An experimental study of positive leaders initiating rocket-triggered lightning

Simultaneous, co-located measurements of ambient, electrostatic-field profiles and rocket-triggered lightning phenomenology beneath Florida thunderstorms are reported. Ambient-field conditions that are sufficient to initiate and sustain the propagation of positive lightning leaders are identified. It is found that lightning can be triggered with grounded triggering wires approximately 400 m long when the ambient fields aloft are as small as 13 kV/m (foul-weather polarity). Ambient potential differences between the height of the triggering wires and ground were as small as −3.6 MV (negative wrt. earth) when lightning occurred. `Precursors,ʹ the first measurable current pulses from the triggering wires, were initiated at similar fields aloft but at wire heights only about half as large, where ambient potentials were as small as −1.3 MV. The mean speed of one `failed leaderʹ is estimated at 1.9×104 m/s over 35 m of propagation. The lengths of `leaderʹ extension and positive-streamer `fanʹ during individual impulses of multiple-pulse precursors are estimated to be 0.7 and 2.0 m, respectively.