MAGNETOSPHERIC FIELD AND CURRENT DISTRIBUTIONS DURING THE SUBSTORM RECOVERY PHASE

We have studied 11 substorm recovery phase events in which magnetic fi eld and energetic particle data were available near the midnight secto r from the GEOS 2 satellite. Comparison with the Tsyganenko magnetic f ield model shows that, after the expansion phase, B(Z) is large and de creases gradually toward the model value during the recovery phase, wh ereas deviations of B(X) and B(Y) relative to the model values are sma ll after the effects of the substorm current wedge have disappeared. W e have modeled this sequence by using temporally evolving current syst ems implemented as additions to the Tsyganenko model. The tail current sheet thickness and the cross-tail current intensity at different rad ial 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 r ecovery phase the current sheet was relatively thick close to the inne r edge of the plasma sheet. Model fittings produced two different fiel d configurations. In seven events the cross-tail current was weak, and the field configuration was highly dipolar. In four events the near-E arth current was weak, but stronger currents remained in the midtail r egion. In these latter events the field configuration at the beginning of the recovery phase included a region where B(Z) was negative. This negative B(Z) and the associated near-Earth neutral line disappeared later as the current system developed toward the quiet time configurat ion. The magnetic field configuration, current distributions, and part icle drift paths during the substorm recovery phase are examined and c ompared with those prevailing during the substorm growth phase.