Upward Leader Research Articles

Modeling of natural first stroke stepped leader characteristics

The paper constitutes a novel physical approach to the different stages of negative leader stepping in cloud-to-ground lightning, starting with determination of the length of the negative streamer zone below the leader tip. The positive space leader, belonging to a pre-existing space leader, propagates towards the main leader tip, bridging the streamer zone. Such propagation time however is too short for leader thermalization. The thermalization, with associated substantial drop in the internal voltage gradient, occurs over a much longer time, through corona current injected at the tip of the moving negative space leader. The latter stage constitutes the determining factor for the time interval between steps. The model has then been applied to leaders with prospective return stroke current of 3–228 kA, leader tip height of 23–2000 m, leader charge linearly decaying to practically vanish at heights of either 2500 m or 4500 m. For the conditions investigated, the step length varies in the range 3.5 m to 39 m, the bridging time of the streamer zone by positive space leader in the range 0.8 - 3 μs, time interval between steps in the range 8.5–92 μs, yielding mean stepped leader speed in the range 1.0 - 11.6 × 105 m/s. Whenever possible the model predictions were satisfactorily compared to field measurements. Interaction between the negative downward leader and an upward connecting leader during the attachment process is out of scope of this work.

Fractal dimension analysis of lightning discharges of various types based on a comprehensive literature review

In this study, photographic records of natural lightning flashes obtained from high-speed video camera observations are analyzed based on an extensive literature review. The purpose of this work is to estimate the fractal dimension of lightning discharges and to analyze and evaluate the effect of lightning type (downward and upward flashes during their propagation as well as return strokes) and lightning polarity (negative and positive). Three methods of fractal dimension estimation are employed namely the: (i) box-counting, (ii) sandbox, and (iii) correlation method, utilizing algorithms developed in MATLAB software. Appropriate image processing techniques are employed to guarantee precision in fractal dimension estimation. A thorough discussion is conducted by comparing the estimated fractal dimension values with those previously documented in the relevant literature based on field observations. The mean fractal dimension of downward negative leaders for the three methods (1.18, 1.31, 1.38) aligns well with previous studies, while positive downward leaders exhibit lower values (1.08, 1.15, 1.26), denoting a reduced branching behavior; upward leaders demonstrate slightly higher fractal dimensions than downward ones. Additionally, an attempt to correlate the lightning return stroke peak current with the fractal dimension is made. The outcomes of this research may assist in facilitating precise modeling of the lightning attachment phenomenon, thereby contributing to the development of safer lightning protection systems.

Attachment processes of negative flashes with multiple return strokes to a tall tower observed at close distances

Two downward negative flashes (F1612 & F1613) with multiple return strokes struck to the 356-m Shenzhen Tower were analyzed. The high-speed camera captured upward positive connecting leaders (UCL) for the first strokes of both flashes and two subsequent strokes of F1612. The observations also show that none of the downward negative leader (DNL) completely propagated to the tower tip before the occurrence of the return stroke (RS), so that we inferred that the UCL initiated from the tower tip might exist for each RS of the two flashes. The possible height of the UCL and DNL connecting point above the tower tip for each RS was estimated, which showed a positive correlation to the speed of corresponding DNL. The striking distances estimated for 8 subsequent RSs in F1612 were in a range from 14.6 m to 85.4 m and that for 3 subsequent RSs in F1613 were from 24.2 m to 66.2 m. There was a weak positive linear relationship between the DNL speed and striking distance for subsequent RSs in the two flashes. The DNL speed showed a positive correlation to the peak current of corresponding RS.

Evolution of the Leader Discharge in Bi‐Directional Propagation System in Altitude‐Triggered Lightning

AbstractOn 18 June 2023, comprehensive observations were conducted to an altitude‐triggered lightning flash. Upward positive leader (UPL) and downward negative leader (DNL) in a bi‐directional development system were detected simultaneously by a high‐speed camera, together with the coordinated measurements of magnetic field and very‐high‐frequency (VHF) emissions. High‐speed images reveal, for the first time, the enhancement of the UPL's propagation speed by DNL in the bi‐directional leader system. Concretely, the upward positive leader initially originates from a suspended wire tip and propagates at a two dimensional (2D) speed of 5.32 × 104 m/s, and after about 6.3 ms, its propagation speed was enhanced to 1.12 × 105 m/s when the stepped DNL started advancing at an average speed of 1.44 × 105 m/s. Additionally, based on the evolution of channel luminosity and the variations of magnetic radiation, it is found that there is a consistency in luminosity variation between the ascending channel and the descending channel in the bi‐directional leader system, and the amplitude of the magnetic field increases when the negative discharges start at the bottom wire end with intensive VHF emissions. Those facts indicate that the DNL has an effect, may be a positive one, on the UPL's development in the early stage of altitude‐triggered lightning.

