the difference between triggered and non-triggered substorm.; We find the ratio of triggered and non-triggered substorm is about 6:4 and this ratio is independent of the distance from satellite to the earth-sun line. More over, it is shown that there are no differences in the tail behavior for triggered and non-triggered substorms.; Using the statistics of point process we find that the association of triggers and onsets is statistically significant at a confidence level so high that there is no possibility that the association is a result of chance. The IMF triggering of substorms is a real phenomenon but no triggers can be identified for a substantial fraction of all substorms.; Finally, we carried out a MHD simulation of IMF triggering that shows a northward turning of the IMF actually delays the onset of tail reconnection rather than triggering it.