Optical observation of needles in upward lightning flashes

Saba, Marcelo M. F.; Paiva, Amanda R. de; Concollato, Luke C.; Warner, Tom A.; Schumann, Carina

doi:10.1038/s41598-020-74597-6

Cited by 20 publications

(101 citation statements)

References 15 publications

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“…If the needles twinkle at a regular rate, and the distance between needles is about equal to the speed of the leader times half the time between twinkles (which is the case in their data), then the twinkles from nearby needles will naturally appear to occur out-of-phase even if the needles have no interaction at all. Close examination of Figure 3 in Saba et al (2020) shows support not only for downward-going waves, but also equal evidence for upward-going waves, which strongly implies that downward-going twinkle-inducing waves are an observational artifact.…”

Section: Needle Structure Along the Positive Leadermentioning

confidence: 83%

“…et al (2020) were somewhat shorter than those observed in Hare et al (2019) and Pu and Cummer (2019). Saba et al (2020) was even able to show that propagation of one 73-m long needle was away from the positive leader with a speed of about 2.7 × 10 5 m/s, and that a negative leader developed from the location of a needle, consistent with Hare et al (2019) and Pu and Cummer (2019), and Saba et al (2020) was able to confirm a hypothesis proposed by Hare et al (2019), that the first needle twinkle occurs about 100 m or so behind the tip of the positive leader. Finally, Saba et al (2020) was also able to show that needles are the result of a corona-brush split, which is where the corona in front of a positive leader splits into two different sections in a failed attempt to branch.…”

mentioning

confidence: 79%

“…Saba et al. (2020) was able to observe optical emissions from needles on upward positive leaders. Saba et al.…”

Section: Introductionmentioning

confidence: 98%

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Needle Propagation and Twinkling Characteristics

et al. 2021

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Needles are a very recently discovered lightning phenomenon, described in Hare et al. (2019), that occur along positive leader channels. They appear like small leader branches, at most around 100-m long, and stick out from the channel. However, unlike leader branches, they exhibit ionization fronts that propagate up each needle, away from the positive leader channel. Hare et al. (2019) referred to these fronts as twinkles, and they occur at a very regular rate, around once per 5 ms. Pu and Cummer (2019) confirmed these findings, and showed that there is a location on the positive leader channel that moves forward along the positive at a regular speed (≈10 5 m/s), where there is no needle activity ahead of this point, and copious needle activity behind it. We call this point the needle production head (since it is the tip of where needles are produced), and we discuss it in more detail in Section 2.5. Paradoxically, despite propagating away from the positive leader channel, Hare et al. (2019) concluded that since needles emit copious VHF while positive leaders do not (Edens et al., 2012; Hare et al., 2019; Shao et al., 1999), needle twinkles must thus be a form of negative propagation. Pu and Cummer (2019) confirmed this by showing that the negatively charged end of a bidirectional leader suppressed needle activity, and by showing a needle that extended into a full negative leader. Saba et al. (2020) was able to observe optical emissions from needles on upward positive leaders. Saba et al. (2020) showed that these needles observed in optical had very similar properties to those reported in Hare et al. (2019) and Pu and Cummer (2019), including that they twinkled multiple times with a few milliseconds between twinkles without growing in length. However, the needles observed by Saba Abstract Recently, a new lightning phenomena, termed needles, has been observed in both VHF and in optical along positive lightning leaders. They appear as small (<100 m) leader branches that undergo dielectric breakdown at regular intervals (called twinkles). Providing a coherent and consistent explanation for this phenomenon is challenging as each twinkle is a form of negative breakdown that propagates away from the positive leader. In this study, we provide detailed observations of needles in VHF, observed during two lightning flashes. We show distributions of different needle properties, including twinkle propagation speeds, time between twinkles, and needle lengths, among others. We show a return stroke and multiple recoil leaders that quench needle activity. We also show that nearby needle activity does not seem to correlate together, and that needle twinkling can slow down by 10%-30% per twinkle. We conclude by presenting possibilities for how the positive leader could induce negative propagation away from the positive channel, and we argue that twinkles can propagate like a stepped leader or like a recoil leader depending on the temperature of the needle, which implies that needle twinkles can probably propagate without emitting ...

