Current Distributions in Lightning Subsequent Return Strokes

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A cloud-to-ground lightning flash consists of a "leader" that progresses in steps from the cloud to the ground followed by three or four successive return strokes. The total time for the process is several tenths of a second. Currents measured in return strokes at the ground rise to a maximum of typically 20 kA in a few microseconds and decline more slowly. These currents are responsible for much of the lightning damage to structures. The rise times and peak currents in lightning strokes as a function of the height above the ground have never been measured directly and are not quantitatively known, though the information is needed for understanding the lightning hazards faced by aircraft and for basic research on the physics of lightning. These properties of lightning return strokes will be inferred from data collected on triggered lightning in Florida in 1987. The data consist of the current and the electric field measured at the ground and stereo photographs of the flashes, from which the three-dimensional geometry of the lightning channels can be reconstructed with high spatial and temporal resolution. The geometry of the lightning path is important in determining the time history of the electric field measured at the ground near the flash. Given the channel configuration and the measured electric field, it is not possible to solve uniquely for the current distribution. Instead, the approach is to assume a current distribution, to use the known path geometry, and to compute the electric field. The results are compared with the observed field and iterated until acceptable agreement is reached. It is anticipated that this will enable the determination of the main features of the return stroke currents, namely the variation with altitude of the peak amplitude, rise time, and propagation velocity. This information is needed for the efficient design of lightning protection for flying aircraft and spacecraft.