Recoil Leaders Research Articles

Constraints on Recoil Leader Properties Estimated from X‐ray Emissions in Aircraft‐Triggered Discharges

AbstractDuring Airbus aircraft campaigns flying into thunderstorms in 2014–2016, X‐rays were observed during two stages of aircraft‐triggered lightning: nanosecond pulses of X‐rays associated with negative leader steps and bursts of X‐rays during recoil events. This work will focus on the observations of X‐ray bursts associated with recoils. Recoils are observed as microsecond‐fast changes in the local electric field, associated with large currents passing through the aircraft, and are found to sometimes be associated with bursts of X‐rays. From over 200 aircraft‐triggered lightning strikes, 54 recoil events were found to be associated with microsecond bursts of X‐rays. The majority of the bursts consist of 1–3 X‐ray pulses, with some bursts containing as many as 29 X‐ray pulses. We compare the observed superposed X‐ray spectrum with modeled spectra using a GEANT4 model of the detector and aircraft, to determine the source potential needed to accelerate the electrons that produce the observed X‐rays. A model of the recoil leader was made to determine the gap distance and gap potential between the recoil leader and the aircraft. From the modeling, we determine a solution space for the gap and leader lengths where the gap length is constrained by the observed minimum and maximum times between the onset of the X‐ray pulses and the onset of the current pulses detected at the aircraft (1 to 93 m). We also find two constraints from the fitting of the modeled spectra to the superposed spectrum, limiting the leader length to between 1 and ∼240 m.

Observation of the Whole Discharge Process During a Multi-stroke Triggered Lightning by Continuous Interferometer

The discharge process is often discussed in the lightning physics. VHF (very high frequency) observations play an important role in studying lightning discharge processes, because they show where air breakdown occurs. Triggered lightning flashes are ideal research objects, because they are generated in a fixed time and place. Regular experiments of triggering lightning have been conducted by Chinese Academy of Meteorological Sciences since 2006, and 189 lightning flashes are triggered successfully. Besides regular observations, such as fast/slow antenna, channel-based current and high-speed video, the continuous interferometer (CINTF) is developed and deployed at the site for triggered lightning after 2016 in order to observe detailed discharge processes. The CINTF can provide high-precision location with time resolution of around 1 μs.A multi-stroke triggered lightning with 8 return strokes is observed by a self-developed lightning continuous interferometer during Guangdong Comprehensive Observation Experiment on Lightning Discharge on 11 June 2019. The whole discharge processes, including precursor current pulse, initial precursor current pulse, initial continuous current and return strokes, last for about 1600 ms. The channel-based current, electric field change and radiation source positions are obtained. CINTF results show that the radiation signal generated from discrete precursor current pulse with a minimum value of about 8 A can be detected. The average transfer charge of initial precursor current pulse is twice as much as that of the precursor current pulse. During the initial continuous current stage after continuously developing of the upward positive leader, the obviously forward channel extension is the main discharge form at the beginning, and then the extension can be accompanied by discontinuously backward propagation, and recoil leader often occurs in the following stage. M-component in the ICC stage is caused by two discharge processes:Forward positive streamer (or leader) or recoil leader generated in the existing channel. Multi recoil leaders occur during the whole M process, which maintain M discharge. The main discharge form in return stroke stage is recoiling discharge. There are multi recoil leaders before return strokes, however, most of them cannot develop to ground, keep as an attempted leader until the last leader-stroke occurs. Adjacent two strokes with a short duration time of 4.5 ms have the same optical channel in optical images, but they are obviously different in development paths, which lead to a very short RS interval.

Characteristics and mechanism of a triggered lightning with two polarity reversals of charges transferred to ground

