Upward Positive Leader Research Articles

Characteristics of Winter Lightning that Occurred on a Windmill and its Lightning Protection Tower in Japan

We have observed lightning that struck a wind turbine and its neighboring lightning-protection tower during the past six winter seasons (2005 to 2010) using various lightning observation instruments. Our results show that the upward lightning from high structures can be classified into self-initiated and other-triggered types according to whether there is a discharge activity prior to the upward lightning. Furthermore, we found that although other-triggered upward lightning can start at a relatively low wind speed, self-initiated upward lightning always started either from the stationary tower under a larger wind speeds or from a rotating wind turbine blade. It appears that the wind does have considerable effect in assisting the initiation of an upward leader. In addition, we found that the self-initiated upward positive leaders from structures with different effective heights exhibited remarkably different initial speeds. Higher structures tend to initiate faster upward leaders. Finally, we discussed the pulse discharges observed in the very initial stages of positive upward leaders and how to protect structures from upward lightning as well.

Observations of the initial, upward‐propagating, positive leader steps in a rocket‐and‐wire triggered lightning discharge

[1] We present high-speed video images (recorded at 300 kfps) of an upward positive leader developing stepwise from the top of a grounded triggering wire. The leader initiated from the wire top at a height of 123 m, and was imaged over a distance of 11 m in ten discrete steps. Unique current pulses were detected at the wire base corresponding to each optical step. Luminosity waves propagated downward from the leader tip. The step lengths ranged from 0.4 to 2.2 m; the interstep time intervals from 16.6 to 30.4μs. The leader's upward development speed increased with height, from 5.5 × 104 m s−1 between heights of 123 m and 134 m, the bottom 11 m, to 2.1 × 105 m s−1at a height of 350 m. The measured wire-base peak currents for the individual steps ranged from 17 to 153 A. After removing from the wire-base current measurements the effect of propagation on the triggering wire and the effect of reflection at the ground connection, we estimate that the peak currents at the wire top, the step current, ranged from 10 to 89 A. The charge lowered to ground following each step ranged from 22 to 107μC. The leader channel contained, on average, 51 μC m−1 of positive charge, a value that is similar to laboratory measurements of long positive polarity sparks.

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.

Exposure of Overhead Conductors to Direct Lightning Strikes: Modeling of Positive Streamer Inhibition

In this paper, it has been shown that a positive upward connecting leader from an overhead conductor spans a considerable part of the lateral attractive distance of the conductor. This justifies the effort to search for means to suppress streamers/upward leaders from such conductors. A new simplified model is introduced to account for the initiation and time growth of glow corona from a cylindrical conductor above ground under linearly rising fields due to cloud charges or negative downward leaders. The model was applied to assess lightning exposure of a streamer-inhibited ground wire as well as its application to lightning protection of adjacent power conductors. The paper confirms the prospect of successful application of streamer inhibition due to space charge shielding (ultra-corona) as a means for lightning protection. Streamer inhibition by using larger conductor diameters or conductors provided with an insulating layer was also considered and the limitations of both techniques were pointed out.

Physical processes during development of upward leaders from tall structures

The objective of our study was to identify and interpret the various processes during development of upward positive leaders from tall structures. We provide a physical interpretation for the varying luminosity of the leader channel during its initial ascent and the pulsing luminosity during much later stages of the established leader. Our analysis confirms that pulsed luminosity is a result of recoil leaders intercepting the current-carrying channel connected to ground. This interception produces an M-component. Applying the bidirectional leader concept and a simplified electrostatic approximation, we introduce physical models of the dart leader-return stroke sequence and the M-component.

