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This study analyzes the two‐dimensional speed profiles of 107 stepped leaders and 93 dart leaders recorded by high‐speed cameras in Utah (USA), together with data from lightning location system. The results shows that the final and average speed of the stepped leader has a very strong (R = 0.82) and strong (R = 0.71) correlation with the peak current of the return stroke. It also shows that 91% of the stepped leaders increased their speed near the ground (average increase of 69%). The same analysis for dart leaders shows weak correlation with the peak current of the prospective return stroke (R = 0.39 to average speed and R = 0.28 to final speed). This paper briefly discusses why peak current is better correlated with final speed than with the average speed, and why stepped leaders exhibit a significant correlation, while dart leaders do not. Plain Language Summary This study looks at how fast stepped leaders and dart leaders of lightning flashes propagate from the cloud base to ground, using high‐speed camera videos and data from a lightning location system. The results show that the final and average speed of the leaders are well correlated to the return stroke current, the return stroke current being more closely related to the final speed than the average speed. In contrast, dart leaders showed a weak correlation between their speed and the return stroke current. It also shows that most stepped leaders sped up as they got closer to the ground. Key Points The return stroke peak current is better correlated with final speed than with the average speed of the stepped leader No significant correlation was found between dart leaders speed and stroke peak current The stepped leaders increase their propagation speed near the ground

Lightning discharges between charged clouds and the Earth’s surface are responsible for considerable damages and casualties. It is therefore important to develop better protection methods in addition to the traditional Franklin rod. Here we present the first demonstration that laser-induced filaments—formed in the sky by short and intense laser pulses—can guide lightning discharges over considerable distances. We believe that this experimental breakthrough will lead to progress in lightning protection and lightning physics. An experimental campaign was conducted on the Säntis mountain in north-eastern Switzerland during the summer of 2021 with a high-repetition-rate terawatt laser. The guiding of an upward negative lightning leader over a distance of 50 m was recorded by two separate high-speed cameras. The guiding of negative lightning leaders by laser filaments was corroborated in three other instances by very-high-frequency interferometric measurements, and the number of X-ray bursts detected during guided lightning events greatly increased. Although this research field has been very active for more than 20 years, this is the first field-result that experimentally demonstrates lightning guided by lasers. This work paves the way for new atmospheric applications of ultrashort lasers and represents an important step forward in the development of a laser based lightning protection for airports, launchpads or large infrastructures.A terawatt laser filament is shown to be able to guide lightning over a distance of 50 m in field trials on the Säntis mountain in the Swiss Alps.

We examined three descending positive leaders at distances of 5–11 km and three descending negative leaders at distances of 6–7 km, all simultaneously imaged by high‐speed framing cameras operating in the visible and UV ranges. UV images (290–370 nm) of the positive leaders each exhibited a strong embellishment at the lower channel end, which was not observed in the corresponding visible images (480–800 nm). In contrast, none of the negative leaders exhibited channel embellishment in the UV range and their morphology in UV was similar to that in the visible. Additionally, no embellishment was seen in four negative leaders imaged in UV only. The observed UV embellishment, which is likely to be the streamer zone at the positive‐leader tip, appeared to undergo expansion‐contraction cycles. We attributed the lack of detectable streamer‐zone emission in the UV range in negative leaders to a much lower streamer generation rate compared to positive leaders. Plain Language Summary Lightning is usually imaged in the visible (400–800 nm) range. In this study, we compare, for the first time, ultraviolet (UV, 290–370 nm) images of positive and negative lightning leaders with the simultaneously recorded visible images. The key findings include the discovery of a pulsating streamer zone at the positive leader tip in UV, which was not detectable in the visible. We did not observe any streamer zone at the negative leader tip in either UV or visible ranges. We explained the observed disparity between positive and negative leaders in terms of different streamer generation rates and offered a hypothetical mechanism of the positive streamer zone pulsation. Key Points UV images of descending positive leaders exhibited a strong and pulsating embellishment at their lower end, not seen in the visible range In contrast, images of negative leaders in the UV range were similar to those in the visible range (no UV embellishment at the tip) The observed polarity asymmetry is likely to be related to very different streamer production rates at positive and negative leader tips

