Recoil Leaders Research Articles

Observational Evidence of Negative Leader Reactivation Processes Following a Negative Return Stroke in Lightning Discharges

AbstractHigh‐time‐resolved mapping results of two rare reactivation processes following a negative return stroke are discovered and analyzed. At first, the discharges prior to the reactivation process were dominated by positive discharges lasting for tens of milliseconds in a limited space in the vicinity of a decayed negative leader channel. The positive discharges produced no detectable electric field change. Then, negative discharges started and propagated along the decayed downward negative leader channels at a speed exceeding 106 m/s for a few microseconds and produced negative electric field changes. The analysis reveals that the processes are physically distinct from recoil leaders and side discharges in terms of propagation behaviors, electromagnetic characteristics, and time scale. The possible mechanisms of the processes are discussed. The observation suggests that the reactivation processes of the negative leaders may lead to subsequent return strokes.

Observation of tropical positive cloud-to-ground flashes accompanied by chaotic and regular pulse trains

This study investigates the occurrence of chaotic pulse trains (CPTs) and regular pulse trains (RPTs) in tropical positive cloud-to-ground (CG) lightning flashes. These flashes are categorized into four types based on the initial polarity of the initial breakdown (IB) pulses and their relationship to the first return stroke (RS). A total of 71 positive CG flashes from five thunderstorm events were analyzed. The analysis reveals instances of CPTs and RPTs both before and after the first positive RS, along with the occurrence of mixed polarities in RPTs. Variations in IB pulse polarities and the presence of CPTs and RPTs before the first positive RS were observed, contrasting with previous findings in negative CG flashes. All positive CG flashes have been detected when cloud top height occurrences were between 12 and 18 km. In contrast, for negative CG flashes with CPTs and RPTs the cloud top height occurrences were between 5 and 12 km. It is interesting that CPTs and RPTs can be detected during IB process of positive CG flashes at relatively high altitude in the thundercloud. Perhaps due to low pressure at higher altitudes in the cloud, electrical process associated with CPTs and RPTs are easily discharged before the occurrence first positive return stroke. The altitudes of cloud top heights for the inverse polarity of IB pulses were located between 16 and 18 km. This research enhances the understanding of positive CG lightning initiation process and their relationship with CPTs and RPTs, as well as the occurrence of recoil leaders.

Ground‐Based Observations of 337 nm and 777 nm Optical Emissions Produced by Lightning

AbstractLightning flashes were observed in Barrancabermeja (Colombia) by two photometers centered at 337 and 777 nm wavelengths, a high‐speed camera, and the Colombia Lightning Mapping Array. Both optical and VHF radio emissions of different lightning processes are investigated. Flash initiations by upward in‐cloud breakdown present a first short optical pulse followed by a period of ∼1 kHz breakdown propagation. The ratio remains at unity or below, associated with the transition to negative horizontal leader propagation. In flash initiations by a fast‐stepped negative downward leader to ground, a dominance of the 337 nm radiance is found. Observations of downward leaders before ground strokes reveal that the luminosity measured by the photometers is dominated most of the time by the light coming from the cloud and not from the visible leader channels. One case of a visible cloud‐to‐ground channel with positive continuing current shows a high similarity in the evolution of the luminosity along the channel, 777 nm radiance, and the energy detected by the Geostationary Lightning Mapper from space. Finally, two types of recoil leaders have been observed: connecting and non‐connecting. Whereas the measured 777 nm peak radiance is similar in both types of recoil leaders, connecting recoil leaders present about seven times higher 337 nm peak radiance. Our observations generally show agreement between the VHF power and optical emissions.

Upgraded Low-Frequency 3D Lightning Mapping System in North China and Observations on Lightning Initiation Processes

The three-dimensional (3D) low-frequency lightning mapping system (LF-LMS) in north China has been updated. The lightning electric field derivative (dE/dt) sensor and continuous acquisition mode has been newly designed to ensure a capability of entire lightning processes detection, especially weak discharges during lightning the initiation process. The twice cross-correlation delay estimation and the grid iteration nested optimization location algorithm are used to realize the 3D location of the discharge channel, and the location resolution and calculation speed are balanced consequently. The location results of the rocket-triggered lightning demonstrated that the system achieved a high-resolution mapping of lightning discharge channels, which coincided well with the optical images. The horizontal and vertical location error for rocket triggered lightning was less than 40 m in both horizontal and vertical. Intracloud (IC) lightning flashes were observed to be initiated by three different discharge processes, initial breakdown pulse (IBP), narrow bipolar event (NBE), and initial E-change (IEC). The corresponding initial height was 10.5 km, 6.9 km, and 9.2 km, respectively. The upward negative leader was initially located, followed by scatter radiation sources and negative recoil leaders in the lower negative charge region for all cases. The electric field characteristics of the IEC and subsequent IBPs indicated that they are different discharge processes with the same current direction. The IEC process might correspond to the discharge process with continuous current and less noticeable current changes.

