Summary
Global thunderstorm activity maintains a variable potential difference of the order of 250 kV between the quasi-equipotentials of the lower reaches of the ionosphere and the Earth. Under fair-weather conditions a downward current of ~3 pA/m2 and a geoelectric field of order 100-200 V/m can be measured at ground level. The time constant of this global circuit is ~20 minutes, thus by monitoring the fair-weather field it is possible to measure the globally integrated electrical intensity of thunderstorm activity. Meteorological influences are a major hindrance to useful ground-based measurements of the geoelectric circuit. Conditions suitable for optical astronomy are generally advantageous for geoelectric field measurements. The high altitude, clear skies and generally low wind speeds of the polar plateau are ideal. Additionally, in polar regions, the interaction of the solar wind and the Earths magnetic field imposes an ionospheric convection pattern of variable form and intensity, generally with a dawn-to-dusk, cross-polar cap potential difference in the range 20 to 60 kV. Using measurements made at Vostok, Antarctica (78.5S. 107E) we have been able to statistically demonstrate the influence of global thunderstorm activity and solar wind on the vertical geoelectric field measured at ground level. In this talk we will show how by additionally measuring the air-earth current and making measurements at more than one Antarctic polar plateau site, we will be able to separate these contributing influences on individual days.