IMP-8 OBSERVATIONS OF TRAVELING COMPRESSION REGIONS IN THE MIDTAIL NEAR SUBSTORM EXPANSION PHASE ONSET

The large data set returned by the IMP 8 magnetic field investigation has been examined to understand the characteristics of traveling compr ession regions (TCRs). Using 15 years of the AL index, we identified 5 65 isolated substorm events with well-developed expansion phase for wh ich IMP 8 was in the mid-tail lobes and providing magnetic field measu rements. From this data set, 17 substorms were found to produce the bi polar B(Z)TCRs frequently observed farther down the tail. However, ano ther 14 cases have field compressions during which the field tilts nor th-then-south, but the maximum B-Z does not reach a positive value, i. e., the B-Z variation is not bipolar. These ''negative B-Z'' compressi on regions (NCRs) are examined in some detail and found to occur at la rger \Z\ values. The usual bipolar B(Z)TCRs, in contrast, are found at smaller \Z\ values closer to lobe-plasma sheet interface. We interpre t an NCR as a TCR which is observed at large \Z\ where the growth phas e flaring is too strong for the lobe field draping about the plasmoid to produce a positive B-Z between the leading edge and the center of t he compression region. Hence, at IMP 8 distances TCRs are often not ac companied by true bipolar B-Z signatures; rather only a north-then-sou th tilting of the field relative to flared lobe field is observed.