Characteristics of a triggered bipolar lightning flash obtained in the Shandong triggering lightning experiment (SHATLE) are analyzed based on simultaneous observation results of lightning very high frequency (VHF) interferometer map, channel-base current, fast electric field change and high-speed optical images. The flash lasts about 315 ms with only the initial continuous current (ICC). As the polarity of charges transferred to the ground varies from negative to positive and then to negative, the ICC can be mainly divided into three stages including the first negative ICC stage, positive ICC stage, and second negative ICC stage, respectively, for 152.755 ms, 87.225 ms and 75.02 ms. Charges transferred to the ground during the three stages are about –40.0 C, +13.3 C and –1.0 C, respectively, with the peak current of about –3.8 kA, 1.6 kA and –2.25 kA. According to the VHF interferometer map, during the first negative ICC stage, the upward positive leader (UPL) initiated from the top of the wire as negative charges starts to be transferred to the ground. The UPL develops at a two-dimensional (2D) speed of 3.7 × 10<sup>4</sup> m/s into the cloud and multiple positive branches develop in the form of small-scale and dense recoil leaders, keeping the increase of negative charge transferred to the ground. Then a negative leader (NL) initiates on a previously ionized positive branch channel and develops into the virgin air horizontally as a floating channel at a 2D propagation speed of 1.59 × 10<sup>5</sup> m/s. About 28.816 ms later after the NL develops, a negative pulse is detected in the fast electric field change, caused by a negative polarity breakdown discharge from the grounding trunk channel to the floating channel which is observed for the first time. Then about 39 μs later, the first polarity of the channel-base current changes from negative to positive, and rapidly reaches a positive peak in 1.75 ms. Subsequently, with the positive ICC decreasing, the negative leader gradually terminates about 65.85 ms after the first polarity reversal, and then 21.38 ms later the channel-base current slowly changes to the recognizable negative polarity as the second polarity reversal. A recoil leader generating in a previously ionized positive branch channel connects to the trunk channel, resulting in the initial continuous current pulse (ICCP) during the second negative ICC stage. Then several negative recoil leaders occur, tracing back to the previous positive channels without obvious current changes until the flash ends. Based on the analysis, the positive branch channel persistently transfers negative charge to the ground in the whole discharge process by the positive breakdown on the tip or the negative recoil leaders retrograding along the previous positive channels. The trunk channel is connected to the floating channel through negative breakdown discharges, linking at the positive charge accumulation area at the tail end of the negative leader or the positive polar end of a bidirectional leader towards the trunk channel, if the negative leader develops as a bidirectional leader later. Then, the net charge transferred to the ground is dominantly positive and the reversal of first channel current polarity occurs. With the negative leader disappearing, the supply of positive charge ceases, so the current polarity reverses again (the second reversal). Connection of the negative leader to the grounding trunk channel and continuous development of the positive leader are inferred to play an important role in reversing the two current polarities. In this case, the negative leader developing in virgin air might be initiated transversely in an ionized positive channel or from the end of a decayed positive leader branch of the ionized positive channel which is small and undistinguishable from the VHF interferometer map.

Needles and Lightning Leader Dynamics Imaged with 100–200 MHz Broadband VHF Interferometry

AbstractA new short‐baseline VHF interferometer with 200 MHz bandwidth images lightning with significantly higher spatial resolution than our previous system with a 60 MHz maximum frequency. Needles (Hare et al., 2019, https://doi.org/10.1038/s41586‐019‐1086‐6), needle‐associated negative leaders, a nearby bidirectional leader, and their interactions were imaged during an intracloud lightning flash. Six recoil leaders traversing the positive leader channel definitively determined the leader path and verified that needles originate from positive channels. Two negative leaders initiated from the positive channel, either from or close to needles, when another active positive leader passed ~1 km away. While the needles develop continuously with a VHF source density decreasing from the front end to the back end, a bidirectional leader initiated nearby and had strong influence on subsequent needle dynamics. These rarely observed yet perhaps common lightning processes are analyzed comprehensively to give new insight into the origin and impact of needles on positive lightning leaders.

Optical characteristics of branched downward positive leader associated with recoil leader activity

The propagation characteristics of one downward positive leader with mainly four forked branches in one natural positive lightning have been investigated based on high-speed video observations. One branch develops horizontally; one branch travels downward eventually reaching the ground, and two faint branches are reilluminated by recoil leader (RL) activity multiple times. Four branches share some interesting correlation during their development. At the early phase, the horizontal branch and descending branch are inactive or propagate slowly when RL activity is more active. After the appearance of RL activity each time, the horizontal branch propagates slightly faster. After that, the two luminous branches propagate more actively and faster, while RL process becomes diminished and suppressed. Both downward and horizontal branches speed up as the downward one is approaching the ground. The two-dimensional (2-D) propagation speed for the downward branch ranges from 4.25 × 104 m/s to 3.85 × 105 m/s, with an average value of 1.01 × 105 m/s, and it ranges from 2.69 × 104 m/s to 3.61 × 105 m/s with an average speed of about 1.22 × 105 m/s for the horizontal one.