Investigation of Upward Leader Development Characteristics by Two Kinds of Charge Density Models in Wind Turbines

ABSTRACTThe upward discharge is a significant factor threatening the operation of wind farms. This paper investigates the inception and development of upward leaders of wind turbines' tip, where blade rotation prevents charge accumulation, using charge density models from laboratory discharges and artificially triggered lightning. The results indicate that larger return stroke currents create higher spatial potentials, and slower downward leader speeds allow for longer development time, favoring the development of upward leaders. As wind turbine sizes increase, the rotation of the blade tips resembles artificially triggered lightning, and the long air gap discharge model may underestimate the development of upward leaders. Calculations from the artificially triggered lightning model reveal improved upward leader formation when further laterally away from the downward leader within a certain distance, elucidating why multiple upward leaders are frequently observed around turbines. The striking distance and attractive radius are largely determined by the development of upward leaders. The absence of corona charge shielding at blade tips means upward leader inception is independent of downward leader velocity, emphasizing downward leaders' significant influence on turbines. Thus, the formula for estimating a wind turbine's lightning incidence requires integrating the return stroke current and velocity of the downward leader.

FDTD Simulation of a Small‐Scale Charged Airplane Model in an Ambient Electric Field between Two Flat Electrodes

When an airplane flies in an electric field under a thundercloud, electric fields at the edges and projected portions of the airplane are enhanced and leaders emanate from there. When a positive leader emanating upward from the airplane connects with a downward negative leader from the bottom of an ordinary thundercloud and a negative leader emanating downward from the airplane connects with an upward positive leader from the ground, a large lightning current flows along the channel bridged between the thundercloud and the ground through the airplane. Recently, a method to reduce the risk of lightning strikes to airplanes has been proposed. It controls the charge on the surface of an airplane to suppress the electric field at edges and projected portions of the airplane. In this paper, an airplane under a thundercloud is represented with a vertical conducting bar or a horizontal conducting bar with a small projected portion, which is placed between impulse‐high‐voltage‐applied two flat electrodes, and it is analyzed using the finite‐difference time‐domain (FDTD) method. Corona and leader discharges emanating from edges and projected portions of an airplane model are considered with their engineering representations: 40‐μS/m and 0.02 S/m conducting regions for corona and leader discharges, respectively. The airplane model is not pre‐charged or pre‐charged negatively. It follows from the FDTD‐computed results that pre‐charging an airplane model with a relevant amount of negative charge can avoid discharges from and to the airplane model. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

Subnanosecond Electromagnetic Pulse Generated by a Long Spark Discharge: Lightning Implication

AbstractThe effects of generating pulsed radiation by a long spark discharge are important for the development of lightning models and applications related to lightning protection. In experiments with a Marx generator simulating a lightning discharge, we detected the radiation in the form of a single ultrawideband electromagnetic pulse (UWB EMP) about 200 ps in duration, and rising time about 100 ps. UWB EMP generation occurs during the breakdown of a “rod–rod” 4 m long gap. Pulses of almost unipolar shape are observed in more than half of all positive discharges. EMP emission occurs before the main stage, and corresponds to the start of the upward leader from a grounded electrode. In negative discharges, pulses are also observed, but less frequently and with a smaller amplitude. The UWB EMPs, given their large amplitude (more than 100 V/m at a distance of 90 m from the discharge), can be considered as possible new lightning damage factors.

