We have reported on the first experimental test of the Hill model, using determinations of from SuperDARN observations. A large dataset of quasi-stable IMF periods and suitable SuperDARN coverage was used to compare of the Hill model ( ) with that derived from the SuperDARN measurements ( ). The Hill model incorporates feedback from the Region 1 current system and correctly predicts saturation of the transpolar potential at high values of the solar wind electric field. Comparison with , however, indicates that some modification of this model may be necessary.
The best-fit solution of the Hill model to () occurs when the constant ionospheric conductance () is 23 S and an added minimum potential () is 17 kV. The value of is in reasonable agreement with the SuperDARN observations and that reported in other studies, however, the value of is higher than expected. The ram pressure dependence of the Hill model also suggests that a lower value of may be more appropriate. Siscoe et al.,  give a detailed description of Equation (1), however, it is possible that some of the assumptions they use to derive (1) are incorrect. For instance, the assumption of a uniform global conductivity may be inadequate, or the manner in which ionospheric conductivity mediates M-I coupling may need to be re-examined.
This work was supported by NSF grant ATM- and NASA grant NAG5-. Operation of the Northern Hemisphere SuperDARN radars is supported by the national funding agencies of the U.S., Canada, the U.K., and France. The Hill model formulation was kindly provided by Dr. George Siscoe. We gratefully acknowledge the ACE/MAG instrument team, the ACE/SWEPAM instrument team, and the ACE Science Center for providing the ACE level 2 data.