Needle Propagation and Twinkling Characteristics

Abstract

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 VHF...

High‐Speed Video Observations of Needles in a Positive Cloud‐to‐Ground Lightning Flash

“…This is different from the pulsating repetition of the needle extension from the surface of hot channel core that was hypothesized by Hare et al. (2019, 2021), Pu and Cummer (2019), and Saba et al. (2020).…”

“…Hare et al. (2021) showed detailed characteristics of needle propagation and flickering and observed that recoil leaders quench needle activity. Saba et al.…”

“…Although the needle‐like structures analyzed by Hare et al. (2019, 2021), Pu and Cummer (2019), and Saba et al. (2020), shared some common features of development and flickering, the physics behind needle production still remains unclear.…”

“…Hare et al. (2021) emphasized the disconnection hypothesis and the influence of the following recoil leaders on the recurrence of the needles and twinkling. Therefore, the production mechanisms and detailed dynamics of needles and the associated flickering events are still not completely understood, and, hence, further research of this elusive phenomenon is needed.…”

Activation of Abundant Recoil Leaders and Their Promotion Effect on the Negative‐End Breakdown in an Intracloud Lightning Flash

“…One of the notable asymmetries is that the positive leader could reactivate in the form of recoil leader after cooling down or being cutoff, while the negative leader rarely does (Mazur, 2002, 2016; Qie et al., 2017). Lightning positive leader propagation draws more attentions these years, and new insights into its physical characteristics and mechanism are revealed with data in different aspects obtained by comprehensive observations (R. Jiang et al., 2020; Hare et al., 2021; Visacro et al., 2017; Zhao et al., 2021). Yuan et al.…”