Three‐dimensional imaging of upward positive leaders in triggered lightning using VHF broadband digital interferometers

Upward positive leaders (UPLs) in two artificially‐initiated lightning flashes were imaged in three dimensions using VHF broadband digital interferometers and a high‐speed video camera with time‐synchronized channel‐base current measurements. Locatable VHF sources of the two UPLs began at 1.1 km and 1.5 km, a few milliseconds after the UPL inception, and ascended to 2.4 km and 3.7 km, respectively, with average 3‐D speeds on the order of 106 ms−1. The initial stage currents for both flashes were unusually large and had peak values of 6 kA and 18 kA. VHF sources associated with positive leader propagation were located when the average current was higher than 3 kA and had significant pulse activity. The source altitudes and channel‐base currents suggest that there might have been a region of significant negative charge at altitudes from 2 to 4 km, which is below the freezing level of typical thunderstorms in Florida.

Dynamic Simulation of Lightning Attachment to Earthed Overhead Transmission Line Structures

In this paper a novel method is presented to investigate lightning stroke attachment to earthed overhead transmission line structures. This method is based on the dynamic simulation of lightning downward leader movement toward the ground in three dimensions. To perform the simulation, a three-dimensional model for clouds, towers, wires, and a perfectly conducting ground is created. A stepping approach is also introduced in accordance with the field observations of negative downward lightning leaders to model the downward leader movement toward the ground. In each step of the leader movement, the electric field is computed numerically in all points of the simulation space and in three dimensions. Meanwhile, stable positive upward leader inception from some test points on earthed structures is tracked by means of an adopted precise model of the leader—streamer system to find the ultimate striking point. The main computational volume is occupied by an electric field calculation, which is performed using the charge simulation method. A direct movement model for the downward leader is also simulated and compared with the stepping approach. The percentage of strokes that attach to the phase wires, ground wires, and towers is extracted using this method. The maximum return stroke current for the strokes that attach to the phase wires is also computed by this dynamic simulation and is compared with the traditional methods that are available for transmission line lightning performance investigation.

High‐speed video observations of rocket‐and‐wire initiated lightning

We present observations of a rocket‐and‐wire triggered lightning flash obtained with high‐speed video cameras recording 5400 and 50000 frames per second (frame times 185 μs and 20 μs) with time‐synchronized current and electric field measurements. Transient leader channels were observed with precursor current pulses occurring before the development of the sustained upward positive leader that initiated the initial continuous current. The sustained upward positive leader stepped with a constant speed of 5.6 × 104 m s−1 over its initial 100 m. The wire destruction occurred discontinuously over a time of 7 ms about 45 ms after sustained upward leader inception, with a small change in channel current. Downward leaders, upward connecting leaders, and filamentary streamers were imaged in the bottom 50 m of the channel. We present the first images of a negative step forming in lightning, apparently involving a space stem similar to steps in meter‐length negative laboratory sparks.

A Modified Version of the Rolling Sphere Method

The calculation of the striking distance can estimate the probability of lightning strike on a structure and thereby evaluate the effectiveness of a lightning protection system (LPS). The dimensioning and the positioning of air-termination on structures is often performed with the Rolling Sphere Method (RSM). RSM originated from the electric power transmission industry and is based on the well-known Electrogeometric Model (EGM). The EGM relates striking distance to the prospective peak stroke current. To apply this technique, an imaginary sphere is rolled over the structure. All surface contact points are deemed to require protection, whilst the unaffected volumes are deemed to be protected. The main drawback of this method is that it disregards the upward leadersiquest development and assumes the same probability for attachment to the ground, to a structure, and to a LPS. The proposed model is based on physical phenomena leading to the formation and the development of positive upward leader in the field produced by the negative downward leader charge distribution and by some other competing upward leaders. Its purpose is to develop a 3-D numerical model in order to improve the interception efficiency of the Lightning Protection System.