Correlated Lightning Electric Field and High-Speed Video Observations of Recoil Leaders Recorded in Rzeszow, Poland

Correlated Lightning Electric Field and High-Speed Video Observations of Recoil Leaders Recorded in Rzeszow, Poland

Lightning Discharge as a Self-Organizing Transport Network. Part 1. The Concept of an Asymmetric Discharge Tree

Lightning is a self-developing transport system of plasma channels demonstrating the ability to self-regulation due to self-consistent maintenance of zero total charge of the entire branched discharge network. The lightning structural evolution is accompanied by significant morphological changes: new plasma channels appear, and some of old ones disappear. The accomplished work was in general aimed at studying intracloud lightning and cloud-to-ground discharges up to the moment the latter come in contact with the ground. It has been shown, as part of the study, that the ability of lightning, as an open system, to maintain its structural “homeostasis” is closely linked with macroscale asymmetry: the lightning morphological and transport properties are caused by loss of electric discharge tree structural symmetry when the discharge polarity changes for the opposite. Indeed, the asymmetry of electric field threshold strength levels needed to support the growth of positive and negative streamers leads to pronounced macroscale effects in the physics of lightning. Ground discharges of negative polarity consist most often of a series of strokes passing through the same channel, while positive flashes are as a rule limited to a single stroke. Negative and positive lightning leaders have significant morphological differences: the growth of positive leaders is accompanied by development of negative recoil leaders, whereas no one has observed positive recoil leaders, if they exist at all. The results of the study are presented in two parts. This article contains an introductory part that sets out the narration general context, and, by analogy with the hierarchical Horton-Strahler scheme used for river systems, analyzes the asymmetry of the spatial distribution of capacitance in different parts of the lightning discharge tree. The concept of a point of zero induced charge or a reversing point is discussed.

Characteristics of recoil leaders as observed by LOFAR

Lightning dart leaders and recoil leaders are current pulses that propagate along previously established lightning channels reionizing the channel. They are poorly understood since it is not known how a previously ionized channel can undergo dielectric breakdown multiple times. In this work we investigate five recoil leaders on two different channels using lightning observations of the LOFAR radio telescope in vhf (where we observe 30--80 MHz) using our TRI-D interferometric imaging algorithm. We show that while recoil leaders do slow down over time on average, they can also clearly accelerate. In addition, the vhf power emitted by the recoil leader is closely correlated with speed. We also investigate the vhf-emitting width of the recoil leaders and show that it is thinner than our meter-scale resolution. This shows that recoil leaders have significant streamer activity in their channel core or at most very inner region of the corona sheath, and that there is either very little, or weakly vhf-emitting, streamer activity in the majority of the corona sheath. Finally, we show that recoil leaders can have small microsecond-scale fluctuations in emitted vhf power that can occur in the same spot across subsequent recoil leaders on the same channel, demonstrating that recoil leader propagation is affected by small-scale channel geometry.

Transient phenomena in positive cloud-to-ground lightning discharges

Positive lightning discharges to ground (+CGs) are relatively rare and considerably less studied than negative ones (-CGs). We present observations of unusual transient phenomena occurring in +CGs and discuss their mechanisms. One of them is a brief electric coupling to a concurrent -CG initiated from a 257-m tall tower located 11 km from the +CG channel. A transient process (stroke) in the -CG flash appears to cause a transient luminosity enhancement (M-component) in the +CG channel. In the course of these essentially simultaneous transients, positive charge is in effect taken from the ground at the position of the tower and injected into the ground at the position of the +CG channel. Recoil leaders reactivating decayed +CG branches near the cloud base are each observed to cause a transient luminosity decrease (dip), as opposed to the expected luminosity increase, in the +CG main channel.

Upward Bipolar Lightning Flashes Originated From the Connection of Recoil Leaders With Intracloud Lightning

AbstractThe present work shows high‐speed videos of two upward flashes that started with positive upward leaders and, instead of being followed by negative subsequent return strokes, they were followed by positive subsequent return strokes. In both cases, after the positive leaders developed, recoil leaders (RL) appeared in their decayed branches as would be usual in negative upward lightning flashes. However, in these flashes the negative end of a recoil leader connected to a positive leader of an intracloud (IC) flash nearby. The connection initiated a downward positive leader that re‐ionized the decayed channel of the upward flash all the way to the tower giving origin to a positive subsequent return stroke. This work shows that RL do play an important role in the occurrence of bipolar upward flashes and their interaction with IC flashes can provide explanations for all types of bipolar upward flashes initiated by upward positive leaders.

