Dependence of the near-Earth magnetotail magnetic field on storm and substorm activities

Equatorial magnetic field data in the near-Earth tail region 3 < R < 23 R-E have been sampled from the large magnetic field database obtained by ISEE 1 for 1978-1987. The dependence of the field magnitude on storm and substor m activities represented by the Dst and AL indices, respectively, has been examined. The data for declining/quiet phases in terms of the AL index have been used; i.e., the data obtained before and during the periods when the auroral electrojet was rapidly increasing have not been used to avoid the p ossible influences of substorm expansion onset. The following two features are noted: (1)Magnetic field magnitude does not depend significantly on sub storm activity (AL) in the near-Earth magnetotail R < 9 R-E, while it incre ases with increasing substorm activity in the midmagnetotail R > 9 R-E. The slope of the regression line for the AL index changes in a step-like fashi on in the vicinity of R = 9 R-E. (2) Field magnitude decreases with increas ing storm activity (Dst) in the region within R similar to 12 R-E, while it increases beyond R similar to 12 R-E. In contrast to the regression slope for the auroral electrojet, the slope for the Dst index changes gradually w ith increasing the distance from the Earth. Considering these points along with the results obtained for the lobe magnetic field, we discuss the chang es in the large-scale distribution of the neutral sheet current during the course of major substorms: During the expansion onset of substorms, the neu tral sheet current increases in the near-Earth magnetotail and decreases in the midmagnetotail: The neutral sheet current subsequently decreases in bo th the near-Earth and middistant tail regions during the recovery phase of substorms, where the current reduction is particularly evident near R = 6 R -E. It is also suggested that not only in the near-Earth magnetotail but in the midmagnetotail the cross-tail current is significantly enhanced during intense storms.