Interannual consistency of bi‐monthly differences in diurnal variations of the ground‐level, vertical electric field

Abstract

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).