Simultaneous Upward Lightning From Small Structures and From the 150 m High Peissenberg Tower, Germany

In this study, we analyze a lightning event, which occurred on March 20, 2020 at 8:05 pm. Three other-triggered upward lightning developed simultaneously from the 150 m high Peissenberg Tower and from two small structures, a 35 m high mast and a 25 m high Church Tower. The velocity of the upward leaders ranged from 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> to 4.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> m/s. In addition, to these upward leaders, the total of 51 isolated leaders were observed. These leaders had no contact to the lightning channel or to other leaders or to objects at ground, i.e., most of them did not develop along a path of the decayed channel of a previous leader. Therefore, it is doubtful whether the isolated leaders may be recoil leaders. The lightning to the Peissenberg Tower was a bipolar flash with a current, which contained several (unusual) positive and negative components. The first component was a negative initial continuous current (ICC), followed by a strong negative current component, with a maximum of 4.1 kA and a duration of 40.2 ms. This current component was superimposed by a pulse train of 26 extremely strong current pulses with peak values up to 1.87 kA. The negative current component was followed by a positive current pulse with a peak value of 31 kA, a 10%–90% risetime of 1.80 ms, an action integral of 0.68 MJ/Ω, a full width on half maximum of 0.67 ms and a duration of 15.7 ms. The positive current pulse was superimposed by strong oscillations with the frequency of about 5 kHz and with amplitudes up to 10.2 kA. After its cessation, the positive current pulse turned into a negative continuing current with the duration of 217 ms, the maximum of 245 A, and the transferred charge of 18.0 C. The negative continuing current was followed by a negative return stroke which in turn was followed by a positive continuing current, which lasted 19.7 ms and had a maximum of 173 A. Finally, a second negative return stroke occurred, which was followed by a short negative continuing current with an M-component.

Multi‐Stroke Positive Cloud‐To‐Ground Lightning Sharing the Same Channel Observed With a VHF Broadband Interferometer

AbstractThis work presents the first observation of a multi‐stroke positive cloud‐to‐ground lightning flash sharing the same channel to ground mapped with a very high frequency broadband interferometer and a Lightning Mapping Array. This type of lightning flash is very rarely observed, and it is currently unclear how frequent it is and even under what conditions it occurs. Our observations indicate a scenario where the first downward positive leader initiates from a decayed negative channel. After the first return stroke, some of the main negative channel branches stop propagating and are likely cut off. A fast recoil leader and/or a fast breakdown play a crucial role in reconnecting these previously decayed leader channels and initiating the subsequent positive stroke. The mechanism we propose to describe the phenomenon allows us to explain its rarity and the discrete positive charge transfer to the ground.

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

AbstractA unique intracloud (IC) lightning flash with multiple positive leader branches spreading out from the same point is studied using simultaneous high‐speed images, electromagnetic (EM) field changes, and radiation source mapping. A channel of the positive leader protruded from the cloud and induced a bidirectional luminous segment at tens of meters ahead of the leader tip. They led to a small‐scale attachment where multiple channels successively emerged and propagated toward diverse directions. The propagation mode of the positive leader experienced a noticeable transition from stable and smooth channel extension to active development accompanied with frequent recoil leaders, attribute to the arrival of an impulsive current wave induced by the in‐cloud activity. We find solid evidence that long recoil leader activated the negative‐end breakdown with a regular EM pulse train. The back‐and‐forth interaction from the opposite ends of the associated channel is revealed.

Recoil Leader and Associated Discharge Features Observed During the Progression of a Multi‐Branched Upward Lightning Flash

AbstractUsing a high‐speed video camera and a high‐speed optical imaging system, we observed a multi‐branched upward lightning flash that exhibited several special discharge features. One special feature is that a series of recoil leaders and the subsequent discharges caused by the connection of some recoil leaders to a conducting channel could extend a discharge channel like a leader. Another special feature is that the subsequent discharge could be similar to either a return stroke (RS) or an M‐component or something between. It seems that when a recoil leader initiated in a more conductive channel with a closer distance to a conducting channel, its subsequent discharge is similar to an M‐component. In cases with opposite conditions, the subsequent discharge will become similar to a RS. In conditions between the above two cases, a discharge that is intermediate between the RS and M‐component will occur.

