Testing the Hill model of transpolar potential saturation

The Hill model predicts that the transpolar potential (Phi(PC)) saturates for strong solar wind electric fields (E-SW) and saturates at higher values for larger solar wind ram pressures (P-SW) and/or lower ionospheric Pedersen conductances (P-P). Three months of Defense Meteorological Satellite Program (DMSP) measurements of polar cap potentials have been compared with predictions of the Hill model. Data from representative days that span a wide range of interplanetary and solar-cycle conditions were selected for presentation. In the comparison we augmented the Hill model by adding a constant term to the magnetospheric potential and expressing Sigma(P) in terms of the 10.7-cm solar radio flux (F-10.7). Temporal variations in observed Phi(PC) are in good agreement with the predictions of the augmented Hill model. Comparison of two events with significantly different average P-SW but similar F-10.7 and E-SW found that DMSP observed higher potentials when P-SW was larger. DMSP also observed higher potentials when Sigma(P) was lower, when comparing two events with significantly different F-10.7 (solar minimum versus solar maximum) but similar P-SW and E-SW. Both comparisons agree with the predictions of the Hill model.