Lightning Leader Research Articles

Study on the effectiveness of lightning rod tips in capturing lightning leaders

This paper addresses the performance of various lightning rod tips: pointed, concave, blunt, flat and conical for lightning air terminal (LAT). To observe the behaviour of the various lightning rod tips, laboratory examination was conducted under two conditions, i.e. corona emission pattern under quasi-static electric field and lightning impulse testing—with and without pre-ionization. The data obtained from the corona emissions were correlated to the surface sharpness area of the tip. Using dimensional analysis, it can be concluded there is strong evidence that increased surface sharpness can lead to an increase in the corona discharge current. In addition, an impulse test was conducted to determine the performance of the LATs with different tip types and sizes. Using the individual and competitive test, it was found that the blunt LAT is the most recommended receptor.

Analysis of shielding failure parameters of high voltage direct current transmission lines

This paper studies the influence of different parameters on the shielding failure of high voltage direct current (HVDC) transmission lines. A numerical analysis model considering the developing process of the downward lightning leader based on electromagnetic field theory is used to estimate the lateral striking distance to the transmission line. The influences of different factors (different parameters of the downward lightning leader, HVDC transmission line, and weather conditions) on the shielding failure of the HVDC transmission line (TL) have been considered in this analysis. The results show that the dimensions of the transmission line strongly affect the analysed results of the HVDC–TL shielding failure. ► We study the influence of different parameters on the shielding failure of HVDC–TL. ► We model the downward lightning leader to estimate the lateral striking distance to the TL. ► The TL dimensions effect seriously on the HVDC–TL shielding failure. ► Weather conditions have effect on the HVDC–TL shielding failure.

Spatial and energy distributions of X‐ray emissions from leaders in natural and rocket triggered lightning

Energetic radiation is known to be produced by lightning. To investigate these emissions, ground‐based observations are being conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL where measurements of energetic radiation from both natural and rocket‐triggered lightning discharges are recorded. In the present study, data from two natural negative cloud‐to‐ground stepped leaders and one rocket‐triggered “chaotic” dart leader are analyzed in detail to investigate X‐ray energy spectra and spatial X‐ray distributions around the source. These measurements are compared with Monte Carlo simulations of runaway electron propagation with the goal of understanding the underlying mechanism of runaway electron production and their role in lightning initiation and propagation. We show that the energetic electrons that produce X‐rays exhibit a characteristic energy less than 3 MeV for two natural and one rocket‐triggered leaders investigated. In addition to studying these three leaders, energetic electron luminosity, total energy, and energetic electron per meter are compared to the following return stroke currents for 28 leaders from 12 different triggered flashes. Electron luminosity is found to increase exponentially with return stroke current up to about 10 kA and to be roughly constant for larger currents. The maximum electron luminosity, which is determined indirectly through X‐ray luminosities, is on the order of 1017 electrons/s, which is less than the value found from theoretical calculations.

Simulation of leader speeds at gigantic jet altitudes

Lightning leaders advance in space by creating a heating conversion zone in their tips (i.e., streamer‐to‐leader transition) in which Joule heating produced by currents of many non‐thermal corona streamers transforms into a hot and conducting leader channel. It is believed that the initial stages of transient luminous events termed gigantic jets (GJs) propagating toward the lower ionosphere are directly related to leaders initiated by conventional intra‐cloud lightning discharges and escaping upward from thundercloud tops. In the present work we provide quantitative description of speeds of these leaders as a function of leader current and ambient air density (altitude). The direct comparisons with available experimental data indicate that the initial speeds of GJs of ∼50 km/s are consistent with leaders possessing currents 2–8 A. The observed acceleration of GJs can be explained by growth of the leader current, and at high altitudes (low air densities) may be significantly affected by predominance of non‐thermal (i.e., streamer) discharge forms.