Activation of abundant recoil leaders and their promotion effect on the negative-end breakdown in an intracloud lightning flash

Activation of abundant recoil leaders and their promotion effect on the negative-end breakdown in an intracloud lightning flash

Activation of abundant recoil leaders and their promotion effect on the negative-end breakdown in an intracloud lightning flash

Activation of abundant recoil leaders and their promotion effect on the negative-end breakdown in an intracloud lightning flash

Bidirectional Recoil Leaders in Upward Lightning Flashes Observed at the Säntis Tower

AbstractWe report the observation of three negative upward flashes recorded by a high‐speed camera at the Säntis Tower during the Summer of 2019. The camera was operating at 10,000 fps and an exposure time of 99 μs. Simultaneous measurements of the lightning current were obtained for all three flashes. The close electric field was measured for one of the observed flashes, which was analyzed in detail. In this flash, we observed 50 recoil leaders developing in the decayed channel with speeds characteristic of dart leader processes. Out of the 50 recoil leaders, 45 ended up as attempted leaders, three developed into M‐component‐type ICC (initial continuous current) processes, and two into return strokes. This study reveals that depending on the spatial and temporal properties of electric field in the area of the event and the main channel condition, recoil leaders can develop into different processes in upward negative lightning flashes such as attempted leaders, M‐components, M‐component‐type ICC events, mixed mode pulses, or return strokes. We observed M‐component‐type ICC events with junction heights as low as 137 m, while all of the observed mixed mode pulses seemed to attach directly to the structure. Bidirectional propagation of the recoil leader was also observed.

Side Discharges From the Active Negative Leaders in a Positive Cloud‐To‐Ground Lightning Flash

AbstractWe found two kinds of side breakdowns from the active horizontal negative channel in a positive cloud‐to‐ground flash. During the propagation of the negative leader in the field‐of‐view, some decayed negative branches were reactivated, and most of them promoted the propagation of advancing negative leaders. The reactivations extended outward from the edge of the negative leader channel with an average speed of 2.2 × 106 m s−1, but they produced no distinct VLF‐VHF radiations, as confirmed by the lightning mapping results. The reactivations of decayed negative branches were different from recoil leaders in the decayed negative leader because we deduce the outward breakdown has changed its polarity. In addition, the negative channel that induced a downward positive leader was hot and conductive, as revealed by its brightness. The results indicate that disconnection or current cutoff on the negatively charged lightning channel is unnecessary for the inception of a downward positive leader.

A Rocket‐Triggered Lightning Flash Containing Negative‐Positive‐Negative Current Polarity Reversal During Its Initial Stage

AbstractA rocket‐triggered lightning flash containing negative‐positive‐negative current polarity reversal during its initial stage is analyzed using multiple synchronized observation data. The flash was triggered under a thunderstorm transition zone between the convective region and the stratiform region. The upward positive leader branched and developed toward the stratiform region. When the negative initial continuous current was decreasing, a negative stepped leader was transformed from a recoil leader on a preexisting positive branch. As the negative leader developed toward the convective region, a breakdown process initiated on a preexisting positive channel and propagated toward the back end of the negative stepped leader, accompanied with a positive electric field change pulse. The current polarity changed from negative to positive about 0.22 ms after the onset of the breakdown process. The negative leader terminated after propagating for 71.08 ms, while scattered discharges on other positive branches maintained to develop, and then the ICC polarity changed from positive to negative.

Needle Propagation and Twinkling Characteristics

AbstractRecently, a new lightning phenomena, termed needles, has been observed in both VHF and in optical along positive lightning leaders. They appear as small (&lt;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.

Lightning as an asymmetric branching network

The polarity asymmetry in lightning macroscopic behavior is well established for both cloud-to-ground and intracloud flashes. Negative polarity discharges to the ground most often consist of a series of strikes passing through one channel, while positive flashes, as a rule, are limited to one stroke; negative and positive leaders of lightning discharges have significant morphological differences; the growth of positive leaders is accompanied by the development of negative recoil leaders, and positive recoil leaders have never been observed (or do not exist). At present a consistent description of the mechanisms of macro-scale manifestations of the lightning discharge asymmetry causes considerable difficulties. Here we use the hierarchical Horton – Strahler scheme to estimate the structural asymmetry of the lightning discharge tree on the analogy with river systems. We postulate that displacement of the bi-directional leader reversing point (zero charge level of the lightning channel corona sheath) from the place of initiation towards the positive leader development is a fundamental manifestation of the lightning polarity asymmetry. This approach not only has the potential to relate the reversing point displacement rate with the difference in peripheral currents, but also helps uncover the mechanism that provide both the competitive suppression of the positive leader side branches and the negative leader renewal. Our results further indicate that the reversing point displacement determines the lightning principal direction of propagation.Lightning structural asymmetry is estimated using Horton – Strahler numbers Lightning reversing point moves towards the positive leader tip The difference in lightning peripheral currents determines reversing point rate Asymmetry provides competitive suppression of the positive leader side branches.