Comparative analysis of the initial stage in two artificially-triggered lightning flashes

The initial discharge stages of two flashes during the Shandong Artificially Triggering Lightning Experiment (SHATLE) are analyzed based on the synchronous data of the current and close electromagnetic field. For a lightning flash, named 0503, the wire was connected, not electrically, but via a 5 m length of nylon, with the lightning rod; while for another, named 0602, the wire was connected with the rod directly. Results show that the discharge processes of the initial stage (IS) in flash 0503 are quite different from that of the usual classical-triggered flash 0602 and altitude-triggered flashes. A large pulse with a current of about 720 A resulted from the breakdown of the 5 m air gap during flash 0503, and the corresponding electric field at 60 m from the lightning rod was 0.38 kV/m. The upward positive leaders (UPLs) propagated continuously from the tip of the rocket after this breakdown. The geometric mean (GM) of the UPL peak current was 23.0 A. Vaporization of the wire occurred during the initial continuous current (ICC) stage and the largest current pulse was about 400 A. Compared with triggered flash 0503, the discharge processes of IS in flash 0602 were simple, only two large pulses similar to each other occurred before dart leader/return stroke sequences. The peak current of the first pulse was 2.1 kA and its corresponding electric field and magnetic field at a distance of 60 m from the lightning rod were 0.98 kV/m and 7.03 μT, respectively. During the second pulse, the wire disintegrated. The current decreased to the background level at the moment of the wire disintegration. The current of the second pulse in triggered flash 0602 was 2.8 kA, and the corresponding electric field and magnetic field at 60 m from the lightning rod were 1.22 kV/m and 9.01 μT, respectively.

Simultaneous optical and electrical observations on the initial processes of altitude-triggered negative lightning

Using a high-speed camera system and two electric field antenna systems, we have documented the initial processes of an altitude-triggered negative lightning (ATNL). The optical records clearly show that ATNL begins with the inception and propagation of an upward positive leader (UPL) and then a simultaneous propagation of UPL and downward negative leader (DNL), known as the bidirectional leader process, follows. Based on the optical records, it is inferred that (1) the triggering height is about 371 m; (2) the two-dimensional (2D) propagation speed of the UPL in its inception phase is about 3.8–5.5 × 10 4 m s − 1 during its propagation from about 393 to 452 m above the ground; (3) the grey levels of the DNL are about one order of magnitude higher than that of the UPL in their inception phase; (4) a discharge phenomenon propagating along the elevated triggering wire part of the lightning channel occurs after the mini-return stroke (MRS), with a 2D propagation speed of about 1.6–2.0 × 10 5 m s − 1 . Combined with the simultaneous electric field change records, it is further inferred that (1) the UPL incepts about 932 μs earlier than the unstable DNL and about 4.1 ms earlier than the stable DNL; (2) the unstable DNL propagates downward intermittently three times with a time interval of about 1 ms, and each propagation contains a different number of steps with an average step length of about 7 m; (3) the stable DNL incepts at the tip of the unstable one, with a 2D propagation speed of about 1.9 × 10 5 m s − 1 , an average step length of about 3 m, and a stepping time interval varying from 6 to 31 µs with a mean value of 15 µs.

Observed characteristics of luminous variation events during the initial stage of upward positive leaders

Observed characteristics of luminous variation events during the initial stage of upward positive leaders

Time dependent evaluation of the lightning upward connecting leader inception

The evaluation of the upward connecting leader inception from a grounded structure has generally been performed neglecting the effect of the propagation of the downward stepped leader. Nevertheless, field observations suggest that the space charge produced by streamer corona and aborted upward leaders during the approach of the downward lightning leader can influence significantly the initiation of stable upward positive leaders. Thus, a physical leader inception model is developed, which takes into account the electric field variations produced by the descending leader during the process of inception. Also, it accounts for the shielding effect produced by streamer corona and unstable leaders formed before the stable leader inception takes place. The model is validated by comparing its predictions with the results obtained in long gap experiments and in an altitude triggered lightning experiment. The model is then used to estimate the leader inception conditions for free standing rods as a function of tip radius and height. It is found that the rod radius slightly affects the height of the downward leader tip necessary to initiate upward leaders. Only an improvement of about 10% on the lightning attractiveness can be reached by using lightning rods with an optimum radius. Based on the obtained results, the field observations of competing lightning rods are explained. Furthermore, the influence of the average stepped leader velocity on the inception of positive upward leaders is evaluated. The results obtained show that the rate of change of the background electric field produced by a downward leader descent largely influences the conditions necessary for upward leader initiation. Estimations of the leader inception conditions for the upper and lower limit of the measured values of the average downward lightning leader velocity differ by more than 80%. In addition, the striking distances calculated taking into account the temporal change of the background field are significantly larger than the ones obtained assuming a static downward leader field. The estimations of the present model are also compared with the existing leader inception models and discussed.

