Conjunction of tail satellites for substorm study: ISTP event of 1997 January 2

The interval of 1997 January 1-2 was identified as a favorable conjunction of Geotail and IMP-8 to examine substorm activity in the mid-tail region prior to data acquisition. On January 2, 1997, global auroral observations from Polar indicated a substorm onset at similar to 0120 UT followed by a substorm intensification at similar to 0154 UT sit a local time spatially separated from the initial substorm activity region. During this event, both Geotail and IMP-8 were in the mid-tail near the midnight meridian (Geotail (X, Y) approximate to (-30,-3)R-E and IMP-8 (X,Y) approximate to (-37,2)R-E). Observations indicated that the substorm onset activity was localized in the postmidnight region. After the onset, Geotail detected a transient dipolarization which was not accompanied by large plasma flows (i.e., \ V-x \less than or equal to 200 km/s). The subsequent substorm intensification produced enduring dipolarization at Geotail and highly fluctuating magnetic field (mostly northward B-z) at IMP-8. Observations for this substorm showed no indication of mid-tail activities occurring prior to auroral brightening for both onset and intensification even though the satellites observed activities subsequently. Close examination of data indicates that the delays were not due to a dawn-dusk expansion of mid-tail activity. These results are consistent with substorm activity beginning in the near-Earth region first, followed by activity in the mid-tail region later.