A case study of the response of the magnetosphere to changes in the interplanetary medium

A detailed analysis of world-wide ground based magnetometer data is presented, together with information on the plasma and magnetic field properties of the interplanetary medium and magnetosheath obtained from the ISEE 1 and 2 and IMP 8 spacecraft. The event concerned exhibited an interval of relatively stable southward IMF followed by a sharp northward turning. It is pointed out that during the interval of southward IMF there were occasional transient northward turnings with significant substorm expansive phase activity appearing to be triggered by these transient northward turnings. The final northward turning of the IMF was linked with an episode of strong magnetospheric substorm expansive phase activity after which the level of high latitude magnetic activity declined to a low level. Evidence is presented indicating that the driven system auroral electrojets begin to decay at the time of the northward turning of the IMF, even as the substorm expansive phase activity is initiated in the midnight sector. The collapse of the substorm current wedge during the final decay of high latitude activity is described in some detail, and it is shown that this collapse occurs progressively from east to west in a series of impulsive episodes.