The ring coupling model predicts that the radius of the polar cap remains appreciable down to very small values of the transpolar potential &PHgr; and then decreases rapidly to zero as &PHgr; goes to zero. Electric field data from the S3-2 spacecraft are shown to be consistent with the prediction in that both the theory and the data show a weak dependence of polar cap radius on potential in the observed range. This behavior leads to theoretical insights about the ground state of the magnetosphere. Because the theoretical ground state (&PHgr;=0) has no polar cap, the model implies that the finite polar cap observed after extended periods of northward interplanetary magnetic field (IMF) is the result of a small potential being generated continually by the solar wind. If it is assumed that this potential is generated on closed magnetic field lines by some viscouslike interaction, and the bulk of potential is generated sporadically by magnetic merging, then the bulk implies that the polar cap retains open flux during extended periods of northward IMF and that it does so because of the presence of the potential generated by the viscouslike interaction.