Research on the upward leader emerging from transmission line by laboratory experiments

The problem whether the upward leader can incept from the transmission lines in lightning shielding failure is still in question, since the upward leader emerging from transmission lines is difficult to be captured under natural circumstance. It is necessary to study the characteristics of upward leader. This paper presents some simulated experimental results of a 6-m long, combined plane-rod (PR) electrode to conductor (ground wire (GW)) air gap. A current measuring instrument with frequency response of DC-6.5MHz was developed and used to measure the leader current. Besides, high speed CCD camera was used to observe the optical phenomenon of the whole process. In the experiment, upward leader emerging from bundle-conductors and GW was found. The velocity, surface electric field and charge of upward leader channel were obtained by analyzing the experimental data. Finally, the surface electric field of conductor for natural lightning circumstance was compared with the field in the experiment, in order to prove that the upward leader can emerge from transmission lines due to the downward leader of lightning.

Calculation of surface electric field on UHV transmission lines under lightning stroke

Lightning is the major cause of interruptions in power transmission lines. Ultra-high voltage (UHV) AC transmission line has been put into operation in China. Due to the large tower height up to 90m and the high operation voltage up to 1000kV, the shielding failure probability increase obviously. Nowadays the leader progression model is an advanced method to evaluate the shielding failure probability of transmission lines. The surface electric field is the key issue for lightning failure analyzing, because the upward leader inception mainly depends on the surface electric field. In this paper, the lightning leader model is introduced. A charge simulation method based piecewise linear function is adopted to analyze the surface electric field on phase conductors and ground wires. The influence factors, such as operation voltage, lightning peak current, lightning down leader position, protection angle, on surface electric field were analyzed.

Lightning morphology and impulse charge moment change of high peak current negative strokes

We have analyzed very high frequency lightning mapping observations and remote magnetic field measurements to investigate connections between lightning morphology and impulse charge moment change (iCMC) of negative cloud‐to‐ground (CG) strokes with high estimated peak currents. Four lightning morphologies are identified for a total of 2126 strokes within optimum detection range of the North Alabama Lightning Mapping Array, and statistical iCMC distributions are given for each of these types. Almost all (>90%) of the largest impulse charge moments (greater than −200 C km in this data set) are not produced by strokes in ordinary negative CG flashes. Instead, negative strokes with the largest iCMCs are almost exclusively associated with two unusual flash types that both initially develop as positive (normal) intracloud lightning. In the first type the negative stroke with high iCMCs results from a negative leader that descends from the midlevel negative charge region after the upper level negative leader ceases propagating. In the second type, the upper level negative leader of the intracloud lightning progresses toward ground as a so‐called bolt from the blue to generate the negative stroke. Measurements of strokes associated with four negative polarity sprites suggest that all four were most likely produced in the first unusual lightning type. Our results highlight that estimated peak current and impulse charge transfer are not always well correlated and that the in‐cloud lightning structure strongly influences charge transfer on short time scales in negative CG strokes.

“Chaotic” dart leaders in triggered lightning: Electric fields, X‐rays, and source locations

We present the first published observations of “chaotic” dart leaders in triggered‐lightning discharges. We examine four leaders that exhibited “chaotic” electric field derivative (dE/dt) signatures in their final 10 to 12 μs. The dE/dt signatures were characterized by bursts exhibiting widths of the order of hundreds of nanoseconds on which were superimposed irregular pulses with widths of the order of tens of nanoseconds. These unique signatures were dissimilar from the dE/dt waveforms observed from dart or dart‐stepped leaders in triggered lightning. Three‐dimensional locations for dE/dt pulses that radiated from the bottom 200 m of the leader channels were determined, as were emission times. Vertical leader speeds for the four “chaotic” dart leaders were estimated to range from 2.0 to 4.3 × 107 m/s. A relatively continuous flux of energetic radiation (X‐rays and gamma rays) was observed during the final 10–13 μs of each “chaotic” dart leader. Some individual photons had energies more than 1 MeV. High‐speed video images of three “chaotic” dart leaders were obtained at a frame rate of 300 kilo‐frames per second (exposure time of 3.33 μs). One image, in the frame immediately prior to the return stroke, shows an upward positive leader 11.5 m in length propagating from the launch facility and a downward negative leader above with a streamer zone length of about 25 m. Channel base currents preceding the four “chaotic” dart leaders were of unusually long duration and large charge transfer, and return strokes following them had larger than usual peak currents.

