Dipolarization: A consequence of substorm expansion onset

Dipolarization observed in the near‐Earth plasma sheet can be initiated by the substorm expansion onset. 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 Alfvén bounce time scale.