Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771

The large data set returned by the IMP 8 magnetic field investigation has been examined to understand the characteristics of traveling compression regions (TCRs). Using 15 years of the AL index, we identified 565 isolated substorm events with well-developed expansion phase for which IMP 8 was in the mid-tail lobes and providing magnetic field measurements. From the data set, 17 substorms were found to produce the bipolar B_Z TCRs frequently observed farther down the tail. However, another 14 cases have field compressions during which the field tilts north-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 " compression regions (NCRs) are examined in some detail and found to occur at large |Z| values. The usual bipolar B_Z TCRs, in contrast, are found at smaller |Z| values closer to lobe-plasma sheet interface. We interpret an NCR as a TCR which is observed at large |Z| where the growth phase 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 the compression region. Hence, at IMP 8 distances TCRs are often not accompanied by true bipolar B_Z signatures; rather only a north-then-south tilting of the field relative to flared lobe field is observed.

Geophys. Res. Lett., 24, No. 4, 353-356, Feb. 1997