Lightning leader stepping, K changes, and other observations near an intracloud flash

[1] An intracloud lightning flash in central New Mexico began with the initiation of a negative stepped leader at an altitude of 8.2 km above sea level. As this leader propagated eastward and upward, at 9.1 km above sea level it passed about 200 m to the north of a balloon-borne, electric field-change instrument (Esonde). After the first leader stopped, a second negative stepped leader began near the point of origin of the first leader, but it propagated away from the Esonde. From the changes in the electric vectors and the locations of impulsive radio frequency sources detected by a lightning-mapping array (LMA), we conclude the following: (1) The first negative stepped leader was not preceded by any significant charge rearrangement due to positive leaders. (2) Each step of the first negative leader had both a forward-going wave and a step recoil wave that propagated simultaneously backwards away from the leader tip along the existing channel. The presence of a step recoil wave during each step leads to an explanation for the existence of stepping. (3) After the first (nearby) leader stopped, step recoil waves from the second (distant) leader may have found their way onto the channel formed by the first leader. (4) After the second leader stopped, waves carrying negative charge propagated along the channel of the first leader, producing strong K changes in the electric field at the Esonde and providing a good record of the wavefront shapes.

A new estimation model of the lightning shielding performance of transmission lines using a fractal approach

The path of the lightning discharge shows characteristics of branching and tortuosity, and the lightning shielding failure of the transmission line also has statistical features. In this paper, an improved leader progression model using a fractal approach is introduced, which considers both the deterministic and stochastic features of the downward lightning leader. Based on the proposed model, the statistical features of lightning shielding failure on transmission lines will be analyzed. First, the fractal dimensions of the downward lightning trajectory in simulation are calculated and the value of breakdown probability constant η will be determined based on the fractal dimensions of natural lightning. Second, using the proposed model the shielding failure probability contour around the transmission line space is also obtained and used to describe the shielding failure zone and then the contour curve is applied to analyzing the characteristics of transmission lines that suffer from direct lightning strokes. Moreover, a method to estimate the lightning shielding failure rate of transmission lines is put forward, and the influence of the line height and the shielding angle of ground wires on the shielding failure rate of transmission lines are also investigated.

Attractive radii of vertical and horizontal conductors evaluated using a self consistent leader inception and propagation model—SLIM

A self consistent lightning connecting leader inception and propagation model (SLIM) is utilized to study the attachment of lightning flashes to grounded structures and hence to evaluate the lightning attractive radii of vertical cylindrical structures and horizontal conductors as a function of height of the structure and peak current of the first return stroke. The attractive radius is defined as the maximum lateral distance from where the structure would be able to attract a down-coming stepped leader. The results are compared with the ones that one would obtain using Electro-Geometrical Method (EGM). The results show that for structure heights of the order of 30m or less the attractive radii obtain from SLIM do not deviate significantly (i.e. the error is less than about 20%) from the EGM values. However, for taller objects SLIM predicts significantly larger attractive radii than EGM and the error increases with increasing height.

