Study of the geoelectric fields at Vostok, Antarctica
In fair weather regions, a downward directed geoelectric field of ~ 100 — 200 Vm-1 can be measured at the surface of the Earth. At high latitudes, the interaction of the solar wind and interplanetary magnetic field with the Earth's magnetosphere and ionosphere imposes a variable dawn-to-dusk po...
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| Format: | Thesis |
| Language: | English |
| Published: |
2002
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| Online Access: | https://eprints.utas.edu.au/19141/ https://eprints.utas.edu.au/19141/1/whole_CorneyRita2002_thesis.pdf |
| Summary: | In fair weather regions, a downward directed geoelectric field of ~ 100 — 200 Vm-1 can be measured at the surface of the Earth. At high latitudes, the interaction of the solar wind and interplanetary magnetic field with the Earth's magnetosphere and ionosphere imposes a variable dawn-to-dusk potential drop of 20 — 150 kV on the geoelectric field. Large scale horizontal electric fields in the ionosphere map into the vertical component of the near surface electric field. A total of 133 fair weather days of vertical geoelectric field data from Vostok, Antarctica (78.5°S, 107°E; corrected geomagnetic latitude, 83.5°S), obtained during 1998, were selected to investigate the suitability of the Vostok site for studies of the global electric circuit and confirm the linkage between the solar wind imposed potential patterns and the near surface electric field. Seasonal variations in the diurnal electric field at Vostok are compared with the Carnegie diurnal curves, after appropriate allowance is made for the influence of the solar wind. Consistent features include the shift to later UT hours of the maximum between Southern Hemisphere (SH) summer and winter months, the seasonal constancy of the timing of the diurnal minimum and the diurnal range of the total dataset. The range of the diurnal curves, expressed as a percentage of the mean, increases by 7 to 9% when the imposed polar cap potential contribution is subtracted. The range of the total diurnal curve increases from 28% to 37% of the mean value, comparable with the Carnegie value of 37%. However, the diurnal range of the Vostok SH summer diurnal curve, 51% of the mean value, is excessive compared to the Carnegie value of 40%. The average magnitude of the electric field at Vostok peaks in the SH winter and is a minimum in the SH summer, consistent with global thunderstorm activity and monthly averages of recently re-analysed Carnegie data [Adlerman and Williams, 1996]. However, the Vostok equinoctal mean is similar to the Vostok SH winter mean while the Carnegie equinoctal mean is similar to the Carnegie SH summer mean. Broad similarity of the Vostok and Carnegie diurnal-seasonal variations confirms Vostok as a suitable site for global circuit studies but the possibility that the comparisons are influenced by seasonal and diurnal variations in the average conductivity at ground level or possible inaccuracies in the ionospheric potential model used to subtract the influence of the solar wind on the Vostok data cannot be excluded. Variations of the vertical electric field measurements are compared with both Weimer [1996] and IZMEM [Papitashvili et al., 1994] model calculations of the imposed polar cap potential above Vostok station. Variations of the electric field measurements are calculated using two previously published procedures, a monthly method as used by Frank-Kamenetsky et al. [2001], and a yearly method as per Frank-Kamenetsky et al. [1999]. By investigating the correlations between these parameters for individual UT hours, we confirm and extend the diurnal range over which sigificant correlations have been obtained. Using the yearly method, nineteen individual UT hours are significantly correlated with the Weimer model predictions and ten with the IZMEM model predictions. The linkage between ionospheric potential differences above Vostok and ground-level vertical electric field is clearly demonstrated. The ratio between the two variables is expected to be contant. Diurnal variations in the magnitude of the ratio for individual UT hours allows both models to be tested against the measured Vostok data, demonstrating that Antarctic polar plateau geoelectric field measurements can be used to investigate polar convection. |
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