ESTIMATES OF CURRENT CHANGES IN GEOMAGNETOTAIL ASSOCIATED WITH A SUBSTORM

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 ∼0.4‐3.5 RE 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. Copyright 1978 by the American Geophysical Union.