High-speed video observations of a lightning stepped leader

[1] High-speed video images of eight branches of a lightning stepped leader recorded at a frame rate of 300 kiloframes per second (kfps) (3.33 μs exposure) are analyzed, representing the fastest published frame rate measurements of stepped leader stepping by about 1 order of magnitude and the first observations of space stems/leaders associated with stepped leaders. Sixteen occurrences of space stems/leaders were imaged in 14 different frames at various distances in front of the descending leader tip. A total of 225 frames (about 0.75 ms) involving 82 steps of the downward moving, negatively charged stepped leader were captured, followed by 45 frames of leader channel illumination by the return stroke after the ground attachment of the primary leader channel. The stepped leader exhibited characteristics similar to those observed in both dart-stepped leaders in triggered lightning and in long laboratory sparks. In most cases, the space stem/leader in one frame connects to the leader tip above in the subsequent frame, extending the leader channel. Most connections are associated with significant isolated brightening of the space stem/leader and the connection region, followed by frames of upward propagating reillumination of the existing leader channel. Assuming the leader to be 1 km distant, we measure the 16 space stems/leaders to be on average 3.9 m in length and separated from the previous leader channel tip by an average of 2.1 m. For the 82 steps, interstep intervals are on average 16.4 μs and step lengths are on average 5.2 m.

Observations of simultaneous upward lightning leaders from multiple tall structures

We present high-speed camera observations (up to 7200 images per second) and correlated electric field measurements of upward lightning leaders initiated simultaneously from multiple tall towers. Four towers spanning a horizontal distance of 2.9km and ranging in height from 121 to 191m, developed upward leaders following a nearby positive cloud-to-ground (+CG) flash on 7/16/09 UT in Rapid City, South Dakota, USA during the summer thunderstorm season. The optical and electric field observations suggest that all four upward propagating leaders were positive polarity (i.e., upward negative lightning) and initiated simultaneously approximately 2ms following the +CG return stroke. There was significant intracloud flash activity prior to the return stroke, and upward leader initiation coincided with the passage of horizontally extensive in-cloud negative breakdown following the +CG return stroke. This observation supports the idea that downward positive cloud-to-ground lightning can trigger upward negative lightning from multiple tall objects. Specifically, the triggering component is an area of horizontally propagating negative breakdown following the +CG return stroke that influences a broad area resulting in simultaneous or near-simultaneous, positive polarity upward leader initiation from multiple tall objects.

Lightning initiation mechanism based on the development of relativistic runaway electron avalanches triggered by background cosmic radiation: Numerical simulation

The mechanism of lightning initiation due to electric field enhancement by the polarization of a conducting channel produced by relativistic runaway electron avalanches triggered by background cosmic radiation has been simulated numerically. It is shown that the fields at which the start of a lightning leader is possible even in the absence of precipitations are locally realized for realistic thundercloud configurations and charges. The computational results agree with the in-situ observations of penetrating radiation enhancement in thunderclouds.

Investigation of Lightning Rod Shielding Angle

This paper studies those parameters affecting the shielding angle of the lightning rod (Franklin Rod) above very tall buildings. It was recommended that the shielding angle of the lightning rod is about 45°∼60°. The downward lightning leader is modeled by using discrete line charges to consider the exponential distribution of charges through the downward leader. The voltage condition used by Rizk is used to investigate the inception of the upward lightning leader. Different air conditions (relative air density and air humidity) are considered for more practical simulation. The influences of lightning parameters and lightning rod height on the shielding angle are studied. The results shows that, lightning leader parameters, lightning rod height and ground slope have series effects on the lightning rod shielding angle. Based on the results, a lightning rod shielding angle for shielding design is recommended to decrease the lightning stroke to the lightning rod.

Air‐density‐dependent model for analysis of air heating associated with streamers, leaders, and transient luminous events