Electromagnetic fields of lightning return stroke to wind turbines with discontinuous impedance model

This study introduces an electric field calculation model for wind turbine return strokes, incorporating impedance discontinuity within the turbine, a feature not considered in traditional tall tower models. The model introduces a reflection coefficient, ρx, at the connection between the blades and the tower. Disregarding the generation of upward leaders, this paper calculates the electric field of subsequent return strokes for different distances from the wind turbine and for different model parameters. The results demonstrate that changes in the reflection coefficient ρx affect oscillation amplitude and also slightly influence the pulse width, with a negative ρx leading to smaller and more frequent pulses. Contrasts with measured waveforms indicate that models with a constant reflection coefficient are inaccurate. Adopting variable reflection coefficient values yields a more accurate match. This approach offers a new perspective for electromagnetic field calculations in lightning-struck structures, emphasizing the significance of impedance dynamics of return stroke channel.

Improving lightning protection with corona minimising air terminals

A comprehensive approach to lightning protection is comprised of four key steps, namely protection against direct lightning strikes, dealing with surges and transients, dissipation of lightning currents via earthing and bonding, and protecting people. This paper deals with new research findings associated with direct-strike protection with lightning rods or “air terminals”, namely the effect of accumulated corona space charge around the tips of these components. There is now a great deal of consensus amongst lightning researchers and practitioners that space charge accumulation reduces the efficiency of an air terminal by inhibiting the initiation and development of an upward leader, a critical stage in the lightning attachment process. The paper describes measurements carried out in a high-voltage laboratory to quantify the amount of corona discharge that would be emitted under thunderstorm conditions from a variety of air terminals of different geometries. A unique, previously unreported aspect of these experiments was the corona testing of air terminals under dry and wet conditions. The results of these experiments showed that corona discharge (and hence space charge accumulation) from a standard Franklin rod is substantially higher than from the range of significantly blunter “corona minimising” air terminals that were tested. The previously reported polarity difference in corona characteristics was also observed, i.e., the magnitude of negative corona was larger than positive corona for the same ambient electric field. Differences in corona discharge were also observed under wet and dry conditions, where wet air terminals were found to produce modestly more corona. The paper then addresses the optimisation of air terminals, i.e., minimising corona discharge, for practical lightning protection applications, where the air terminal radius of curvature is tailored to its height and position of installation. Various researchers have made these calculations, the outcomes of which are summarised in this paper. In general, radii of curvature in the range 1–100 mm are required, depending on the installation height and location of the air terminal.

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.

New insight of two kinds of M components in a multi-branch rocket triggered lightning flash

After the first and second return strokes (RSs) of a multi-branch triggered lightning flash, the M component of two different initiation mechanisms burned: (1) Upward leader (UL)-M component, (2) Downward leader (DL)-M component. The UL-M component burned immediately after the RS (about 0.5 ms after the RS). At this time, an UL developing to the cloud generated outside the main channel of the continuous current (CC). With the development of the UL, the M component is formed in the CC channel. The obvious DL process can be observed during DL-M component. The speed of DL is about 2 × 107 m/s, when DL was intercepted by CC channel, M component will generate. The current peak and electric field peak of the UL-M component were larger than those of the DL-M component. It was worth noting that the M component formed by the UL slightly developing to the ground at the head of it did not produce an electric field change at the observation point 2 (1550 m horizontally from the rocket triggered lightning point), which was a result of the combined effect of the positive charge in the branch head and the negative charge in the main channel at the same height. The reflection phenomenon of the M component was also observed in a DL-M component process. The CC in the channel after the RS plays a key role in the formation of the M component. It forms the M component by intercepting the current wave transmitted to the ground from the cloud or the lightning branch.

Direct observations of X-rays produced by upward positive lightning

X-rays have been observed in natural downward cloud-to-ground lightning for over 20 years and in rocket-triggered lightning for slightly less. In both cases, this energetic radiation has been detected during the stepped and dart leader phases of downward negative flashes. More recently, X-rays have also been reported during the dart leader phase of upward negative flashes. In this study, we present the observations of four upward positive lightning flashes from the Säntis Tower (2.5 km ASL) in Switzerland. These consist of the simultaneous records of electric current passing through the tower, and electric field strength and X-ray flux 20 m from the tower base. One of the flashes was captured by a high-speed camera operating at 24,000 frames per second, stills from which are also presented. We detected X-rays during the initial phase of upward negative leader propagation, which can be associated with the leader-stepping process from electric field and current waveforms. To the best of our knowledge, this is the first time that such measurements are reported in the literature. The obtained time-synchronised data confirm that the X-ray emissions detected are associated with the initial steps of the upward negative leader. The frequency and energy of X-ray pulses appear to decrease as functions of time, with pulses disappearing altogether within the first millisecond of the leader initiation. X-ray emission also appears to be correlated with the maximum current-derivative and the electric field change of leader steps, consistent with cold electron runaway. These observations contribute to improving our understanding of upward lightning, which is a primary source of damage to tall structures such as wind turbines and telecommunications towers, as well as aircraft during takeoff and landing.

