Variations in the atmospheric, near-surface vertical electric field component Ez measured at the Russian Antarctic station Vostok in 1998 are analyzed in conjunction with changes of the interplanetary magnetic field (IMF). A total of 134 days were selected which satisfied the fair weather conditions, that is, days with absence of high winds, falling or drifting snow, clouds, and electric field pollution from the station's power plant. It is shown that the average diurnal variation of Ez for these days follows the global geoelectric field fair-weather diurnal variation: the Carnegie curve, which describes the global electric circuit formed by the thunderstorm activity occurring mostly over equatorial regions. The Ez diurnal variation shows strong seasonal dependence: it is maximal (~40% of the average daily magnitude) in summer but gradually reduces through the equinoctial months and is almost negligible during the austral winter. Ez at Vostok is strongly affected by variations in both the IMF By and Bz components. The influence of By is dominant during geomagnetic daytime hours (1100--1400 UT at Vostok): Ez increases with By in the range from -10 to +10 nT. The IMF Bz effect is mainly seen at dawn (Ez decreases with Bz) and dusk (Ez increases with Bz). ¿ 2001 American Geophysical Union