Blue and gigantic jets are transient luminous events in the middle atmosphere that form when conventional lightning leaders escape upward from the thundercloud. The conditions in the Earth's atmosphere (i.e., air density, reduced electric field, etc.) leading to conversion of hot leader channels driven by thermal ionization near cloud tops to nonthermal streamer forms observed at higher altitudes are not understood at present. This paper presents a formulation of a streamer‐to‐spark transition model that allows studies of gas dynamics and chemical kinetics involved in heating of air in streamer channels for a given air density N under assumption of constant applied electric field E. The model accounts for the dynamic expansion of the heated air in the streamer channel and resultant effects of E/N variations on plasma kinetics, the vibrational excitation of nitrogen molecules N2(v), effects of gains in electron energy in collisions with N2(v), and associative ionization processes involving N2(A3Σu+) and N2(a′1Σu−) species. The results are in excellent agreement with available experimental data at ground and near‐ground air pressures and demonstrate that for the air densities corresponding to 0–70 km altitudes the kinetic effects lead to a significant acceleration of the heating, with effective heating times scaling closer to 1/N than to 1/N2 predicted on the basis of similarity laws for Joule heating. This acceleration is attributed to a strong reduction in electron losses due to three‐body attachment and electron‐ion recombination processes with reduction of air pressure.

Terrestrial gamma ray flash production by active lightning leader channels

The production of terrestrial gamma ray flashes (TGFs) requires a seed energetic electron source and a strong electric field. Lightning leaders naturally provide seed electrons by cold runaway and strong electric fields by charge accumulation on the channel. We model possible TGF production in such fields by simulating the charges and currents on the channel. The resulting electric fields then drive simulations of runaway relativistic electron avalanche and photon emission. Photon spectra and directional distributions produced by the model agree qualitatively with observations. Simulations with a variety of initial conditions indicate sufficient electric fields are produced if an unbranched channel supports a current pulse of at least 100 kA such as occurs if the channel is at least 1 km long and embedded in an ambient electric field of at least 100 kV m−1. The mechanism does not strongly depend on altitude as friction and characteristic electric field strengths scale similarly. Seed particle production is not directly simulated, but estimates of seed production rates suggest current pulse activity of ∼1 ms duration can account for TGF‐scale emission.

Recent advances in theory of transient luminous events

This paper presents an overview of the recent modeling efforts directed to the interpretation of observed features of transient luminous events termed sprites, blue jets, and gigantic jets. The primary emphasis is placed on interpretation of various emissions documented to date from sprites and comparison of exiting models with recent high‐speed video and satellite‐based observations of sprites. We also discuss the recently advanced theories of blue jet and gigantic jet discharges describing electrodynamic conditions, which facilitate escape of conventional lightning leaders from thundercloud tops and their upward propagation toward the ionosphere. The paper concludes with a brief survey of the recent literature on chemical and global electric circuit effects of sprite and jet discharges.

Statistical analysis of the lightning performance of high voltage OHLs using dynamic simulation of lightning leaders movements

In this paper a simulation ground is designed to track the lightning path from cloud to the striking point on high voltage overhead line (OHL) structures. By using this simulation method, the strikes to towers, phase wires, ground wires and earth is counted. The method also reveals the property of strokes attaching the phase wires like their path, their peak current and their distance from earth in final jump. The simulation environment consists of two towers, ground wires, phase wires and earth comprising a full span. The downward lightning leader is simulated based on field observations by stepping line charges coming down. The inception of upward leader is tested in each step movement to find the target striking point. Using this method, lightning performance of an OHL is investigated and the number of strikes as well as highest lightning current attaching the phase wires is extracted. The results, yield by proposed method, are also compared with the existing methods of OHL lightning performance analysis.

Shielding failure rate calculation by means of downward and upward lightning leader movement models: Effect of environmental conditions

In this paper the effects of environmental conditions on shielding failure rate (SFR) of transmission lines are investigated. The study utilizes a previously published work in which leader progression model for lightning upward and downward leaders are used to calculate the SFR. Taking into account the effects of reduced air density and humidity on the parameters of upward leader model and wind pressure on the movement of lightning leaders and wires, SFR and maximum lightning stroke current causing shielding failure are computed. The electric field in all simulations is calculated by means of charge simulation method. The results of simulation show that the effects of relative air density and height of installation are quite higher than that of the wind pressure and humidity while the humidity has the lowest impact on the SFR of investigated transmission line.