Magnetospheric field and current distributions during the substorm recovery phase

We have studied 11 substorm recovery phase events in which magnetic field and energetic particle data were available near the midnight sector from the GEOS 2 satellite. Comparison with the Tsyganenko magnetic field model shows that, after the expansion phase, BZ is large and decreases gradually toward the model value during the recovery phase, whereas deviations of BX and BY relative to the model values are small after the effects of the substorm current wedge have disappeared. We have modeled this sequence by using temporally evolving current systems implemented as additions to the Tsyganenko model. The tail current sheet thickness and the cross-tail current intensity at different radial distances were varied using six free parameters in the model. The parameters were evaluated using a least squares fit for each of the 11 events separately. The results suggest that at the beginning of the recovery phase the current sheet was relatively thick close to the inner edge of the plasma sheet. Model fittings produced two different field configurations. In seven events the cross-tail current was weak, and the field configuration was highly dipolar. In four events the near-Earth current was weak, but stronger currents remained in the midtail region. In these latter events the field configuration at the beginning of the recovery phase included a region where BZ was negative. This negative BZ and the associated near-Earth neutral line disappeared later as the current system developed toward the quiet time configuration. The magnetic field configuration, current distributions, and particle drift paths during the substorm recovery phase are examined and compared with those prevailing during the substorm growth phase.