PEAK-CURRENT VARIATIONS OF LIGHTNING RETURN STROKES AS A FUNCTION OF LATITUDE

LATITUDINAL variations in lightning-flash characteristics have been hypothesized1,2, but have not been found to exist3,4. This may be because lightning characteristics are constant over the Earth, or because the observations have been made over too small an area of the globe. To investigate the question of latitudinal variation, I have used a network of thirty-six gated wideband magnetic direction finders covering the eastern United States to measure the characteristics of cloud-to-ground lightning flashes. These characteristics include the flash location, the flash polarity, the number of strokes in the flash and the peak magnetic radiation field in the first return stroke of the flash. From the peak radiation field, I estimate the peak current5. More than five million lightning ground flashes were recorded by the network in 1988. An examination of the mean peak current in the first return stroke shows that the peak current varies by almost a factor of two, from 25k A in New England to 40-45 kA in northern Florida. Apparently, this is the first report of an important lightning parameter, the return stroke current, varying over a large area of the globe. It seems that the generation of lightning strokes and their characteristics may be a sensitive function of latitude and hence of temperature. The results presented here indicate that even higher peak currents will be observed if this research is extended into the equatorial regions. © 1990 Nature Publishing Group.