Characterization of return-stroke currents in rocket-triggered lightning

We present a statistical analysis of the salient characteristics of current waveforms for 206 return strokes in 46 rocket‐triggered lightning flashes. The flashes were triggered during a variety of experiments related to the interaction of lightning with power lines that were conducted from 1999 through 2004 at the International Center for Lightning Research and Testing at Camp Blanding, Florida. The return‐stroke current, after measurement, was injected into either one of two test power lines or into the Earth near a power line via a grounding system of the rocket launcher. Statistical information is presented for return‐stroke peak current, charge transfer, half‐peak width, and 10%–90% risetime. Our return‐stroke peak current statistics are found to be generally consistent with those reported from other triggered‐lightning studies and appear to be independent of electrical properties of the strike object, as previously found in another study. We found significant correlation (R2 = 0.76) between lightning return‐stroke peak current and the corresponding charge transfer within 1 ms after return‐stroke initiation. The dependence is surprisingly similar to that found by Berger and co‐workers for the natural first return‐stroke peak currents and 1‐ms charge transfers. The means of the 10%–90% current risetimes for strikes to the power line (geometric mean 1.2 μs) and for strikes to the Earth (geometric mean 0.4 μs) are significantly different which indicates that the electrical properties of the strike object affect the risetime. This effect is likely related to the impedance seen by lightning at the strike point and/or to reflections at impedance discontinuities within the strike object, larger effective impedances apparently resulting in larger risetimes. A dependence of the return‐stroke current half‐peak width on the electrical properties of the strike object was not observed in our direct and nearby‐strike experiments.