High‐Speed Video Observations of Recoil Leaders Producing and Not Producing Return Strokes in a Canton‐Tower Upward Flash

AbstractHigh‐speed video and electric field change data have been used to examine the initiation and propagation of 21 recoil leaders, 7 of which evolved into dart (or dart‐stepped) leaders (DLs) initiating return strokes and 14 were attempted leaders (ALs), in a Canton‐Tower upward flash. Three DLs and two ALs clearly exhibited bidirectional extension. Each DL was preceded by one or more ALs and initiated near the extremity of the positive end of the preceding AL. The positive end of each bidirectional DL generally appeared to be inactive (stationary) or intermittently propagated along the positive part of the preceding AL channel and extended into the virgin air. A sequence of two floating channel segments was formed ahead of the approaching positive end of one DL, causing its abrupt elongation.

Development of Side Bidirectional Leader and Its Effect on Channel Branching of the Progressing Positive Leader of Lightning

AbstractFine time‐resolved images of the development of lightning bidirectional leaders below the cloud base were captured by a high‐speed camera in association with multiband electromagnetic field measurements. The bileaders tended to originate at a radial distance of about 200 m from a progressing positive leader channel, propagating more or less radially toward and away from the main channel. Significant asymmetrical channel extensions at opposite ends were clearly recognized. The positive ends of the bileaders propagated in single paths with average speed of 6.1 × 104 m/s, while the opposite negative end involved obvious branching and stepping propagation with average speed of 1.6 × 105 m/s. The bidirectional leaders eventually connected to the existing main positive leader, producing new branches of the flash. Another cause of positive leader branching was that the positive leader split at its head and developed into different branches synchronously, which were sometimes invisible but were revealed by recoil leaders.

Propagation of positive, negative, and recoil leaders in upward lightning flashes

Leader propagation is a fundamental issue in lightning physics. The propagation characteristics of positive leaders and negative leaders are summarized and compared based on data from high-speed camera and electromagnetic field in rocket-triggered lightning and tower-initiated lightning discharges; available channel base current data recorded in rocket-triggered lightning are also used. The negative leaders propagate in a stepped fashion accompanied by many branches. The stems ahead of the negative leader tip determine the manner and direction of the leader propagation, and even the branching and winding of the lightning channel. The impulsive current, electromagnetic field, and related optical images suggest that the positive leader may develop in a step-like fashion at its initial stage of triggered lightning. However, the stepping processes of the positive leader are obviously different from those of the negative leader. Tower-initiated lightning revealed that the most conspicuous characteristics of the stepwise positive leader involve the intermittent brush-like corona zone in front of the leader tip and the luminosity enhancement of the channel behind the tip. In rocket-triggered lightning flashes, the charge transferred during an individual step for the negative leader was nearly an order greater than for the positive counterpart. The successive streamers ahead of the leader tip are essential for both negative and positive leader propagation, and the stems could be formed from one or more streamers in the previous negative streamer zone with the main leader channel dim. High-resolution observation of tower lightning also revealed a new type of bidirectional recoil leader, with polarity contrary to the traditional one, traversing in negative channels associated with tower-initiated and rocket-triggered lightning.

Theroy and observation of bidirectional leader of lightning: Polarity asymmetry, instability, and intermittency

A lightning discharge in a thundercloud usually starts with a locally breakdown process (preliminary breakdown) followed by a widely extending leader process. In the early 1960 s, from the view of fundamental electrostatics[Kasemir H W 1960 J. Geophys. Res. 65 1873] suggested that the lightning leader is initiated by an electrodeless discharge with a zero-net-charge conductive channel extending bidirectionally in the ambient electric field of the thundercloud, i.e.the bidirectional leader theory. However, the bidirectional leader theory has just been recognized by lightning researchers since the late 1980 s, when airplane[Mazur V 1989 J. Geophys. Res. 94 3326] and rocket-triggered lightning experiments have proven that the bidirectional leader theory provides the best common physical basis for explaining a variety of lightning processes. Nevertheless, challenges still remain in other properties of the bidirectional leader theory, such as the polarity asymmetry, the sustainability, instability and restrike of a leader channel, which are all the key concerns of lightning researches. In the present paper, we first briefly review the concept and development of the bidirectional leader theory and its appearances in various lightning processes, especially in the negative stepped leader and recoil leader process. By reviewing a variety of field observation data of inception thresholds and propagation properties of long gap spark discharges, we put forward and emphasize an alternative polarity asymmetry of the lightning leader:the polarity asymmetry of continuity, i.e., the continuity of positive leader and the intermittency of negative leader. Based on the detailed discussion, we then argue that the stepwise positive leader may be restrikes due to temporary instability of a continuous positive leader, while the intermittency of negative leader manifests not only a relatively regular stepped leader but also a more erratic negative recoil leader. We further propose that the positive leader-streamer may play a unique role in igniting all kinds of intermittent events in lightning, including the initiation of lightning with the fast positive streamer, the formation of space leader in front of a negative stepped leader with the secondary positive streamer, and the reformation of recoil-leader in an instability leader channel due to its low threshold of inception and propagation. Finally, we discuss the self-adjustable ability of a bidirectional leader, suggesting that the potential drop along the streamer channel in front of the leader tip may be an important factor for stabilizing the bidirectional leader channel.

