Magnetic field variations in the near-earth magnetotail during a weak magnetospheric substorm are studied by two approximate methods to estimate quantitatively the changes in the cross-tail current. Analysis of the event indicates that the magnetic field variations can be interpreted as a tailward retreat of the cross-tail current sheet location by ~0.4--3.5RE and an overall reduction of the current density by ~5--25%. These changes are accompanied by enhancement of magnetic field-aligned currents and earthward flowing plasma. The general features of the observations are consistent with the idea that the cross-tail currents is being diverted into the ionosphere at the substorm onset in association with earthward drainage of plasma in the plasma sheet.

The magnetosphere can be regarded as a magnetohydrodynamic (MHD) dynamo which converts the streaming energy of the solar wind plasma into electric current (or, equivalently, magnetic) energy. The cross-tail current in the magnetotail is a resistive portion of the current solenoids driven by this MHD dynamo (Bostrom, 1974; Akasofu, 1977; Alfven, 1977). Disruption, or less drastically, reduction of the cross-tail current has long been recognized as a viable process to account for the abundant energy release during magnetospheric substorms, both from theoretical considerations (Atkinson, 1967; Siscoe and Cummings, 1969; Bostrom, 1974; Kan and Akasofu, 1978) and on observational grounds (Cummings and Coleman, 1968; Bonnevier et al., 1970; Bostrom, 1971; Akasofu, 1972; McPherron et al., 1973; Lui and Burrows, 1978). Monitoring the changes in the cross-tail current from in situ particle measurements has so far been unsuccessful, and even if it is achieved, it yields at best local disturbances only. Large-scale changes in the current system in the magnetotail can be and have been inferred from the observed magnetic field perturbations. At the synchronous distance and in the near-earth magnetotail near the equatorial plane, magnetic field observations have indicated that the magnetosphere relaxes to a more dipolar field configuration during the substorm expansion (Cummings and Coleman, 1968; McPherron et al., 1973). This observation and the growing realization that the substorm auroral electrojet is linked with magnetospheric currents have led many to the suggestion that the near-earth portion of the cross-tail current is interrupted and diverted into the ionosphere at the onset of magnetospheric substorms. Studies of the magnetic field-aligned current system in the near-earth magnetotail have yielded results which are consistent with this idea (Aubry et al., 1972; Fairfield, 1973; Sugiura, 1975). At the high latitude boundary of the plasma sheet, the field-aligned currents tend to flow toward the earth in the postmidnight sector and away from the earth inthe premidnight sector. This current pattern, although just part of the complete field-aligned current system in the nightside magnetosphere, can be considered as a partial or total diversion of a portion of the cross-tail current (Atkinson, 1978).

In this letter, a quantitive approach is taken to estimate the substorm changes of the cross-tail currents and field-aligned currents based on Imp-6 magnetic field observations in the near-earth magnetotail.