Observations of the initial, upward-propagating, positive leader steps in a rocket-and-wire triggered lightning discharge

We present high-speed video images (recorded at 300 kfps) of an upward positive leader developing stepwise from the top of a grounded triggering wire. The leader initiated from the wire top at a height of 123 m, and was imaged over a distance of 11 m in ten discrete steps. Unique current pulses were detected at the wire base corresponding to each optical step. Luminosity waves propagated downward from the leader tip. The step lengths ranged from 0.4 to 2.2 m; the interstep time intervals from 16.6 to 30.4 ms. The leader's upward development speed increased with height, from 5.5 x 10(4) m s(-1) between heights of 123 m and 134 m, the bottom 11 m, to 2.1 x 10(5) m s(-1) at a height of 350 m. The measured wire-base peak currents for the individual steps ranged from 17 to 153 A. After removing from the wire-base current measurements the effect of propagation on the triggering wire and the effect of reflection at the ground connection, we estimate that the peak currents at the wire top, the step current, ranged from 10 to 89 A. The charge lowered to ground following each step ranged from 22 to 107 mC. The leader channel contained, on average, 51 mu C m(-1) of positive charge, a value that is similar to laboratory measurements of long positive polarity sparks. Citation: Biagi, C. J., M. A. Uman, J. D. Hill, and D. M. Jordan (2011), Observations of the initial, upward-propagating, positive leader steps in a rocket-and-wire triggered lightning discharge, Geophys. Res. Lett., 38, L24809, doi: 10.1029/2011GL049944.