Polar cap potential saturation during large geomagnetic storms

We present evidence that the cross polar cap potential saturates during the Bastille Day geomagnetic storm which is characterized by large values of the driving interplanetary electric and magnetic fields. Empirical models that were derived and constrained for more benign solar wind conditions severely over-predict the cross polar cap potential for this event. Global simulation results show that the magnetopause moves significantly inward as a result of compression and erosion of the magnetosphere. We propose a model that explains the potential saturation primarily as an effect of the shortened x-line. An appropriately modified empirical model gives much better potential prediction, supporting our hypothesis.