Recoil leaders develop in lightning flash decayed channels. The propagation of a recoil leader depends on the charges stored at its tip and the conductivity of the decayed channel. When the recoil leader propagates over the entire channel, a subsequent return stroke happens. Recoil leaders very often cease propagating before they reach the ground, that is, only part of the decayed channel is reionized. The present work aims to analyze the herein named secondary recoil leader that connect with the primary recoil leaders and cause them to start propagating again. We believe that the secondary recoil leader injects additional charge into the primary recoil leader, allowing the recoil leader reionize the whole decayed channel of the lightning flash. High‐speed videos analysis of upward lightning flashes shows that secondary recoil leaders play an important role on the formation and progression of dart leaders/subsequent return strokes. Plain Language Summary The recoil leader is a phenomenon that occurs in all types of lightning flashes (upward, downward and intracloud flashes). They arise in the remnants of decayed channels of positive leaders, partially or completely rebuilding these channels. The recoil leaders are responsible for some physical processes observed in lightning flashes. Thus, understanding how these physical processes originate is of significant importance. This work presents the role of secondary recoil leaders (recoil leaders that connect to preexisting recoil leaders) in the integral reconstruction of the decayed channels of the analyzed lightning flashes. Key Points Use of high‐speed cameras to study recoil leaders in upward lightning flashes Secondary recoil leaders boost the development of previous recoil leaders Secondary recoil leaders likely influence the development of dart leaders/subsequent return strokes

Positive leaders branch less frequently than negative counterpart, and the physical processes and properties of positive leader branching remain a mystery. We investigated 10 m laboratory discharges under four positive voltages using a high‐speed video camera. Positive leaders differ from negative leaders by either directly splitting or connecting with floating bidirectional leaders to form branching, and the number of leader branches shows a positive correlation with the applied voltage, that is, the branched channels increased from 1 to 4 when the voltage increased by a factor of 1.5. Grounding points are positioned beneath the electrode and are more concentrated with lower voltage. During the stable progression of the leader, there is a slight increase in its development speed as the applied voltage rises. When the voltage is increased by 70%, the average breakdown time decreases by 40%. These characteristics provide insights into the branching mechanism of positive leaders. Plain Language Summary The discharge mechanisms in natural and laboratory discharges are highly complex. This study aims to unravel the branching mechanism of positive leaders through laboratory experiments. A specific 10 m rod‐to‐plate discharge gap was designed to vary the maximum applied voltage, making it possible to examine the effects of applied voltage on the leader discharging trunk and branched channels. The statistical results indicate that as the peak voltage increases, the number of leader channels during the discharging process also increases. In natural lightning, positive leader channels generally exhibit low bifurcation rates, whereas in our study, the maximum number of discharging channels reached four with a sufficiently high applied voltage. The strike ground points of the trunk channel tend to be more dispersed when the applied voltage increases. Additionally, as the voltage rises, the average propagation speed of the leader trunk channel also increases. Our study presents the first systematic findings in this field, with crucial implications for a comprehensive understanding on the leader branching process and the development of discharge models. Key Points The positive correlation between the number of positive leader branches and applied voltage is demonstrated for the first time As the applied voltage increased, the strike ground points of the discharge channel became more dispersed The average 3‐D speed of the positive leader trunk channel exhibits a slight increase with the rising applied voltage

Positive lightning leaders present reactivation behaviors during their propagation processes. The mechanism underlying the reactivation of positive leaders is yet unclear. Using the synchronous measurement of optical observation, discharge voltage, and current, a novel phenomenon related to the reactivation of positive leaders is reported in laboratory. It is disclosed that separate luminous structures, which lead to positive leader steps, can form ahead of decayed leaders. The single positive leader branch can be reactivated by a separate luminous structure accompanied by a steep‐rising current pulse superimposed on zero background current. When positive leaders show decayed branches during their development, separate luminous structures can reactivate decayed branches and result in alternating and competitive development among branches. This process is accompanied by a sudden elongation in length. The reactivation scenario reported here in laboratory helps to interpret that in natural lightning.

Terrestrial gamma-ray flashes (TGFs) are transient gamma-ray emissions from thunderstorms, generated by electrons accelerated to relativistic energies in electric fields. Elves are ultraviolet and optical emissions excited in the lower ionosphere by electromagnetic waves radiated from lightning current pulses. We observed a TGF and an associated elve using the Atmosphere-Space Interactions Monitor on the International Space Station. The TGF occurred at the onset of a lightning current pulse that generated an elve, in the early stage of a lightning flash. Our measurements suggest that the current onset is fast and has a high amplitude—a prerequisite for elves—and that the TGF is generated in the electric fields associated with the lightning leader.

