Dipolarization observed in the near-Earth plasma sheet can be initiated by the substorm expansion ofset. Field-aligned currents at the substorm expansion onset are enhanced first in the ionosphere in response to an enhanced magnetospheric convection. Closure of substorm field-aligned currents through the plasma sheet can disrupt the cross-tail current to cause dipolarization in the plasma sheet. The electric field induced by the current disruption can launch a fast-mode compression wave to result in the plasma injection event. A new X line can form tailward of the dipolarization region during the substorm expansion phase. A timing sequence of substorm events is inferred from the proposed conceptual model of substorms based on the magnetosphere-ionosphere coupling process on the Alfv¿n bounce time scale. ¿American Geophysical Union 1991