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

Abstract

Abstract. 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 107 rn s '1 , while the triggered dart leader average 2-D speed is 1.3 + 0.1 x 107 rn 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 Its. The optical risetime is evaluated from short channel segments (length of 3.2 + 1.7 m) located within 100 rn of the ground. The corresponding risetime for triggered leaders is 1.4 + 0.4 its. 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.

Progression Features of Dart Leaders in Natural Negative Cloud‐to‐Ground Lightning Flashes

“…No significant correlation is found for the brightness ratio and height. The average brightness ratio in this study is close to that of the triggered flashes (1.4) reported by Mach and Rust (). Similar to that of Mach and Rust (), we found no significant correlation for the brightness ratio and height.…”

“…In this study, we note that the risetime of the light signal recorded by us was caused by two factors: (1) the discharge process at the leader tip and (2) the dart leader propagation through a channel section. In contrast, Mach and Rust () found no significant relationship between the optical risetime and propagation speed of dart leaders for both natural and triggered lightning. Based this, Mach and Rust () claimed that the dart leader electric field wave front is responsible for the optical signal risetime.…”

“…Jordan et al () described the dart leader light waveforms as sharp pulses with 20‐to‐80% risetimes of about 0.5–1 μs and widths of 2–6 μs followed by a more or less constant light level (plateau). Mach and Rust () found that the 10–90% risetime is 2.6± 0.4 μs for natural dart leaders and 1.5± 0.4 μs for triggered dart leaders. They found no correlation between the dart leader optical risetime and the propagation speed.…”

Observed one‐dimensional return stroke propagation speeds in the bottom 170 m of a rocket‐triggered lightning channel

“…The use of film as a recording medium has some drawbacks, including limited resolution (both temporal and spatial) and non-linear response to incident light intensity. The recent advent of digital sampling and recording methods, along with the development of solid-state photodetectors, has allowed researchers to achieve finer resolution and increased linearity of data collection, typified by the work of Mach and Rust [1997, 1989a, 1989b and by the ALPS system employed by, among others, Wang et al [1999aWang et al [ , 1999bWang et al [ , 2004. The use of rocket-triggered lightning allows researchers to predict and determine more accurately the time and location of the termination of the lightning channel, and to prepare data collection instruments prior to the event.…”

On β2 stepped leaders in negative cloud‐to‐ground lightning

“…These authors also note that the RLs develop in a retrograde fashion, i.e., propagate toward the leader origin [ Saba et al , , Figures 4d, 4e, and 4f] at a minimum speed of 4 × 10 6 m s −1 . All the above speed estimates are in fair agreement with what is observed not only for the dart leaders in negative CG flashes [e.g., Schonland et al , ; Orville and Idone , ; Jordan et al , ; Mach and Rust , ; Campos et al , ] but also in the one dart streamer in a type β 2 negative leader, whose speed could be estimated by Schonland et al []. Warner [] has observed recoil activity in the branches of upward propagating, positive leaders whose initiation is induced in response to negative cloud charge transferred to ground by positive cloud‐to‐ground flashes in the vicinity of tall towers.…”