Based on the high‐speed video observations obtained at the Tall‐Object Lightning Observatory in Guangzhou (TOLOG), the attachment processes of three lightning flashes seem to belong to the connections between “the lateral surface of negative leader and positive leader tip” which had never been reported before were investigated. The leader connection processes of these flashes were examined in detail, and a mechanism behind connecting behavior between the lateral surface of negative leader and positive leader tip was discussed. Observation shows that negative leaders usually have abundant branches. Although some may extinguish, they still maintain a certain conductivity. When the positive leader tip comes close to the lateral surface of the negative leader (usually a few tens of meters), the previously disappeared short branch can be reactivated. It is easier for the positive leader tip to connect the tips of these branches (whether they have been extinguished or not). Plain Language Summary In the lightning attachment process, the leader connecting behavior is an interesting topic. In the attachment process of a negative cloud‐to‐ground lightning flash, the “Tip to Tip (negative leader tip to positive leader tip)” and “Tip to the lateral surface (negative leader tip to the lateral surface of positive leader)” connection types have been widely observed, and researchers have carried out a series of studies and discussions on the characteristics and the physical mechanisms of the leader connecting behavior. However, is there an opposite polarity connection type, like “the lateral surface and tip (the lateral surface of negative leader and positive leader tip)” connecting behavior? In this study, the attachment processes of three lightning flashes that seem to belong to the connections between “the lateral surface of negative leader and positive leader tip” were analyzed. The leader connection processes of these flashes were examined in detail. The results allow one to see the details of leader connecting behavior needed for improving our understanding of lightning attachment process mechanisms. Key Points For the connection between negative and positive leaders, no convincing “lateral surface and tip” type is observed Positive leader tip is easier to connect to the tip of negative leader branch, whether it has been extinguished or not A short gap of a few tens of meters is required for the reactivation of short leader branch on the lateral surface of the negative leader

The dissonant development of positive and negative lightning leaders is a central question in atmospheric electricity. It is also the likely root cause of other reported asymmetries between positive and negative lightning flashes, including the ones regarding: stroke multiplicity, recoil activity, leader velocities, and emission of energetic radiation. In an effort to contrast lightning leaders of different polarities, we highlight the staggering differences between two rocket‐triggered lightning flashes. The flash beginning with upward positive leaders exhibits an initial continuous current stage followed by multiple sequences of dart leaders and return strokes. On the other, in its opposite‐polarity counterpart, the upward development of negative leaders is by itself the entire flash. As a result, the flash with negative leaders is faster, briefer, transfers less charge to the ground, has lower currents, and smaller spatial extent. We conclude by presenting a discussion on the three fundamental leader propagation modes. Plain Language Summary Lightning flashes that carry positive and negative charges are completely different. In this article, we report on lightning triggered by launching a rocket tethered to the ground toward an electrified cloud. The staggering differences between positive and negative flashes are exposed by a three‐dimensional radio location system and by the current transferred to ground via the trailing wire. Key Points Triggered flashes with positive and negative leaders are contrastingly different with the latter being faster, briefer, and more compact The channel behind triggered positive leaders decays engendering dart leaders and return strokes, which is unparalleled in the negative case Average conductivity is higher in the negative leader channel despite the lack of return strokes and the lower charge transferred to ground

Lightning is a dangerous yet poorly understood natural phenomenon. Lightning forms a network of plasma channels propagating away from the initiation point with both positively and negatively charged ends—called positive and negative leaders 1 . Negative leaders propagate in discrete steps, emitting copious radio pulses in the 30–300-megahertz frequency band 2 – 8 that can be remotely sensed and imaged with high spatial and temporal resolution 9 – 11 . Positive leaders propagate more continuously and thus emit very little high-frequency radiation 12 . Radio emission from positive leaders has nevertheless been mapped 13 – 15 , and exhibits a pattern that is different from that of negative leaders 11 – 13 , 16 , 17 . Furthermore, it has been inferred that positive leaders can become transiently disconnected from negative leaders 9 , 12 , 16 , 18 – 20 , which may lead to current pulses that both reconnect positive leaders to negative leaders 11 , 16 , 17 , 20 – 22 and cause multiple cloud-to-ground lightning events 1 . The disconnection process is thought to be due to negative differential resistance 18 , but this does not explain why the disconnections form primarily on positive leaders 22 , or why the current in cloud-to-ground lightning never goes to zero 23 . Indeed, it is still not understood how positive leaders emit radio-frequency radiation or why they behave differently from negative leaders. Here we report three-dimensional radio interferometric observations of lightning over the Netherlands with unprecedented spatiotemporal resolution. We find small plasma structures—which we call ‘needles’—that are the dominant source of radio emission from the positive leaders. These structures appear to drain charge from the leader, and are probably the reason why positive leaders disconnect from negative ones, and why cloud-to-ground lightning connects to the ground multiple times. Radio interferometric observations of lightning over the Netherlands reveal small needle-shaped plasma structures associated with the positive leader channels, explaining why cloud-to-ground lightning connects to the ground multiple times.