CHARACTERIZATION OF VERTICAL ELECTRIC-FIELDS 500 M AND 30 M FROM TRIGGERED LIGHTNING

Vertical electric field waveforms of leader-return stroke sequences measured 500 m and 30 m from rocket-triggered lightning are presented. The 500-m data were recorded during the summer of 1986, the 30-m data during the summer of 1991, both at the NASA Kennedy Space Center, Florida. The 40 leader-return stroke field waveforms at 500 m and the 8 waveforms at 30 m all appear as asymmetrical V-shaped pulses, the bottom of the V being associated with the transition from the leader to the return stroke. Only two waveforms at 30 m were suitable for quantitative analysis. The widths of the V at half of peak value for these are 1.8 and 5.0 mu s, while for the 500-m data they are 1 to 2 orders of magnitude greater, with a median value of 100 mu s. Applying a widely used and simple leader model to the measured leader electric fields at 500 m, we infer, for the bottom kilometer or so of the leader channel, leader speeds between 2x10(6) and 2x10(7) mis and leader charges per unit length of 0.02x10(3) to 0.08x10(3) C/m. From the two measured leader electric field changes at 30 m we infer, using the same leader model, for the bottom 100 meters or so of the leader channel, speeds of 3x10(7) and 1x10(7) m/s (the corresponding measured waveform half widths are 1.8 mu s and 5.0 mu s) and charges per unit length of 0.14x10(3) and 0.02x10(-3) C/m (the corresponding measured leader field changes are 81 kV/m and 12 kV/m). The corresponding measured return stroke peak currents for the above two cases are 40 kA and 7 kA, respectively. A positive correlation is observed between the magnitude of the leader field change at 500 m and the ensuing return stroke current peak.