Natural Lightning Flashes Research Articles

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.

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.

Spatiotemporal Correlation between Artificially Triggered and Adjacent Natural Lightning Flashes

A triggered lightning flash (TLF) provides a unique perspective on the relationship between spatiotemporal proximity flashes, owing to its determined location and time, convenient direction measurement, and explicit association with the charge region. In this study, 3-D lightning location, current measurement, and atmospheric average electric field (AAEF) data were used to investigate the spatiotemporal relationship of TLFs (68 samples in South China) with adjacent natural lightning flashes (NLFs). The TLF-related negative charge regions had an average core height of 5.2 km and ambient temperature of approximately −1.7 °C. The effective negative charge region (the charge density that was high enough for the occurrence of lightning discharge) can be approximately equivalent to a circle with an average diameter of 10.3 km. For approximately 93% of (all) the TLFs, no NLF channel (initiation) was located within 5 km of the flash-triggered position, within 5 s before and after their occurrence. In situations where spatiotemporally adjacent NLFs and TLFs occurred, they were either associated with different charge layers or the same charge layer but different charge positions. Most NLFs that caused significantly sharp AAEF changes just before or after the TLFs were not associated with the TLF-related negative charge. Therefore, the recovery of the AAEF, which has usually been referenced as the timing choice of the triggering operation, was not directly associated with the TLF-related charge region. The average interval between the TLFs and NLFs that occurred within 10 min before and after the TLFs and neutralized the TLF-related negative charge was approximately 145 s.

Ground‐Based Observation of a TGF Occurring Between Opposite Polarity Strokes of a Bipolar Cloud‐To‐Ground Lightning Flash

AbstractWe present the ground‐based observation of a terrestrial gamma‐ray flash (TGF) that occurred between positive (second) and negative (third) strokes of a five‐stroke bipolar cloud‐to‐ground lightning flash. Those two strokes shared the same channel to ground at a distance of 200 m or so from the Lightning Observatory in Gainesville (LOG), Florida. Earlier TGF observations at ground level in Florida (a total of four) were either associated with the initial continuous current (ICC) of rocket‐triggered lightning flashes or occurred during the relatively steady current following the return‐stroke current peak in natural lightning flashes; that is, in the presence of current‐carrying channel to ground. The TGF presented here occurred in a different context: at the early (in‐cloud) stage of negative leader entering the remnants of the channel previously created by the positive stroke. The TGF had a duration of 35 μs and consisted of 18 pulses with amplitudes ranging from 114 to 912 keV. The overall flash context in which the TGF occurred was as follows. The first (143‐kA) stroke was negative and terminated on ground 1.4 km from LOG, the second (12‐kA) stroke was positive and forged a new path to ground about 200 m from LOG, and the third (28‐kA) stroke (TGF producer) was negative and followed the path of the second (positive) stroke. The fourth (43‐kA) stroke was negative and created a new termination on ground about 900 m from LOG, and the fifth (4.5‐kA) stroke was also negative and followed the fourth‐stroke channel.

Measurement of Return Stroke Current With Magnetic Sensor in Triggered Lightning

Based on the magnetic field recorded with a magnetic sensor deployed at 130-m distance from the channel base, we remotely measured the return stroke current waveform in classical rocket-triggering lightning flashes, including 40 M-components and 48 return strokes (RSs) in eight triggered flashes observed during the summer of 2019. The current waveform retrieved from the close magnetic field is in excellent agreement with the coaxial shunt measurement. For return strokes, the amplitude error of the peak current is less than 1 kA. The error of the 30%-90% risetime is less than 0.1 μs. The error of the half-peak width is less than 1 μs. The error of charge transfer within 0.1 ms of the return strokes is less than 0.1 C. For M-components, the amplitude error of the peak current is about less than 0.3 kA. The error of the 30%-90% risetime is generally less than 4%. The error of the half-peak width is approximately less than 3%. The error of charge transfer within 0.1 ms of M-components is less than 0.02 C. The magnetic sensor is likely applicable to measure lightning current without direct current measurement at near range in rocket-triggered lightning or natural lightning flashes.

Progression Features of Dart Leaders in Natural Negative Cloud‐to‐Ground Lightning Flashes

AbstractWe have analyzed the propagation process of nine dart leaders in four natural negative lightning flashes observed by a high speed optical system LAPOS 5. The propagation speeds range from 0.6 to 3.7×107 m/s with an average value of 1.5×107 m/s. The leader luminosity peaks at the front part of the leader head and decreases gradually backward within the head. The leader channel behind the leader head has a relatively low and constant luminosity. The average luminosity of the leader head is about 1.3 times brighter than that of the leader channel behind. The head lengths of seven dart leaders were estimated to range from 21 to 88 m. Our results show that as the ground is approached, the dart leader propagates faster with its leader head growing brighter and longer. For different strokes within an individual lightning flash, it was found that the larger change ratio of leader head brightness relates with the larger change ratio of propagation speed and the larger change ratio of leader head length.

