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 pr ior to data acquisition. On January 2, 1997, global auroral observations fr om Polar indicated a substorm onset at similar to 0120 UT followed by a sub storm intensification at similar to 0154 UT sit a local time spatially sepa rated from the initial substorm activity region. During this event, both Ge otail 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 t he postmidnight region. After the onset, Geotail detected a transient dipol arization which was not accompanied by large plasma flows (i.e., \ V-x \les s than or equal to 200 km/s). The subsequent substorm intensification produ ced enduring dipolarization at Geotail and highly fluctuating magnetic fiel d (mostly northward B-z) at IMP-8. Observations for this substorm showed no indication of mid-tail activities occurring prior to auroral brightening f or both onset and intensification even though the satellites observed activ ities subsequently. Close examination of data indicates that the delays wer e 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.