Characterization of the initial stage of negative rocket-triggered lightning

We performed a statistical study on the initial stage (IS) of negative rocket-triggered lightning using 37 channel-base current recordings obtained during the summer of 1994 at Fort McClellan, Alabama, and during the summers of 1996 and 1997 at Camp Blanding, Florida. The IS can be viewed as composed of an upward positive leader (UPL) followed by an initial continuous current (ICC). The IS has a geometric mean (GM) duration of 279 ms and lowers a GM charge of 27 C to the ground. The average IS current in an individual lightning discharge varies from a minimum of 27 A to a maximum of 316 A with a GM value of 96 A for the entire sample of 37 discharges. We examined the current variation at the beginning of the IS in 24 flashes. In 22 out of 24 cases this initial current variation (ICV) includes a current drop, probably associated with the disintegration of the copper triggering wire and the subsequent current reestablishment. The GM time interval between the onset of the initial stage and the abrupt decrease in current is 8.6 ms, and the GM current level just prior to the current decrease is 312 A, a value about 3 times the GM value of average current for the whole IS, 96 A. Before this abrupt current decrease, a GM charge of 0.8 C has been lowered to ground with a corresponding GM action integral of 110 A(2) s. The abrupt current decrease takes typically several hundred microseconds and is followed, immediately or after a time interval up to several hundred microseconds, by a pulse with a typical peak of about 1 kA and a typical risetime of less than 100 mu s. The ICC usually includes impulsive processes that resemble the M processes observed during the continuing currents that follow return strokes in both natural and triggered lightning. We present statistics for the following parameters of current pulses superimposed on the ICC: magnitude, risetime, half-peak width, duration, charge transferred, preceding continuous current level, interpulse interval, and time interval between the onset of the IS and the first ICC pulse. The observed characteristics of ICC pulses varied significantly among the three data sets. For all data combined, the characteristics of the ICC pulses are similar to those of the M-component current pulses studied by Thottappillil et al. [1995]. This latter finding suggests that ICC impulsive processes are of the same nature as M processes.