We have analyzed Super Dual Auroral Radar Network (SuperDARN) data between February 1998 and December 2000 to determine the statistical characteristics of the total variation in the high-latitude ionospheric electric potential, or cross polar cap potential, ΦPC. Periods are chosen to satisfy the criteria that (1) the solar wind and interplanetary magnetic field (IMF) are quasi-stable for ≥40 min and (2) sufficient SuperDARN data exist to adequately determine ΦPC. A total of 9464 individual 10-min periods satisfying the first criteria are analyzed. A subset of 2721 periods satisfy both criteria, of which 1638 are considered high-confidence periods. The resulting data set shows that for quasi-steady solar wind and IMF, ΦPC (1) is nonlinear in the expression for the effective interplanetary electric field EKL, (2) saturates at high values of EKL, and (3) is highly variable for any given value of EKL. These results indicate that simple formulations involving the upstream solar wind and IMF conditions are inadequate to describe the instantaneous ΦPC and that the inclusion of internal and coupling processes between the magnetosphere and ionosphere may be necessary.