Submicrosecond fields radiated during the onset of first return strokes in cloud‐to‐ground lightning

Abstract

An experiment to measure the electric field E and dE/dr signatures that are radiated by the first return stroke in cloud-to-ground lightning was conducted on the eastern tip of Cape Canaveral, Florida, during the summer of 1984. At this site, there was minimal distortion in the fields due to ground wave propagation when the lightning struck within a few tens of kilometers to the east over the Atlantic Ocean. Biases that are introduced by a finite threshold in the triggered recording system 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/dr during the initial onset of 63 first strokes were found to be normally distributed with a mean and standard deviation of 39 _+ 11 V m-• /xs -• after they were normalized to a range of 100 km using an inverse distance relation. Values of the full width at half maximum (FWHM) of the initial half-cycle of dE/dr in 61 first strokes had a mean and standard deviation of 100 +_ 20 ns and were approximately Gaussian. When these results are interpreted using the simple transmission line model, after correcting for the effects of propagation over 35 km of seawater, the average value of the maximum current derivative, (dI/dt)p, and its standard deviation are inferred to be 115 + 32 kA/•s -•, with a systematic uncertainty of about 30%. The FWHM after correction for propagation is about 75 +_ 15 ns. The inferred values of (dI/dt)p are significantly higher than most previous measurements of natural first strokes during direct strikes to instrumented towers but are in good agreement with direct measurements of dI/dt during subsequent return strokes in rocket-triggered discharges in Florida. 1589 1590 KRIDER ET AL.: SUBMICROSECOND FIELDS RADIATED BY LIGHTNING ET AL.' SUBMICROSECOND FIELDS RADIATED BY LIGHTNING 1593 80-70-E 50ß -4.0-'o 313-20-10-0 25 ß ß ß ß ß ß ßß ß ß ß i 315 i i 30 40 45 50 Range (kin) Ip 12 P KRIDER ET AL.' SUBMICROSECOND FIELDS RADIATED BY LIGHTNING 1595 KRIDER ET AL.: SUBMICROSECOND FIELDS RADIATED BY LIGHTNING 1597

Multiple pulses in dE/dt and the fine-structure of E during the onset of first return strokes in cloud-to-ocean lightning

“…The waveforms were recorded at ranges of 4 to 40 km and, as we stated in section 2.0, they have not been corrected for the effects of ground-wave propagation because the propagation paths were almost entirely over ocean water. Krider et al (1996) have estimated that propagation over a smooth ocean surface will reduce the peak dE/dt and increase the full-width-at-half-maximum (of the negative half-cycle) by about 6% and 11%, respectively, at a range of 10 km, and by 15% and 33%, respectively, at a range of 35 km (see their Table 2). In order to evaluate whether such effects might be present in our dataset, Fig.…”

“…Indeed, Krider et al (1996) have previously argued that E and dE/dt measurements made at a similar site in 1984 were not significantly affected by propagation. Willett et al (1998, A block schematic diagram of the data acquisition system has been given by Willett et al (1989, Fig.…”

“…Both digitizers were triggered on the output of a wideband RF receiver tuned to 5 MHz in order to minimize any biases that might be introduced by the finite trigger threshold. Krider et al (1996) and Willett et al (1998) Plots of all dE/dt and E waveforms in our dataset, together with the outputs of the RF receiver and the slow-E antenna, are given in technical reports by Bailey and Willett (1989) and Izumi and Willett (1991). The type of lightning process that triggered the recording system was determined from the overall shape and structure of the E and slow-E waveforms.…”

“…The lightning locations were measured using a network of three gated, wideband magnetic direction-finders that provided an overall location accuracy of about 1 km (Maier and Jafferis 1985;Krider et al 1996). The flashes occured at ranges of 5 to 50 km on three thunderstorm days, and maps showing the locations of the experiment site and the lightning strike points are given in Willett et al (1990, Fig.…”

“…Submicrosecond measurements of the electric field, E, and dE/dt waveforms that are radiated during the onset of first return strokes in cloud-to-ocean lightning have been reported previously by Weidman and Krider (1978;1984), Krider et al (1996), Willett et al (1995;, and Willett and Krider (2000). Weidman and Krider (1978; noted that the shape of the initial E field, when recorded under conditions where there is minimal distortion due to the effects of ground-wave propagation, typically begins with a slow, concave front that lasts for several microseconds.…”