PARAMETERS OF GLOBAL THUNDERSTORM ACTIVITY DEDUCED FROM THE LONG-TERMSCHUMANN RESONANCE RECORDS

Parameters of global thunderstorm activity are deduced from long-term frequency and amplitude records of the first three Schumann resonance (SR) modes performed at the Nagycenk Observatory (47.6 degrees N, 16.7 degrees E), Hungary, from May 1993 to May 1996. The variations of the daily frequency range of each mode, which are characteristic of the c hanges of the effective source width, exhibit annual and semiannual va riations. Annual and semiannual variations are present in both the rec orded data and the parameters of global thunderstorm activity. The eff ective source width varies over 3-4 h at solstices to 5-6 h during the equinox conditions. A period of about four months also appeared in th e spectra of the recorded data. A similarity was found between the pat terns of the annual and semiannual variations of the daily frequency r ange, and the changes in median solar time, apart from a three-month p hase lead of the astronomical data. The similarity might be due to an unknown solar-terrestrial link. Two approaches were used for the param eterisation of the global thunderstorm activity. One of these employs the preliminary evaluation of the source-observer distance (SOD) using the ratio of individual mode amplitudes. Variations of the source int ensity are obtained after dividing the amplitude recorded by that calc ulated for the established SOD. The second approach applies a direct s ummation of the individual mode intensities. Numerical simulations sho wed that such cumulative intensity depends on the level of the global thunderstorms rather than on the SOD. The two estimates were compared and the conclusion was made that these coincide to within +/-30%. The maximum of the global thunderstorm activity occurs in summer, in the N orthern hemisphere, and the minimum is observed in winter with a two-f old characteristic variance. (C) 1997 Elsevier Science Ltd. All rights reserved.