Monitoring Thunderstorm Activity With Sferics
About 100 lightning strokes per second are generated all over the world excited by thunderstorms located mainly in the continental areas at low and middle latitudes. In order to monitor the thunderstorm activity, sferics are the appropriate means.
Measurements of Schumann resonances at only a few stations around the world can monitor the global lightning activity fairly well. One can apply the dispersive property of the Earth-ionosphere waveguide by measuring the group velocity of a sferic signal at different frequencies together with its direction of arrival. The group time delay difference of neighbouring frequencies in the lower VLF band is directly proportional to the distance of the source. Since the attenuation of VLF waves is smaller for west- to east propagation and during the night, thunderstorm activity up to distances of about 10000 km can be observed for signals arriving from the west during night time conditions. Otherwise, the transmission range is of the order of 5000 km.
For the regional range (< 1000 km), the usual way is magnetic direction finding as well as time of arrival measurements of a sferic signal observed simultaneously at several stations. Presumption of such measurements is the concentration on one individual impulse. If one measures simultaneously several pulses, interference takes place with a beat frequency equal to the inversal average sequence time of the pulses.
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