Reexamination of driven and unloading aspects of magnetospheric substorms

It is widely accepted that substorms consist of both directly driven and loading-unloading processes. However, a recent study has presented results which suggested that over 90% of the auroral electrojet (AE) variation was directly predictable from the solar wind variations alone. This would imply that only a small residual in the AE variability is due to internal magnetospheric dynamics. The present paper considers nonlinear dynamical models of the global solar wind-magnetosphere interaction and uses the observed, highly variable solar wind electric field (VBs) to drive the Faraday loop analogue model. It is found 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, one does not obtain realistic time behavior of the model AL index. Thus these results show quite clearly that both driven and unloading processes must be included in a realistic model of geomagnetic activity.