2 SUBSTORM INTENSIFICATIONS COMPARED - ONSET, EXPANSION, AND GLOBAL CONSEQUENCES

We present observations of two sequential substorm onsets on May 15, 1 996. The first event occurred during persistently negative IMF B-Z, wh ereas the second expansion followed a northward turning of the interpl anetary magnetic field (IMF). While the first onset remained localized , the second event led to a major reconfiguration of the magnetotail, The two very different events are contrasted, and it is suggested that the IMF direction controls the evolution of the expansion phase after the initial onset. Magnetic field modeling and field-aligned mappings are used to find the high-altitude source region of the auroral featu res and currents giving rise to ground magnetic disturbances: It is sh own that the auroral brightening is related to processes near the inne r edge of the plasma sheet but that the initial field-aligned currents couple to the midtail region. Ground magnetograms show an abrupt, lar ge-scale weakening of the electrojet during the recovery phase. This e vent is followed by eastward drifting omega bands in a double-oval con figuration, During that period, the Geotail plasma data show oscillati ons at <100 km/s amplitude. We argue that both these features are conn ected with the global tail evolution as the neutral line ceases to be active and reforms in the distant tail.