On the nature of bipolar flashes that share the same channel to ground

The paper examines the relationship between sequential events in bipolar flashes of two groups sharing the same channel to ground: one initiated by an upward positive leader, the other by a natural flash. Two quite different mechanisms are involved in the occurrence of the bipolar flashes of each group: The bipolar events that start as a natural flash are results of recoil leaders sharing parts of the initial branching structure; the bipolar events that start as upward positive leaders are the result of the interaction between these leaders' initial branching structures and the branches of the intracloud flashes they triggered.

Bidirectional leader development in a preexisting channel as observed in rocket‐triggered lightning flashes

AbstractTwo cases of bidirectional leader, starting almost immediately below a decaying dart or dart‐stepped leader which terminated before reaching the ground, were identified, for the first time, in rocket‐triggered lightning flashes based on high‐speed video, electric field changes, and channel base current, indicating that a bidirectional leader can be excited by a decayed dart leader and propagate in a preexisting discharge channel. The positive end of the bidirectional leader moved upward along the terminated downward dart leader path, and the negative end extended downward along the decayed initial continuous current or preceding stroke path to the ground and culminated in a return stroke. The positive leader propagated with an average speed of 1.3 × 106 m/s and 2.2 × 106 m/s in the two cases, roughly twice as fast as its negative counterpart with speed of 7.8 × 105 m/s and 1.0 × 106 m/s, respectively. The positive end started earlier than the negative end with a preceding time less than 40.3 µs. The bidirectional leader can be regarded as a recoil leader with the positive end retrogressing along a negative leader channel, whose polarity is contrary to the traditional recoil leader with negative leader end retrogressing along an existing positive leader channel. Weak luminosity was continuously observed during the terminated dart leader and bidirectional leader stage, but the channel current was less than the minimum detection capability of 9.3 A.

Upward lightning flashes characteristics from high‐speed videos

AbstractOne hundred high‐speed video recordings (72 cases in Brazil and 28 cases in USA) of negative upward lightning flashes were analyzed. All upward flashes were triggered by another discharge, most of them positive CG flashes. A negative leader passing over the tower(s) was frequently seen in the high‐speed video recordings before the initiation of the upward leader. One triggering component can sometimes initiate upward leader in several towers. Characteristics of leader branching, ICC pulses, recoil leader incidence, and interpulse interval are presented in this work. A comparison of the results is done for data obtained in Brazil and USA. The duration of ICC and the total flash duration are on average longer in Brazil than in USA. Only one fourth of all upward leaders are followed by any return strokes both in Brazil and USA, and the average number of return strokes following each upward leader is very low. The presence and duration of CC following return strokes in Brazil is more than two times larger than in USA. Several parameters of upward flashes were compared with similar ones from cloud‐to‐ground flashes.

The physical concept of recoil leader formation

The concept of recoil leader formation is based upon their nature as bidirectional and bipolar leaders. The analogy of the unipolar leaders to free-burning arcs was applied to interpret the processes during current cutoff in individual leader branches. These processes are essential to locating the origin and the timing of recoil leader formation. The difference in the density of the branching structures of positively- and negatively charged leaders is shown as the decisive factor affecting the occurrence of recoil leaders in positively charged channels, absence of them in negatively charged channels, and the lack of multiple return strokes in positive cloud-to-ground flashes.

