Correlated time derivatives of current, electric field intensity, and magnetic flux density for triggered lightning at 15 m

[1] We present measured current and its time derivative correlated with the corresponding electric field intensity and magnetic flux density and their time derivatives measured at 15 m for two lightning return strokes triggered in 1999 at Camp Blanding, Florida. Lightning was triggered to a vertical 2-m rod mounted at the center of a 70 70 m buried metallic grid. The rocket-launching system was located underground at the center of the grid. The experiment was designed to minimize any influence of either the strike object or a finite-conducting Earth (ground surface arcing and propagation effects) on the fields and field derivatives. The measured current derivative waveform and the return stroke portion of the magnetic flux density derivative and electric field intensity derivative waveforms associated with the two strokes are observed to be essentially unipolar pulses that have similar waveshapes for the first 150 ns or so, including the initial rising portion, the peak, and about 50 ns after the peak. The current and magnetic field derivative waveshapes are very similar for their total duration, and both decay to near zero about 200 ns after the peak derivative is reached. The electric field derivative decays more slowly than the current derivative after about 150 ns, taking about 500 ns to decay to near zero. The transmission-line model, the simplest available and most used return stroke model, is employed to calculate the return stroke field derivatives, given the measured current derivative as a model input, for return stroke speeds of 1 x 10(8) m s(-1),2 x 10(8) m s(-1), and 3 x 10(8) m s(-1) (the speed of light). A reasonable match between calculated and measured field derivative waveshapes is achieved for both strokes for a return stroke speed between 2 10 8 m s 1 and 3 x 10(8) m s(-1). Although the measured field and current derivatives have similar waveshapes for about 150 ns, which might appear to be consistent with the hypothesis that the radiation field component of the total field derivative is dominant for that time, transmission line model calculations indicate that this is not the case. Further, electric field derivatives measured simultaneously at 15 m and at 30 m for many strokes are observed to have similar waveshapes, which also might appear to be consistent with the hypothesis that the derivatives are dominated by the radiation field component; however, according to transmission line model calculations, while the calculated total field derivative waveshapes are similar at 15 and 30 m, all field components significantly contribute to the waveforms at both distances, and the mix of field components at 15 m and at 30 m is quite different.