Two-dimensional speed and optical risetime estimates for natural and triggered dart leaders

We report velocities, risetimes, and other optical measurements of a set of 35 natural and 26 triggered dart leaders. All of the dart leaders are from negative strokes. The data were taken with our return stroke velocity device mounted on the National Severe Storms Laboratory mobile laboratory. The average two-dimensional (2-D) speed for the natural leaders is 1.9 +/- 0.2 x 10(7) m s(-1), while the triggered dart leader average 2-D speed is 1.3 +/- 0.1 x 10(7) m s(-1). These two averages are significantly different. We find no significant change in the dart leader 2-D speed with height. The mean 10-90% optical risetime for the natural dart leaders is 2.6 +/- 0.4 mu s. The optical risetime is evaluated from short channel segments (length of 3.2 +/- 1.7 m) located within 100 m of the ground. The corresponding risetime for triggered leaders is 1.4 +/- 0.4 mu s. These averages are significantly different. We calculated an average dart leader head length of 35 +/- 5.4 m. We find no significant difference between natural and triggered dart leader head lengths. The channel traversed by the dart leader and before the return stroke is often bright enough to be detected by our instrument. We find that the ratio of the channel brightness after the dart leader head to the dart leader head is closer to unity for triggered dart leaders. We see no clear relationship between dart leader optical risetimes and subsequent return stroke peak transmission line model (TLM) current. We find that natural dart leader speeds are correlated with the return stroke peak TLM currents: the peak TLM current increases with increasing natural dart leader 2-D speed. We see no correlation between triggered dart leader speeds and subsequent return stroke peak TLM currents. At a given dart leader speed, natural return strokes will have smaller peak TLM currents. Overall, these results indicate that there are significant differences between natural and triggered dart leaders.