Annual Variations of the Ionospheric Potential, the Air-Earth Current Density and the Columnar Resistance Measured by Radiosondes
It was found that the horizontal visibility at the surface is related to the air-earth current density in the stratosphere (6), and it is expected that the air-earth current density has an annual variation. Statistical results show that indeed the air-earth current density has a pronounced annual variation, high in winter and low in summer. The columnar resistance has also an annual variation, reverse to the variation of the air-earth current.
The ionospheric potentials over Tateno and Hachijojima did not show clear annual variation, and no change over Syowa-Base (Antarctica). The product of thunderstorm days and their areas, on the basis of the World Distribution of Thunderstorm Days published by the WMO (World Meteorological Organization), shows a maximum in July and minima in December and April. According to Mühleisen and Fischer, clear annual variation of the ionospheric potential was observed at Weissenau, minimum in June and maximum in December.
Above-mentioned facts suggest that variation of the ionospheric potential does not seem simply to be explained by the thunderstorm activity.
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