On The Transition From Precursors To The Initial Upward Positive Leader In Negative Rocket-Triggered Lightning

Precursory current pulses (precursors) and initial upward leader pulses are examined to characterize the charge transfer of precursors and the sustained upward leader during the early stage of rocket-triggered lightning. According to the analysis of six negative triggered flashes, it is found that both precursors and the initial upward positive leader (iUPL) pulse are led by a small deflection that appears at about 25 mu s prior to the major pulses. The characteristics of precursors and iUPL pulses are very similar except that the precursors generally transfer less electric charge (35.07 mu C on average) than iUPL pulses do (64.73 mu C on average). Some marginal differences in the parameters (e.g., rise time, duration) may be attributed to the modification of the current pulses caused by the different impedances of the leader channel and the steel wire. The ascending rocket causes the enhancement of electric field (E-field) in the close vicinity of the wire tip, and therefore the breakdown of the iUPL is initiated by a stronger E-field than that of the precursors. The step lengths of the precursors (defined as the extended length of the wire tip lifted by the rocket between two adjacent isolated precursors) are similar to that of iUPL pulses. Our analyses indicate that the transition from precursors to the iUPL occurs when the E-field around the wire tip is strong enough, by the accumulation of more positive charge, to launch a meter-scale breakdown and the ensuing charge transfer to ground.