A remote sensing study of the dynamics of the cross-tail current has been performed using ISEE magnetic field data during a 12-hour period when these spacecraft were in the tail lobe. IMP 8 data taken simultaneously in the interplanetary medium showed that the solar wind was continuously supplying power to the magnetosphere. On a global scale the magnetosphere responded directly, as evidenced by the good general correlation between the auroral electrojets and the interplanetary magnetic field (IMF). A detailed study of the ISEE and ground magnetic fields reveals the occurrence of four main successive cross-tail current disruptions associated with substorm onsets. As a general trend, each disruption was followed by a cross-tail current increase while in the meantime the intensity of the auroral electrojet decreased. A new substorm started when the cross-tail current again reached a high value and while the polar ionosphere was relaxing to a quieter state. These observations also apply to the last substorm of the series, which coincided with a northward turning of the IMF. These results strongly support a cross-tail current threshold instability as the main cause of auroral substorms. ¿ American Geophysical Union 1994