Lightning return stroke current waveforms aloft from measured field change, current, and channel geometry

Abstract

Three-dimensional reconstructions of six rocket-triggered lightning channels are derived from stereo photographs. These reconstructed channels are used to infer the behavior of the current in return strokes above the ground from current waveforms measured at the channel base and electric field change waveforms measured at a range of 5.2 km for 24 return strokes in these channels. Streak photographs of 14 of the same strokes are analyzed to determine the risetimes, propagation speeds, and amplitudes of relative light intensity for comparison with the electrical inferences. Results include the following: (1) The fine structure of the field change waveforms that were radiated by these subsequent return strokes can be explained, in large part, by channel geometry. (2) The average 10-90% risetime of the stroke current increased by about a factor of seven in our sample, from an observed 0.31 ± 0.17 μs at the surface to an inferred 2.2 ± 0.5 μs at 1 km path length above the surface. (3) The three-dimensional propagation speed of the current front averaged 1.80 ± 0.24 × 10 8 m/s over channel lengths typically greater than 1 km. (4) Assuming that the measured current was entirely due to the return stroke forced an unreasonably large and abrupt reduction in inferred current amplitude over the first few tens of meters above the surface, especially in cases when the leader was bright relative to its stroke. Therefore a significant fraction of the current at the surface was probably due to the leader in such cases. (5) Independent of the above, peak return stroke currents decreased by approximately 37 ± 12% between 100 m and 1 km of path length above the surface. Because of uncertainty about how to partition the measured current between leader and return stroke, however, we are unable to infer the variation of current amplitude near the ground. Copyright 2008 by the American Geophysical Union.