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 exam- ine substorm activity in the mid-tail region prior to data acquisition. On January 2, 1997, global auroral observa- tions from Polar indicated a substorm onset at ∼0120 UT followed by a substorm intensification at ∼0154 UT ata local time spatially separated from the initial substorm ac- tivity region. During this event, both Geotail and IMP-8 were in the mid-tail near the midnight meridian (Geotail (X,Y ) ≈ (−30, −3)RE and IMP-8 (X,Y ) ≈ (−37, 2)RE). Observations indicated that the substorm onset activity was localized in the postmidnight region. After the onset, Geo- tail detected a transient dipolarization which was not ac- companied by large plasma flows (i.e., | Vx |≤ 200 km/s). The subsequent substorm intensification produced endur- ing dipolarization at Geotail and highly fluctuating mag- netic field (mostly northward Bz) atIMP-8. Observat ions 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.