Scaled experiment and observation of the lightning discharge process of rotating wind turbines under different shapes of high-voltage electrodes in the laboratory

In this study, a discharge test for scaled wind turbine is performed to examine the lightning discharge characteristics of rotating wind turbines, and an observation platform is designed. Moreover, the wind turbines are subjected to discharge simulation with rod and arc electrodes, and high-speed and single-lens reflex cameras are used to observe the physical discharge process. Under the rod electrode, the downward leader is initiated first, and its length occupies a larger proportion of the discharge gap. The discharge contains multiple stepped leaders. When the gap distance is less than 4 m, the probability of occurrence of negative stepped leaders may be smaller. As the gap distance increases, the number of stepped leaders increases significantly. Under the arc electrode, the upward leader is initiated first, and its length accounts for a larger proportion of the discharge gap. With the rotation of the wind turbine, the upward leader is initiated earlier and the length is longer. Furthermore, the speed of its development does not differ significantly. The discharge simulation process under the arc electrode is similar to natural lightning strikes on wind turbines.

Nanosecond photography observation of the discharge process across the 220 kV/500 kV insulators in lightning impulse

Long air gap discharge is the basis for the study of lightning protection and external insulation design of high voltage lines. However, there is a lack of detailed and comprehensive observation of the long air gap discharge process at the ns-level, as well as higher voltage insulator discharge observations. In this paper, the lightning impulse discharge/flashover tests are conducted on 220 kV glass insulators and 500 kV glass/composite insulators. Firstly, the U50% test is performed to determine the U50% value of each insulator under standard lightning impulse, and 1.1U50%, slightly higher than U50%, is applied to ensure the breakdown of the insulator. Subsequently, emICCD is utilized with a very short exposure time (minimum 10 ns) to obtain repeated single shots of discharge images under different delays, exposure times and light intensity gain. These images were spliced together based on time sequence to create a continuous discharge process. The discharge processes of 220 kV glass insulator and 500 kV glass/composite insulator are compared and analyzed. The results show that in the early stage of discharge, the discharge processes at the low voltage end of both 220 kV and 500 kV glass insulators are more intense. Additionally, the downward leader is faster than the upward leader in the discharge process of 500 kV composite insulators.

Research on the Initiation of Multiple Upward Leaders From an Isolated Building Based on an Improved Lightning Attachment Model

AbstractMore and more optical records have exhibited that multiple upward leaders (MULs) occur frequently on a building in the flash attachment process. An interesting issue is why a building can continue to launch upward leader (UL) after the first one appears. This phenomenon is analyzed in the present paper. Considering the influence of the leader behaviors on the ambient electric field, an improved 3‐D fine‐resolution lightning attachment model with MULs is established to simulate cloud‐to‐ground flash events with diverse leader spatial morphologies. The simulation results show that MULs may initiate almost simultaneously or with an obvious delay and the variation range of UL length is large. From this, the flash events of lightning terminating on a building are divided into four scenarios and each scenario is analyzed. It was found that the spatial location of downward leader, the length and propagation direction of the first UL and the time interval from the inception of the first UL to final jump significantly affect the electric fields at top corners of building and further affect the inception of the second UL. Based on qualitative analysis, four factors are proposed to explain why the above four scenarios happen.

On the Question of the Formation of the Lightning Current

The bipolar lightning development model was used to study the dependence of the potential that is transported to the earth by the downward leader channel. It was shown that this parameter strongly depends on the starting position of the lightning and on the trajectories of formation of its bipolar leaders. It was shown that the main reason for the change in potential is not the loss of voltage in the lightning channel with a finite conductivity but its polarization in the electric field of the storm cloud. An estimate was made of the range of potential variation in the channel with ideal conductivity depending on the starting position and trajectory of the lightning at a constant charge in the thunderstorm cell. It was shown that, for the variation of the lighting current within two orders of magnitude, a mere twofold change in the charge of the thunderstorm cell is sufficient. The preferable starting position is found for the lightning whose upward leader can penetrate into the upper layers of the troposphere, turning into a blue jet.

