On the polarity and continuing currents in unusually large lightning flashes deduced from ELF events

An analysis of ELF events has enabled the magnitude and duration of the continuing currents in 274 unusually large lightning flashes to be estimated. An exponential decay model of the variation of the current moment of the source lightning stroke with time was fitted to the measured spectra using an optimisation algorithm. Of the events analysed, 212 were of positive polarity (i.e. the initial polarity of the electric field was positive) and 62 were negative. The average magnitude of the peak current moment for positive events was 5.1 x 10(7) Am with a standard deviation of 3.2 x 10(7) Am while that for negative events was 2.9 x 10(7) Am with a standard deviation of 0.6 x 10(7) Am. The average time constant T of our data set is 32 ms with a standard deviation of 6 ms for positive events and 26 ms with a standard deviation of 5 ms for negative events. Positive events are therefore associated with somewhat larger and more variable current moments and slightly longer continuing currents. We believe that these positive events are most likely to be produced by positive cloud to ground lightning.