The solar wind-magnetosphere-ionosphere coupling is studied using an equivalent circuit model, in which the energy transfer from the solar wind to the innter magnetosphere and the ionosphere is assumed to consist of two parts: (1) the direct coupling between the solar wind and the polar ionosphere through the tail magnetosphere; and (2) the indirect coupling through the magnetotail plasma sheet. The direct coupling leads to a highly enhanced level for region 1 field-aligned currents and the auroral electrojet, while the indirect coupling through the plasma sheet leads to to the occurrence of impulsive components, which are assumed to result from the current interruption of the tail circuit. A set of circuit parameters is carfully determined on the basis of the observed plasma parameters. The circuit model thus constructed can successfully reproduce several observed behaviors of the magnetosphere during major storms and substorms. The model predicts that the maximum potential ϕmax across the plasma sheet after the onset of substorm can be 2--3 times the potential (ϕCT) imposed at the tail magnetopause. The impulsive variations in the energy (WRC) stored in the ring current belt are found to be much smaller than the variations in the auroral electrojet and the region 1 field-aligned currents. ¿ American Geophysical Union 1988