An Updated Model of the Substorm Current Wedge.

The Substorm Current Wedge (SCW) was developed over 40 years ago to explain the magnetic signatures observed on the ground and in geosynchronous orbit during the substorm expansion phase. The ensuing decades of new observations, including radar and low-altitude spacecraft, combined with insight gained from MHD simulations and theoretical considerations have tremendously advanced our understanding of this important magnetosphere-ionosphere system. In this paper, we present a revised, time-dependent picture of the substorm current wedge that follows its evolution from the initial substorm flows through substorm expansion and recovery. We describe the updated ionospheric current closure, which is far more complicated than the original picture, as well as an updated magnetospheric portion that includes two sets of field-aligned currents. Finally, we conclude by identifying open questions.