Observations of stepping mechanisms in a rocket‐and‐wire triggered lightning flash

Abstract

[1] We present 10 high-speed video images that depict the bottom 150 m of a downwardnegative, dart-stepped leader in a rocket-and-wire triggered flash, recorded at 240 kiloframes per second (4.17 ms frame integration time), along with correlated measurements of the X-ray emission at 50 m, electric field derivative (dE/dt) at 80 m, and the rocketlaunch-tower current beneath the leader. We observed discrete segments of secondary channel that exhibited luminosity above that of the surrounding corona streamers and were distinctly separate and beneath the downward-extending leader channel. These segments appear similar to the space stems or space leaders that have been imaged in long negative laboratory sparks. Multiple simultaneous pulses in X-ray emission, dE/dt, and launch tower current were recorded during the time that the leader steps were imaged. The leader extended at an average downward speed between 2.7 × 10 6 and 3.4 × 10 6 m s −1 .

High‐speed video observations of a natural negative stepped leader and subsequent dart‐stepped leader

“…In their study, disconnected luminous segments interpreted to be space leaders can also be seen to form ahead of the leader tip in the decaying channel remnants. However, Biagi et al [2010] also recorded what appears to be a streamer zone ahead of the dart leader tip, a significant difference from the present study where no such streamer zone was observed.…”

“…We interpret these segments as analogous to the space leader segments observed during the initial negative stepped leader. The image of the dart-stepped leader also shows a striking lack of an observable corona streamer zone in the volume just ahead of the leader tip, in contrast to the observation by Biagi et al[2010] in which a corona streamer zone can be clearly seen ahead of the tip of a descending dart-stepped leader. Another lower luminosity zone was also observed to extend beyond the low-luminosity breakdown region of the dart-stepped leader tip, a feature that to our knowledge has not been reported in previous literature.…”

“…[20] It is interesting to compare these observations to those of Biagi et al [2010], where high-speed video was used to observe a descending dart-stepped leader that occurred as part of an artificially triggered lightning flash. In their study, disconnected luminous segments interpreted to be space leaders can also be seen to form ahead of the leader tip in the decaying channel remnants.…”

“…Hill et al [2011] have presented perhaps the earliest such observation, showing visible but poorly resolved features of the breakdown process at a distance of approximately 1 km and at a camera frame rate of 300,000 frames per second. Triggered lightning has been better studied using high-speed video, including observations by Biagi et al [2009Biagi et al [ , 2010 that show highly resolved features of the breakdown process in front of a dart-stepped leader near ground at frame rates of 50,000 and 240,000 frames per second. However, it is unclear to what extent the processes may differ due to the additional influence of the preconditioned channel and the vaporized wire on the breakdown process.…”

Modeling of the Stepping Process of Negative Lightning Stepped Leaders

“…Photographic evidence show that these leaders travel towards ground in intermittent steps and experimental data on the time intervals between the steps and the lengths of the individual steps are available in the literature [1,2]. The available information show that the time interval between steps span from 10 μs to 100 μs and the lengths of the steps span from 5 m to about 200 m [1,2,3,4]. More recently, detailed development of the stepping process in negative lightning leaders has been observed using high speed photography [4].…”