Testing the Potential Saturation Predictions of the Hill Model Using Observations From the March 31, 2001 Storm

Recently Siscoe et al. [JGR, 2001JA000109, 2002] extended Hill's hypothesis for saturation of the transpolar potential for high solar wind electric field intensities. For extreme southward IMF conditions the Region 1 current system takes over the role of the Chapman-Ferraro current system of deflecting the solar wind. The high-altitude cusps move toward the equator and a shoulder develops poleward of the cusp. The region 1 currents weaken the magnetic field at the subsolar magnetopause and the nose of the magnetosphere is indented relative to the high latitude magnetopause. A feature of the Hill model (as described by Siscoe et al.) is that the saturation potential increases with increasing solar wind dynamic pressure. Observations of the March 31, 2001 magnetic storm provide a multi-location test of the predictions of the Hill model. The low latitude location of the high-altitude cusp, and the formation of a magnetopause shoulder poleward of the cusp was observed by Polar. GOES data show the reduction of the subsolar magnetic field strength below dipole values. The dynamic pressure and the IMF vary considerably over the day, and DMSP data is used to show that the polar cap potential stayed within the saturation limits defined by the Hill model. MHD simulations using the MRC Integrated Space Weather Model provide the large-scale context for understanding the satellite measurements.