Speed and current for lightning stepped leaders near ground as determined from electric field records
The speed and current during the final stages of the lightning stepped leader were determined from features of the electric field waveform. The average speed of the leader tip was found below the height where dE/dt was zero for close lightning. This height was in the range 0.6-2.0 km for flashes at the distances considered here, 3.6-5.7 km. For 10 stepped leaders observed near Tampa, Florida, the mean final one-dimensional speed was 4.4×105 m/s, the standard deviation was 3.8×105 m/s, and the range was 1.0-14×105 m/s. These speeds are consistent with other stepped leader speeds reported in the literature that were found using optical techniques. The current was estimated when the leader was near ground from dE/dt during the last hundred microseconds before the return stroke. For 62 stepped leaders observed at distances of 3.6-14 km, the mean final leader current was 1.3 kA, the standard deviation was 1.0 kA, the geometric mean was 1.0 kA, and the range was 110 A to 5.0 kA. Errors resulting from uncertainties in distance, charge center height, calibration, ground channel inclination, and channel branching were found to be about 40-50% rms for both speed and current.