SUBMICROSECOND FIELDS RADIATED DURING THE ONSET OF FIRST RETURN STROKES IN CLOUD-TO-GROUND LIGHTNING

An experiment to measure the electric field E and dE/dt signatures tha t are radiated by the first return stroke in cloud-to-ground lightning was conducted on the eastern tip of Cape Canaveral, Florida, during t he summer of 1984. At this site, there was minimal distortion in the f ields due to ground wave propagation when the lightning struck within a few tens of kilometers to the east over the Atlantic Ocean. Biases t hat are introduced by a finite threshold in the triggered recording sy stem were kept to a minimum by triggering this system on the output of a wideband RF receiver tuned to 5 MHz. Values of the peak dE/dt durin g the initial onset of 63 first strokes were found to be normally dist ributed with a mean and standard deviation of 39 +/- 11 V m(-1) mu s(- 1) after they were normalized to a range of 100 km using an inverse di stance relation, Values of the full width at half maximum (FWHM) of th e initial half-cycle of dE/dt in 61 first strokes had a mean and stand ard deviation of 100 +/- 20 ns and were approximately Gaussian. When t hese results are interpreted using the simple transmission line model, after correcting for the effects of propagation over 35 km of seawate r, the average value of the maximum current derivative, (dI/dt)(p), an d its standard deviation are inferred to be 115 +/- 32 kA mu s(-1), wi th a systematic uncertainty of about 30%. The FWHM after correction fo r propagation is about 75 +/- 15 ns. The inferred values of (dI/dt)(p) are significantly higher than most previous measurements of natural f irst strokes during direct strikes to instrumented towers but are in g ood agreement with direct measurements of dI/dt during subsequent retu rn strokes in rocket-triggered discharges in Florida.