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Section: Needle Structure Along the Positive Leadermentioning

confidence: 83%

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confidence: 79%

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Needle Propagation and Twinkling Characteristics

et al. 2021

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“…The new positive leader propagation mechanism that is here suggested does not only have just implications for long air discharges in the laboratory. As the streamers optically observed at the heads of natural positive leaders 11 – 13 show some similarities with those in the laboratory discharge, the proposed mechanism also introduces a new physical process which needs to be assessed when studying leader-related lightning phenomena such as intermittent propagation of positive leader 39 – 44 , recoil leaders 45 – 47 and needles 12 , 48 , 49 . According to this new mechanism, the leader will propagate stepwise using short steps, as previously suggested by Bazelyan 17 .…”

Section: Discussionmentioning

confidence: 94%

Elongation and branching of stem channels produced by positive streamers in long air gaps

Zhao

Becerra

Yang

et al. 2021

Sci Rep

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The elongation and branching of long positive spark discharges in the laboratory and in lightning have been attributed to the formation of thermalized channels inside a diffuse, glow-like streamer section at the leader head. It is experimentally shown here that the structured morphology of streamers produce low-density stem channels that elongate and branch well before a new leader channel section is formed. These non-thermalized stems are also shown to develop ahead of a developing leader channel. These findings are based on high-speed photography and Schlieren imaging used to visualize both the morphology of streamer filaments and stem channels. Numerical analysis is also performed to estimate the axial temperature and density of the stem channels. A stem-driven mechanism for the propagation and branching of positive long air gap discharges is proposed and discussed based on the presence of not-yet thermalized, low density channels formed by streamer ensembles at the leader head.

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“…More recently, Saba et al. (2020) reported clear corona brush (corona streamer formation) surrounding and diverging outward from the positive leader head, as captured by Phantom cameras at distances of 0.3 and 1.9 km. Because the observations of CSZ with fine time resolution are still lacking, the physical characteristics of CSZ is not very clear yet.…”

Section: Discussionmentioning

confidence: 99%

Fine Structure of the Breakthrough Phase of the Attachment Process in a Natural Lightning Flash

Jiang

Srivastava

Qie

et al. 2021

Geophysical Research Letters

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The attachment process in cloud-to-ground lightning flashes is a crucial process that eventually switch on the discharge route between the cloud and the ground, acting as a transition from the leader stage with a peak current of 1-2 kA to the return stroke stage with a peak current of several tens of kiloamperes (Pu et al., 2019; Rakov & Tran, 2019). In response to the downward leaders starting from the charged clouds, upward connecting leaders (UCLs) of the opposite polarity are initiated from the grounded objects, extend their channels toward the approaching downward leader and make contact, followed by the collision and return stroke (Saba et al., 2017). The progress of the knowledge of lightning attachment experienced the identification of the existence of UCL at the early stage (Golde, 1967; Orville, 1968), later the investigation of the characteristics of the leaders that determine the connection point or influence the selection of contacted leaders (Jiang et al., 2015; Lu et al, 2013, 2016; Qie & Zhang, 2019), and more recently the exploring of the so called breakthrough phase (BTP) which is one of the most poorly understood processes in lightning physics (Tran & Rakov, 2017).

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Optical observation of needles in upward lightning flashes

Cited by 20 publications

References 15 publications

Needle Propagation and Twinkling Characteristics

Needle Propagation and Twinkling Characteristics

Elongation and branching of stem channels produced by positive streamers in long air gaps

Fine Structure of the Breakthrough Phase of the Attachment Process in a Natural Lightning Flash

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