Insights on Space‐Leader Characteristics and Evolution in Natural Negative Cloud‐to‐Ground Lightning

Abstract

A space stem is a luminous plasma segment that has been observed to form ahead of the leader tip during the negative leader stepping process (Biagi et al., 2009(Biagi et al., , 2010Gorin et al., 1976). A space stem may eventually thermalize, undergo a sharp increase in conductivity, and develop into a space leader. Due to the intensification of electric field at its extremities, a space leader can start extending toward the pre-existing leader channel (PELC), eventually connecting with the PELC, leading to its extension and the completion of a new leader step. Characterizing the details of the formation and progression of space leaders is, therefore, essential for understanding the leader stepping process as well as the extent to which electrical characteristics of the PELC influence the formation and extension of space leaders. Gorin et al. (1976) first observed in laboratory spark experiments, space stems which were incepted in the vicinity of the negative leader tip and later evolved into space leaders that merged with the negative primary leader channel, leading to its extension. Biagi et al. (2009Biagi et al. ( , 2010 were the first to report separated luminous segments that they believed were space stems and space leaders in dart-stepped leaders of triggered lightning flashes using high-speed video cameras; they observed 1-4 m long luminous segments 1-10 m ahead of the leaders. Biagi et al. (2014) observed eight luminous segments 1-6 m in length and 3-8 m ahead of the downward stepped leader preceding the first stroke of an altitude-triggered flash. Hill et al. (2011) and

Separate luminous structures leading positive leader steps

“…The average forward development velocity of the separate luminous structure was obviously greater than the average backward development velocity. The average bi-directional development velocity of the separate luminous structure is estimated to be less than 9 × 10 5 m/s and greater than 4.5 × 10 5 m/s, which is close to the leader-step formation speed of negative lightning leaders reported in recent literature 32 .…”

Unusual Plasma Formations Produced by Positive Streamers Entering the Cloud of Negatively Charged Water Droplets

“…The first reasonably complete and clear description of the negative leader stepping process (including the space leader) is due to Gorin et al (1976, Figure 2a). Space leaders in negative natural and rocket-triggered lightning were observed by Biagi et al (2010Biagi et al ( , 2014, Petersen and Beasley (2013), Tran et al (2014), Gamerota et al (2014), Qi et al (2016), and Khounate et al (2021), among others, and modeled by Syssoev et al (2020). There is a growing body of evidence that positive leaders in long sparks and lightning can also involve space leaders formed inside or near the streamer zone extending from the leader tip (see Huang et al, 2021;Kostinskiy et al, 2018).…”

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

“…Acting as a bipolar floating leader that emerged ahead of the growing leader tip, it seems an analog of space leader formation which typically occurs in front of the negative stepped leader tip. However, the length of the negative leader step is generally several to a few tens of meters, and the most common distance from the space leader initiation to the leader tip is within 10 m (Hill et al., 2011; R. Jiang et al., 2017; Qi et al., 2016), with just a few strong cases involving distance up to a dozen meters (Khounate et al., 2021). A typical step cycle in negative leader lasts a dozen to 20 µm, considerably shorter than the present 0.46‐ms development from the bidirectional leader initiation to the attachment.…”