Interyear consistency is demonstrated in the bi-monthly average diurnal vertical electric field measured over ~720 fair-weather days collected during a 5-year interval (1998 to 2002) at Vostok (78.5¿S, 107¿E; magnetic latitude 83.6¿S), Antarctica. After correcting for the influence of polar-cap convection, seasonal peaks in the average electric field values occur around July-August with a diurnal maximum at ~ 2050 UT while lowest average magnitudes are measured near November-December when the associated diurnal maximum occurs at ~1830 UT. These variations are consistent with expected seasonal changes in global thunderstorm activity. Comparisons of ground-level vertical electric field measurements (Vm-1) with Weimer-1996 model cross-polar cap potentials above Vostok (kV) for individual UT hours yield significant correlations over all hours but with reduced standard errors around local magnetic noon (~1300 UT). This implies a more direct linkage between solar wind parameters and the cross-polar cap potential near magnetic noon, for this site (magnetic latitude: 83.6¿S). An all hours--all seasons linkage factor of 0.76 ¿ 0.06 Vm-1 per kV is determined, broadly consistent with an average ionosphere-ground potential difference of ~250 kV and a measured average vertical electric field of 185 Vm-1. Evidence is presented supporting a seasonal variation in this linkage factor, with generally lower magnitudes in the austral winter (May to August).