Based on the ionospheric currents determined from ground-based magnetometer data, we propose a global substorm current system. The proposed substorm current system is composed of (a) the region-1 and -2 currents, (b) the growth-phase current loops, (c) the substorm current wedge. The growth-phase current loops are initiated during the growth phase due to reconfiguration of the magnetosphere driven by enhanced magnetospheric convection. The substorm current wedge can be initiated by the expansion onset due to the ionospheric response to the enhanced magnetospheric convection. During the growth phase, the current system consists of currents in (a) and (b). During the expansion phase, the substorm current system consists of currents in (a), (b) and (c). Energy dissipated during the substorm expansion phase is supplied by both the directly driven process and the unloading process. This is supported by the observations that depolarization occurs inside the current wedge, while thinning proceeds on the dawn and dusk sides of the current wedge. The ratio of the unloading current to the directly-driven current is ~2 during the expansion phase of a substorm. ¿ American Geophysical Union 1992