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

Abstract

Equatorial magnetic field data in the near‐Earth tail region 3 < R < 23 RE 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 substorm 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 possible influences of substorm expansion onset. The following two features are noted: (1) Magnetic field magnitude does not depend significantly on substorm activity (AL) in the near‐Earth magnetotail R < 9 RE, while it increases with increasing substorm activity in the midmagnetotail R > 9 RE. The slope of the regression line for the AL index changes in a step‐like fashion in the vicinity of R = 9 RE. (2) Field magnitude decreases with increasing storm activity (Dst) in the region within R ∼ 12 RE, while it increases beyond R ∼ 12 RE. In contrast to the regression slope for the auroral electrojet, the slope for the Dst index changes gradually with increasing the distance from the Earth. Considering these points along with the results obtained for the lobe magnetic field, we discuss the changes in the large‐scale distribution of the neutral sheet current during the course of major substorms: During the expansion onset of substorms, the neutral sheet current increases in the near‐Earth magnetotail and decreases in the midmagnetotail: The neutral sheet current subsequently decreases in both the near‐Earth and middistant tail regions during the recovery phase of substorms, where the current reduction is particularly evident near R = 6 RE. 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.