Statistical characteristics of the electric and magnetic fields and their time derivatives 15 m and 30 m from triggered lightning

[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 x 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).