A self-consistent upward leader propagation model

The knowledge of the initiation and propagation of an upward moving connecting leader in the presence of a downward moving lightning stepped leader is a must in the determination of the lateral attraction distance of a lightning flash by any grounded structure. Even though different models that simulate this phenomenon are available in the literature, they do not take into account the latest developments in the physics of leader discharges. The leader model proposed here simulates the advancement of positive upward leaders by appealing to the presently understood physics of that process. The model properly simulates the upward continuous progression of the positive connecting leaders from its inception to the final connection with the downward stepped leader (final jump). Thus, the main physical properties of upward leaders, namely the charge per unit length, the injected current, the channel gradient and the leader velocity are self-consistently obtained. The obtained results are compared with an altitude triggered lightning experiment and there is good agreement between the model predictions and the measured leader current and the experimentally inferred spatial and temporal location of the final jump. It is also found that the usual assumption of constant charge per unit length, based on laboratory experiments, is not valid for lightning upward connecting leaders.

A Study of Upward Positive Leaders Based on Simultaneous Observation of E-Fields and High-Speed Images

We have measured the correlated sub-microsecond E-fields and high-speed images of nine upward positive leaders occurred in winter from high grounded —structures at Hokuriku areas of Japan. It is found that all the leaders are initiated without any in-cloud discharge activity as their triggers. The average leader speeds we measured are around 106 m/s. All the leaders appear to contain a few pulses soon after their initiations. Pulse rise time ranges from 3.1 μs to 71.4 μs with a GM (Geometric Mean) value of 27.2 μs. Pulse decay time varies from 3.7 μs to 148.8 μs with a GM value of 30.9 μs. Pulse time duration distributes from 12.6 μs to 208.3 μs with a GM value of 59.8 μs. Most of the pulses are the same type of ICC (Initial Continuous Current) pulses. In addition, several correlations such as the light intensity just prior to each pulse versus pulse peak intensity are presented for the pulses.

Spectral Patterns of Lightning Radiations in Intervals of 25 to 100 MHz

Characteristic average power spectra in intervals from 25 to 100MHz are presented for electromagnetic radiation fields from some specific lightning events: preliminary breakdown processes, downward negative stepped- and dart- leaders, return strokes, K changes in cloud-to-ground lightning (CG) and in intra-cloud lightning (IC) flashes, and upward positive leaders in artificially triggered lightning flashes. The spectra are obtained by means of the Fourier transformation of wideband records of electric fields from lightning. Results show that the spectral patterns for these events are obviously dissimilar in the frequency band studied. The spectrum for the IC, the preliminary breakdown process and the K change usually peaks once around 60MHz, while that for the downward negative leader peaks initially around 60-70MHz then around 26MHz as the leader propagates down out the clouds. The spectrum for the upward positive leader in artificially triggered lightning, however, peaks at about 26MHz, whether the leader is propagating below or within the clouds. The dissimilarities in the spectral pattern in the range of 25-100MHz between different lightning events may imply that these events have dissimilar discharge mechanisms and provide a basis for identifying these different discharge events.

