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Abstract.Assuming that the lightning return stroke transmission-line model is applicable, we derive an expression for the return-stroke magnetic field for an arbitrary return stroke speed and from that expression show that for a return stroke speed equal to the speed of light c the electric and magnetic field waveforms at all points in space and the

[1] 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 (R 2 = 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 ms) and for strikes to the Earth (geometric mean 0.4 ms) 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.

[1] We present a statistical analysis of the salient characteristics of the electric and magnetic fields and their derivatives at distances of 15 m and 30 m from triggered lightning strokes that lowered negative charge to ground. Return stroke current and current derivative characteristics are also presented. The measurements were made during the summers of 1999 and 2000 at Camp Blanding, Florida. Lightning was triggered to a 1 to 2 m strike object at the center of a 70 m  70 m metal-grid ground plane that was buried beneath a few centimeters of soil. The strike object was mounted on the rocket launching system that was located below ground level in a pit. The experiment was designed (1) to minimize the influence of the strike object on the field and field derivative waveforms an (2) to eliminate potential distortions of the field and field derivative waveforms both due to ground surface arcing and due to the propagation of the field being over imperfectly conducting ground. Measurements were made on about 100 return strokes, although not all field quantities were successfully recorded for each stroke. We present histograms and parameters of statistical distributions for the following 28 waveform characteristics: current peak, risetime, and width; current derivative peak, risetime, and width; return-stroke electric field change and field pulse width at 15 m and at 30 m; electric field derivative peak, risetime, and width at 15 m and at 30 m; magnetic field peak, risetime, and width at 15 m and at 30 m; and magnetic field derivative peak, risetime, and width at 15 m and at 30 m. We compare our results with those from previous studies. From this comparison we infer, among other results, that for strikes to our buried metal-grid ground plane the current risetime and width are, on average, smaller than for strikes to concentrated grounding electrodes (vertical ground rods).