Characteristics of a bipolar cloud-to-ground lightning flash containing a positive stroke followed by three negative strokes

Using time-correlated high-speed video images at 3200 frames per second, broadband electric field change data and low-frequency magnetic fields, a natural bipolar cloud-to-ground (CG) lightning flash with one first positive stroke followed by three subsequent negative strokes is analyzed. All of these four strokes transferred electric charge to the ground through the same lower channel with a time interval of 328ms between the positive stroke and the first negative stroke. The flash onset was followed by several positive leaders that extended below the cloud base, one of which descended to culminate in a positive stroke with a continuing current. Another positive leader extended horizontally to a distant negative cloud region and induced several recoil leaders that intermittently retrograded along the leader channel. Eventually, three recoil leaders successively traversed along the path of positive stroke to produce respective negative strokes, resulting in the polarity reversal of charge transferred to the ground. The average two-dimensional (2-D) speed of the positive leader was 1.1×105m/s, while for the 3 negative leaders was 6.7×106m/s. The zero-crossing time and rise time of the radiation field waveform for the 3 negative strokes are smaller than the typical negative subsequent strokes, making them hard to be recognized as return strokes by the CG lightning location network.

Concurrent Upward Lightning Flashes from Two Towers

Upward lightning flashes initiated simultane- ously from two towers separated by a distance of 3420 m were analyzed in detail based on high-speed camera images and S-band Doppler radar echo intensity. Both discharges lasted more than 250 ms and were self-initiated from the towers in the form of upward positive leaders with a time difference of less than 4 ms. Abundant recoil leaders oc- curred transiently in the remnant channel sections during the development of the upward lightning. The number of recoil leaders over the lower tower was greater than over the higher tower. When the concurrent upward flashes occurred, the radar echo intensity in the area of the towers was no more than 45 dBZ and the towers were separately located beneath two echo centers with low altitudes of 2-3 km. Keywords: tower, upward lightning, leader, discharge Citation: Jiang, R.-B., Z.-L. Sun, and Z.-J. Wu, 2014: Concurrent upward lightning flashes from two towers, Atmos. Oceanic Sci. Lett., 7, 260-264, doi:10.3878/j.issn.

Initial results of LF sensor network for lightning observation and characteristics of lightning emission in LF band

AbstractWe have been developing an LF sensor network called Broadband Observation network for Lightning and Thunderstorm (BOLT), to image the structure of lightning discharges in 3D using time of arrival (TOA) technique. This paper documents initial results and characteristics of BOLT source locations for further understanding of LF radiation associated with lightning. Theoretical reduced Chi‐square distribution fitted to BOLT observation data indicates a root mean square (rms) timing error of about 200 ns for each sensor. Monte Carlo simulations of BOLT indicate that at an altitude of 5000 m the standard deviations for horizontal differences between a known source and a location that the BOLT algorithm produces are less than 200 m, and vertical differences are less than 400 m in most of the network. Furthermore, comparison of BOLT and VHF source locations arriving at the same site within 5 µs indicates that the average difference for elevation direction is 0.73° with a standard deviation of 3.64°, and that for the azimuth direction is 0.58° with a standard deviation of 1.98°. Lightning flash processes of intracloud (IC) and cloud‐to‐ground (CG) flashes, including preliminary breakdown pulses, negative leaders, breakdown in the negative charge region (NCR), and an attempted leader, are well imaged by BOLT. Normalized amplitudes in E‐field change waveform of BOLT sources associated with negative leaders and breakdown occurred in the NCR do not have significant difference, implying that most BOLT sources in the NCR might be associated with negative recoil leaders.

On β2 stepped leaders in negative cloud‐to‐ground lightning

AbstractIn their seminal lightning studies using streak cameras, Schonland et al. (1938) identified four negative stepped leader events that they term “β2,” a “rather rare variant of the type β leader”, and in it, “the second and slower stage of the leader is associated with the appearance of one or more fast dart streamers, which travel rapidly down from the cloud along the previously formed track and cease when they have caught up with the slower leader tip.” Seven negative downward leaders that agreed with the description given by Schonland et al. for type β2 were recorded in Tucson, Arizona, USA, and in São José dos Campos, São Paulo, Brazil. All cases were recorded by a high‐speed camera operating at 4000 frames per second, and electric field changes were measured for three of them. Their “dart streamers” had speeds between 106 and 107 m s−1, compatible with previous observations of recoil leaders (RLs). Also, during the development of the three cases with correlated electric field changes, it was possible to identify sequences of microsecond‐scale pulses preceding the propagation of a dart streamer in the channel. It is proposed that the luminous process that occurs during the development of a type β2 stepped leader is the visible manifestation of one or more RLs that begin inside the cloud and connect to the in‐cloud, positive portion of the bipolar, bidirectional leader, and then travel downward to the lower end of the negative stepped leader path.

