Reexamination of Driven and Unloading Aspects of Magnetospheric Substorms

Magnetospheric substorms represent a global interaction between the solar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is episodically stored in the magnetosphere, with a large fraction of the energy often being in the form of excess magnetic flux in the magnetotail lobes. This stored energy is then explosively dissipated in the near-Earth, nightside region at substorm onset. It is generally accepted, therefore, that substorms consist of both directly driven and loading-unloading processes. However, a recent study has presented results in which nearly 90% of the auroral electrojet (AE) variation was directly predictable from the solar wind variations alone. This would suggest that only a small residual in the AE variability is due to internal magnetospheric dynamics. We consider nonlinear dynamical models of the global solar wind-magnetosphere interaction. In the present work we use the observed, highly variable solar wind electric field (VB(S)) to drive the Faraday loop analogue model. We find that it is critically important to include magnetotail unloading in the model in order to replicate the main features of geomagnetic activity: With just the driven response in the model, we do not obtain realistic time behavior of the model AL index.