On the Question of the Formation of the Lightning Current

The bipolar lightning development model was used to study the dependence of the potential that is transported to the earth by the downward leader channel. It was shown that this parameter strongly depends on the starting position of the lightning and on the trajectories of formation of its bipolar leaders. It was shown that the main reason for the change in potential is not the loss of voltage in the lightning channel with a finite conductivity but its polarization in the electric field of the storm cloud. An estimate was made of the range of potential variation in the channel with ideal conductivity depending on the starting position and trajectory of the lightning at a constant charge in the thunderstorm cell. It was shown that, for the variation of the lighting current within two orders of magnitude, a mere twofold change in the charge of the thunderstorm cell is sufficient. The preferable starting position is found for the lightning whose upward leader can penetrate into the upper layers of the troposphere, turning into a blue jet.

Influence of the −10 °C isotherm altitudes on winter lightning incidence at wind turbines in coastal areas of the Sea of Japan

Winter lightning poses the greatest threat to wind turbines in coastal areas of the Sea of Japan. The lower altitude of winter thunderclouds initiates an upward lightning leader. The number of lightning discharges striking wind turbines directly during the winter lightning season (WLS) was significantly larger than in seasons other than winter (SOW). Winter lightning also has more energy which causes catastrophic damage to wind turbines. The characteristics of lightning discharges detected near wind turbines in coastal areas of the Sea of Japan were investigated using the Japanese Lightning Detection Network (JLDN). The JLDN detected 46% and 9% of all lightning discharges near wind turbines during the WLS and SOW, respectively. Most of the lightning discharges during the WLS were upward lightning. The probability of lightning discharges striking tall structures such as wind turbines increases as the −10 °C isotherm altitude drops. The characteristics of the winter lightning observed at Nikaho in Akita, Japan were compared to those worldwide. Winter lightning initiated from tall structures in Japan and at the Gaisberg Tower in Austria differed from those initiated in other regions. In locations exposed to similar winter lightning occurring in coastal areas of the Sea of Japan, lightning protection design for wind turbines should consider huge charge transfers of a lightning flash.

Low Frequency Magnetic Field Observations of Natural Positive Leaders Featuring Stepwise Propagation

AbstractWe examine the low‐frequency (LF) magnetic pulses recorded for the positive leader in two natural lightning flashes as recorded on the high‐speed video camera. The captured positive leader in both cases propagated in a stepwise manner at least over several milliseconds according to the concurrent sequence of LF magnetic pulses. By comparing with previous studies on positive leaders under different situations, the average inter‐pulse intervals (11.7 and 20.3 μs, respectively) obtained for our natural cases are shorter than that of upward positive leaders in rocket‐triggered (typically >25 μs) and tower‐initiated lightning (62 μs in one case). Hence, the stepwise propagation of positive leaders in natural lightning is likely more frequent than that in object‐induced lightning, reflecting a possible influence of ambient electric field and other meteorological factors on the stepwise feature of positive leader. The stepwise extension might be common for positive leaders developing in various scenarios.

Quantitative Relationship Between Current Pulses and Associated Low‐Frequency Magnetic Fields During Initial Stage of Rocket‐Triggered Lightning

AbstractThe quantitative relationship between the channel‐base current and the associated low‐frequency magnetic field (B‐field) during the early stage of rocket‐triggered lightning was examined based on field experiments and numerical simulation. There is a good correlation between the current pulse and the corresponding B‐field pulse in terms of amplitude and duration. In specific, the duration of current pulse is approximately proportional to that of the corresponding B‐field pulse for precursors and initial upward leaders; as for the pulse amplitude, the linear correlation is more apparent for the initial upward leaders when compared to the precursors, with the ratio of B‐field pulse and current pulse between 1.7 and 2.0, which is always greater than that (0.97–1.32) for the precursors. A response function is established to show the quantitative relationship in the time domain between the current pulse and the associated B‐field pulse, which is considered as the convolution of the current pulse and the response function. Meanwhile, the current waveform can be obtained if the measured B‐field pulse is de‐convolved with the response function. The simulation results are in good agreement with the measurement, which proves that our approach is accurate and efficient to quantify the relationship between the current and the B‐field pulse of the initial leader discharges.