Fig. 1

The substorm current wedge diverts tail current through the ionosphere. A schematic representation of the substorm current wedge as originally envisioned (McPherron et al. 1973). At the inner edge of the plasma sheet tail current is diverted along field lines closing westward in the midnight ionosphere and returning to the tail from the westward surge. Previously stretched field lines collapse to a more dipolar configuration (dipolarization)