Radiation field wave forms produced by lightning stepped leaders

Broad band electric field data are presented which show stepped leader wave forms preceding records of distant lightning return strokes. The majority of leader pulses are characterized by a large initial peak with a small and slow opposite overshoot. Total pulse durations range from 15–40 μs several milliseconds before the return stroke to 2–10 μs immediately preceding the return stroke. Close to the ground the stepped leader pulses occur at regular 10- to 20-μs intervals and are almost unipolar with rise times of about 1 MS and full widths at half maximum in the range from 1 to 3 μs. The ratio of the peak of the last leader pulse to the subsequent return stroke peak is typically 0.1, which suggests a peak step current near the ground of about 10% of the return stroke peak current.