Department of Electronic Engineering, University of Electro-Communications, Tokyo, Japan

The characteristics of traveling compression regions (TCRs) in the midtail lobes near expansion-phase onsets have been analyzed utilizing the data set returned by the IMP 8 magnetic field investigation between 1973 and 1990. Examination of the 15 years for which we have the AL index led to the identification of 565 isolated substorm events with expansion phases for which IMP 8 was in the tail lobes and returned magnetic field measurements. For 31 of the 565 events a typical TCR variation was observed at IMP 8. These TCR perturbations have been previously studied in the ISEE 3 distant tail measurements and consist of a field compression during which B_Z increases and then decreases after peaking, although B_Z may not reach a positive value in many cases. For 59 of the substorms a field compression was seen at IMP 8 during which the B_Z peak before the field maximum is lacking, i.e., the B_Z just decreases rapidly, and the variation is unipolar. These 90 (31 plus 59) substorms occurred in the region for X = -26 to -38 R_E, |Y| £ 15 R_E, and Z_N £ 20 R_E all in the solar wind aberrated GSM coordinate system. The "unipolar B_Z compression region" has a phase of flat B_Z preceding the sharp southward tilting interval. The typical timescale of this phase is 60-100 s. We interpret this flat B_Z phase as a field compression created by the rapid growth of the plasmoid bulge in the Z dimension underneath IMP 8 just prior to the plasmoid being ejected tailward. The fractional occurrence frequency of the TCRs observed in our substorm database for each year was calculated by dividing the number of the substorms producing TCRs by all of the IMP 8 lobe substorm events for each year. This ratio tends to be high (maximum 41.7%) in the years of low sunspot number, and low (minimum 7.7%) in the years of high sunspot number. For years when the occurrence ratio is low, the TCRs are observed primarily within |Y| £ 10 R_E. When the occurrence ratio is high, the TCRs are found across a wider region in Y with a peak in the premidnight. These results suggest that the plasmoid formation is controlled by solar cycle related parameters in such a manner that the near-Earth neutral line occurs at a wider region in the dawn-duck direction for the years of low sunspot number.

J. Geophys. Res., 103, No. A8, 17641-17650, Aug. 1998.