Data processing procedure using distribution of slopes of phase differences for broadband VHF interferometer

AbstractThe upgraded VHF digital interferometer (VHF DITF) system is introduced which can continuously sample the radiation associated with lightning. A new processing technique was implemented which uses the distribution of slopes of the phase difference versus frequency to locate the radiation source. By using this technique, frequency components which are not due to lightning can be excluded and low as well as high amplitude sources are located. As a result, both positive breakdown and negative breakdown are located, and negative recoil leaders (recoil leaders) are visualized in great detail. The recoil leaders which continue into the positive charge region are seen to slow their propagation and dim their radiation as they cross the flash initiation region. Analysis of the relative received power of the different breakdown types, negative leaders, recoil leaders, and positive leaders, also can be made. In both the intracloud and cloud‐to‐ground flash, the modes of the distributions of received power for negative leaders, recoil leaders, and positive leaders were approximately the same. The brightest emissions seen from the positive leader were substantially lower than the brightest emission seen from the negative leader. The results also indicate that positive leaders as well as lower elevation negative leader emit more low frequency radiation than recoil leaders and high‐elevation negative leaders. By continuously sampling the VHF waveform, the upgraded VHF DITF locates many weak sources which the previous system was not capable of locating.

High‐speed video and electromagnetic analysis of two natural bipolar cloud‐to‐ground lightning flashes

AbstractHigh‐speed video records of two bipolar cloud‐to‐ground flashes were analyzed in detail. They both began with a single positive return stroke that was followed by more than one subsequent weak negative stroke. Due to the elevated cloud base height of its parent thunderstorm, the preparatory processes of each subsequent negative stroke were documented optically below cloud base. In the first event (Case 1) it was observed that all four subsequent negative strokes were initiated by recoil leaders that retraced one horizontal channel segment previously ionized by the positive leader. Those recoil leaders connected to the original vertical channel segment and propagated toward ground, producing four subsequent strokes that had the same ground contact point as the original positive discharge. The second event (Case 2), in contrast, presented 15 subsequent strokes that were initiated by recoil leaders that did not reach the original channel of the positive stroke. They diverged vertically toward ground, making contact approximately 11 km away from the original positive strike point. These results constitute the first optical evidence that both single‐ and multiple‐channel bipolar flashes occur as a consequence of recoil leader activity in the branches of the initial positive return stroke. For both events their total channel length increased continuously at a rate of the order of 104 m s−1, comparable to speeds reported for typical positive leaders.

Performance characteristics of the NLDN for return strokes and pulses superimposed on steady currents, based on rocket‐triggered lightning data acquired in Florida in 2004–2012

AbstractWe present a detailed evaluation of performance characteristics of the U.S. National Lightning Detection Network (NLDN) using, as ground truth, Florida rocket‐triggered lightning data acquired in 2004–2012. The overall data set includes 78 flashes containing both the initial stage and leader/return‐stroke sequences and 2 flashes composed of the initial stage only. In these 80 flashes, there are a total of 326 return strokes (directly measured channel‐base currents are available for 290 of them) and 173 kiloampere‐scale (≥1 kA) superimposed pulses, including 58 initial continuous current pulses and 115 M components. All these events transported negative charge to the ground. The NLDN detected 245 return strokes and 9 superimposed pulses. The resultant NLDN flash detection efficiency is 94%, return‐stroke detection efficiency is 75%, and detection efficiency for superimposed pulses is 5% for peak currents ≥1 kA and 32% for peak currents ≥5 kA. For return strokes, the median location error is 334 m and the median value of absolute peak current estimation error is 14%. The percentage of misclassified events is 4%, all of them being return strokes. The median value of absolute event‐time mismatch (the difference in times at which the event is reported to occur by the NLDN and recorded at the lightning triggering facility) for return strokes is 2.8 µs. For two out of the nine superimposed pulses detected by the NLDN, we found optical evidence of a reilluminated branch (recoil leader) coming in contact with the existing grounded channel at an altitude of a few hundred meters above ground.