Summary
Variations in the atmospheric, near-surface vertical electric field measured at the Southern near-pole station Vostok (78.5(S, 107(E, L=75.0) in 1998, 1999 and 2000 have been analyzed in conjunction with changes of the ionospheric electric field and interplanetary magnetic field (IMF). Comparison of 1-minute data on electric field and meteorological observations makes it possible to establish quantitative criteria for "fair weather" conditions. More then 200 days satisfied the "fair weather" conditions have been selected, i.e. 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, which describes the global electric circuit formed by the thunderstorm activity. The Ez diurnal variation shows strong seasonal dependence: it is maximal in summer but gradually reduces through the equinoctial months and is almost negligible during the austral winter. We demonstrate the direct association of the geoelectric field with the cross polar cap potential difference: Ez at Vostok is strongly affected by variations in IMF Bz component. The IMF Bz effect is mainly seen at dawn (Ez decreases with Bz ) and dusk (Ez increases with Bz). The PC index of magnetic activity, characterizing the general state of the magnetosphere, can be used a proxy for the atmospheric electric field in these time intervals. 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.