It is proposed here that the expansion phase of substorms results from a reduction in the large-scale electric field imparted to the magnetosphere from the solar wind, following a >30-min growth phase due to an enhancement in this electric field. The reduction in the electric field is assumed to propagate antisunward within the magnetosphere. Triggering by a reduction in the electric field is suggested by the observation that substorms are often triggered by northward turnings of the interplanetary magnetic field (IMF). However, under the theory presented here, substorms may be triggered by anything that causes an electric field reduction such as a reduction in the magnitude of the y component of the IMF. A reduction in the large-scale electric field disrupts both the inward motion and energization of plasma sheet particles that occurs during the growth phase. It is suggested here that this can lead to formation of the expansion phase current wedge and active aurora. The current wedge results from the magnetic drift of ions, which has a speed proportional to particle energy, and a large azimuthal gradient in mean particle energy that is expected to develop in the vicinity of magnetic midnight during the growth phase. Current wedge formation will most likely be initiated near the radial distance (~6--10 RE) of the peak in the growth phase plasma pressure distribution, and then propagate tailward from that region. Order-of-magnitude calculations show that the above proposal can account for the rapid development of the expansion phase relative to the growth phase, the magnitude of the reduction in the cross-tail current within the current wedge, the speeds of tailward and westward expansion of the current reduction region, the speeds of poleward and westward motion of active aurora in the ionosphere, and the magnitude of wedge field-aligned currents that connect the ionospheric region of active auroral to the divergent cross-tail current within the magnetosphere. ¿ American Geophysical Union 1995.