High‐speed video observation of stepwise propagation of a natural upward positive leader

Abstract

Using a high‐speed video camera operated at 150,000 frames per second, we have documented the upward propagation of an initial positive leader from a 325 m meteorological tower in Beijing. The stepwise development of the upward positive leader was clearly revealed during its self‐sustained propagation toward the cloud base, providing the first optical evidence for the stepping processes of a positive leader. The leader developed with definitive intermittent pauses and re‐establishment with abrupt jump of the leader top. Obvious brush‐like corona zone emitting outward from the leader top was identified in the frame of stepping, and the luminosity waves propagated downward along the already formed channel from the leader top immediately following the stepping, indicating that a current wave was generated at the leader top and subsequently traveled down the channel. The channel luminosity decreased during the leader pause stage, with the corona zone dimmed correspondingly. The positive leader experienced an average interstep interval of 61.7 µs (ranging between 30 µs and 120 µs) and an average 2‐D speed of 8.1 × 104 m/s. The transient speed of the step jump was estimated to be larger than 7.3 × 105 m/s with an average step length of 4.9 m. The need of sufficient positive charge accumulation at the leader top could be the main cause for the stepping characteristics.

Features of Upward Positive Leaders Initiated From Towers in Natural Cloud‐to‐Ground Lightning Based on Simultaneous High‐Speed Videos, Measured Currents, and Electric Fields

“…In laboratory experiments, no mechanism for stepped propagation of positive leaders has been identified, unlike the finding of stepping mechanism in the propagation of negative leaders, corresponding to the instantaneous bridging of the gap between the negative tip of the leader and a plasma stem with bipolar streamers, positive streamers facing toward the leader tip, and negative streamers facing outward. This stepping mechanism in negative leaders has also been identified in nature, as reported in several works, such as those by Biagi et al (), Hill et al (), Petersen and Beasley (), Qi et al (), and Wang et al ().…”

Abrupt Elongation (Stepping) of Negative and Positive Leaders Culminating in an Intense Corona Streamer Burst: Observations in Long Sparks and Implications for Lightning

“…It looks like a space leader inside the positive streamer zone, with its forward extending positive part being pronounced and branched and the negative part being undetectable. Alternatively, the floating formation was actually connected to the primary channel, in which case it was part of the transition from the corona streamer burst to the regular positive streamer zone. Comparison with the features of positive and negative leaders observed in lightning shows that for either polarity, the stepping in long sparks appears to be qualitatively similar to the stepping in lightning. Our results for positive‐leader stepping, together with those published by Les Renardieres Group for positive sparks and by Wang et al () for UPL in natural upward lightning, argue against the notion (e.g., Saba et al, ; Visacro et al, ) that positive leader cannot (or unlikely to) produce true steps in positive leaders, only fluctuations of current and luminosity induced by a simultaneously developing negative stepped leader.…”

“…Space stems/leaders have never been observed in positive leaders. Further, Baldo and Rea (, ) and Wang et al () reported that the positive‐leader step extended in the forward direction. On the other hand, Pilkey (, Figures A‐1 and A‐2) reported on a floating channel segment, similar to a space leader, but developing on a time scale of the order of hundreds of microseconds and making connection to the lateral surface of a cloud‐discharge channel that was inferred to be positively charged.…”

“…These luminosity pulses were similar in all respects to those recorded by the same instrument for negative downward lightning leaders, except for the pulse 10–90% rise time and half‐peak width: the geometric mean values were 2.0 and 3.4 μs for the positive leader versus 0.4 and 1.1 μs for the negative leader studied by Lu et al (), respectively. On the other hand, Saba et al () presented evidence that the electric field pulses prior to the positive return‐stroke waveform can be “due solely to the upward propagation or a negative connecting leader.” This means that what appears to be positive leader stepping can be induced by a simultaneously developing negative stepped leader, although the works of Les Renardieres Group and Wang et al () present direct evidence of true (as opposed to “induced”) steps in positive leaders. There are apparently three types of positive leaders that occur in response to an approaching negative stepped leader and can exhibit fluctuations of current and luminosity induced by negative‐leader steps.…”

“…Wang et al (), for a UPL developing from the 325‐m‐high meteorological tower in Beijing, China, presented perhaps the most detailed high‐speed video camera data on stepping in positive leaders in natural lightning. Their exposure time was 6.18 μs, and dead time was 0.49 μs.…”