Cutoff and reestablishment of current in rocket‐triggered lightning

A total of three negative rocket‐triggered lightning flashes without return strokes (two from 1997 and one from 1993) are analyzed in this paper in order to study the processes associated with the disintegration of the triggering wire and its replacement by an air‐plasma channel. It appears that the gap resulting from the vaporization of the triggering wire by the upward‐positive leader current is bridged by a leader/return‐stroke type process. Electric fields at distances of 50, 110, and 500 m, the corresponding magnetic fields at 500 m, and the currents to ground are examined for the two 1997 flashes. The electric field prior to the triggering wire's vaporization in these flashes exhibits a positive (atmospheric electricity sign convention) millisecond‐scale ramp due to the upward‐extending positive leader. The electric field changes observed at the three distances just prior to wire vaporization are consistent with an equivalent point charge of about 0.3 C at a height of 1.2 to 1.5 km, suggesting that the charge density distribution at that time is strongly skewed toward the upward positive leader tip. The length of the triggering wire at the time of its vaporization was estimated from still photographs to be about 210–220 m. Following the ramp, a microsecond‐scale V‐shaped negative pulse, which resembles the close electric field signature of a small dart‐leader/return‐stroke sequence, is observed. The corresponding magnetic field decreases abruptly, simultaneously with the onset of the leading edge of the V‐shaped pulse, to values near zero and remains there for tens of microseconds, indicating the attempted interruption (cutoff) of the upward positive leader current flow to ground through the triggering wire as it is vaporized by this current. Following the abrupt decrease, the magnetic field exhibits a rapid increase at a time corresponding to the trailing edge of the V‐shaped electric field pulse, suggesting that the vaporized triggering wire is replaced by an air‐plasma channel that becomes part of the upward positive leader channel when electrical connection to ground is restored. For the third triggered lightning flash, from 1993, similar inferences regarding the processes involved in the replacement of the triggering wire by an air‐plasma channel are made from high‐speed (streak) photography and from measurements of the current to ground and electric field at 30 m. In this flash, the upward positive leader exhibited very pronounced stepping: step current pulses had peaks, as measured at the ground, that were up to a few kiloamperes, and step charges that were up to 100 mC. Characterization of the attempted interruption and the following reestablishment of current to ground in rocket‐triggered lightning may have important implications for the understanding of channel current cutoff in natural lightning flashes and may provide new insights into the formation of strokes observed to occur in the same channel within a millisecond or less.

Characteristics of Leader Processes in Altitude‐Triggered Lightning

AbstractThe physical processes of leader in one triggered lightning flash using the altitude‐triggering technique are studied. The main characteristics involved in the triggered lightning are inferred from data of simultaneous electric field change measurements at two stations and highly time‐resolved optical signals. The results reveal the characteristics of leader propagation in triggered lightning. When rocket attached thin steel wire is ascending to the height of several hundred meters, a bi‐directional leader is initiated at top and bottom end of the wire. When the negative leader propagating downward approaches to the ground, a positive connecting leader is initiated from the ground and propagates upwardly. The mean speed of the negative and connecting positive leader is around 0.86×105 m/s. A mini‐return stroke is produced with the connection to the ground of the downward negative leader. The upward positive leader initiated from the top end of wire propagates at a speed of about 1.1×105 m/s.

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse-I: experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse-II: modeling

We have used the Bondio-Gallimberti model of positive leader propagation to simulate laboratory experiments of laser triggering and guiding of upward leaders initiated from a ground rod. The model proves to be capable of reproducing all the important features of laser-guided leader propagation that have been observed experimentally. The leader guiding effect of the laser-created plasma channel is taken into account in the model by adjusting the value of the charge per unit length of the leader, which has been measured in the laboratory to be lower for a laser-guided leader than for an ordinary one. The charge per unit length of the leader is related in the model to the critical temperature at which the air in the transition region at the leader tip must be heated to be conductive enough to become a new leader portion. For an ordinary leader, this critical temperature is 1500 K, at which the electrons all detach from the negative ions in the leader corona, increasing the air conductivity. We give the interpretation that in the case of the laser-guided leaders, because of the relatively high density of negative ions per unit length in the laser-ionized channel, the right conditions of conductivity can be met in the transition region without the electrons being all detached from the ions, allowing a reduction of the critical temperature and of the charge per unit length.