Characteristics of Negative Flashes With Multiple Ground Strike Points Located by the Japanese Lightning Detection Network

Approximately 50% of natural lightning flashes make multiple contacts on the ground. Lightning-induced voltage by nearby lightning stroke is the major cause of overvoltage. The electromagnetic waves emitted by lightning flashes with multiple ground strike points (GSPs) increase new risks to damage electronics information devices located in the vicinity of the GSPs. Therefore, it is very important for electromagnetic compatibility engineering to clarify the characteristics of lightning flashes with multiple GSPs. We have observed lightning discharges over the whole area of Japan detected by the Japanese Lightning Detection Network (JLDN). Specifically, we analyzed negative cloud-to-ground (CG) flashes observed by JLDN within 80 km of Sagamihara in summer. We investigated the median location accuracy of the JLDN in the observation area to evaluate characteristics of spatial distribution of subsequent strokes hitting the GSPs. The median location accuracy varied between 0.2 and 0.4 km in the observed area. The mean multiplicity of negative CG flash observed by JLDN was 3.5. The average inter-stroke interval of new ground contact (NGC) strokes was longer than that of pre-existing channel (PEC) strokes. The ratio of peak current to the first return stroke was higher in the NGC than that in the PECs. Some of flashes hit the ground at several points. The mean number of GSPs was 2.0 per flash, with the mean number of 1.6 subsequent strokes per a GSP. Furthermore, the number of strokes per a GSP has not been previously reported. We also observed that the subsequent strokes altered between different GSPs.

Acoustical Measurement of Natural Lightning Flashes: Reconstructions and Statistical Analysis of Energy Spectra

AbstractTwenty‐seven natural flashes of three thunderstorms which occurred during October 2012 in Southern France are acoustically reconstructed and analyzed in the [0.1 − 180]‐Hz frequency bandwidth and the [0.3 − 20]‐km distance range. A 50‐m triangular array of four recalibrated microphones sampled at 500 Hz has been used for the recording. A novel method of separation of return strokes from intracloud discharges within the acoustical signal is detailed and systematically applied. It shows the possibility to separate nearby cloud‐to‐ground (CG) events or nearby and distant CGs of the same flash. The separation method yields a total of 36 return stroke signals and spectra, along with some intracloud signals. The combination of reconstruction, separation, and frequency analysis provides new insights on the origin of thunder infrasound, showing unambiguously that thunder infrasound originate dominantly from return strokes. Apart from the higher amplitude of CGs, no clear difference between intracloud and CG spectra is observed. No sharp frequency peaks can be put into evidence. The spectral variability with distance is highlighted, especially for the total acoustic energy and the center frequency and bandwidth. A link between acoustic energy and impulse charge moment change is also found, though only by a small number of data.

On the origin of thunder: Reconstruction of lightning flashes, statistical analysis, and modeling

Thunder from 27 natural lightning flashes of three thunderstorms has been recorded in 2012 in Southern France in the 0.1–180 Hz frequency bandwidth, using a 50 m-wide triangular array of 4 recalibrated microphones in the 0.3–20 km distance range from lightning. Source reconstruction allows to separate, within the acoustical signal, Cloud-to-Ground (CG) from Intra-Cloud (IC) parts of the discharge. The possibility to separate nearby CG events is shown. A total of 36 CG signals and associated spectra is obtained, along with some IC signals. The combination of reconstruction, separation, and frequency analysis provides new insights on the origin of thunder. Thunder infrasound is shown unambiguously to originate dominantly from return strokes. Spectra of CGs and ICs are similar, but of higher amplitude for CGs. No sharp frequency peaks can be clearly evidenced. The influence of distance, therefore of propagation effects, is pointed out. Best fits of energy and frequency gravity center dependence with distance...

Attachment process in subsequent strokes and residual channel luminosity between strokes of natural lightning

AbstractWe examined high‐speed video and electric field records of 23 subsequent strokes following the previous stroke channel in three natural negative lightning flashes, which were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Five strokes exhibited faintly luminous formations (FLFs) occurring in a single, pre‐return‐stroke frame and ranging from 130 to 908 m in length between the lower end of downward leader and the prospective strike point. The FLFs were inferred to be not streamers (as in first strokes) but manifestations of an increase in conduction current in the defunct channel between the leader and ground in response to the increasing electric field produced by the descending leader. Further, in eight strokes, we observed residual channel luminosity persisting over many frames (for 4.7 to 18 ms), through the pre‐return‐stroke frame. The residual luminosity was apparently associated with a stronger channel heating and a larger channel radius (and hence a lower temperature decay rate), both associated with the relatively long preceding continuing current. Presence of either FLF or residual channel luminosity did not appear to significantly influence the mode and velocity of propagation of the descending leader.

Lightning attachment processes of three natural lightning discharges

Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the attachment process for six strokes in three natural lightning flashes. All strokes initiate at a height above ground and propagate bidirectionally from that height, similar to the return strokes of artificially initiated (triggered) lightning previously reported by Wang et al. (2013, 2014). Though the data are quite limited, these natural return strokes suggest a correlation between larger peak current and greater initiation height. Initiation heights determined here span 12–60 m with a typical uncertainty of less than 10 m. The initial upward return stroke luminosity speeds range from (0.8 ± 0.2) to (2.0 ± 0.4) × 108 m/s. Two first return strokes downward luminosity speeds are assessed as (1.6 ± 0.3) × 107 m/s and (1.4 ± 0.3) × 108 m/s. One of the first return strokes appeared to be initiated with a stepping pulse discharge of its leader as an inseparable part of the return stroke.

Theoretical and experimental study of the formation conditions of stepped leaders in negative flashes

Natural lightning flashes are stochastic and uncontrollable, and thus, it is difficult to observe the formation process of a downward negative stepped leader (NSL) directly and in detail. This situation has led to some dispute over the actual NSL formation mechanism, and thus has hindered improvements in the lightning shielding analysis model. In this paper, on the basis of controllable long air gap discharge experiments, the formation conditions required for NSLs in negative flashes have been studied. First, a series of simulation experiments on varying scales were designed and carried out. The NSL formation processes were observed, and several of the characteristic process parameters, including the scale, the propagation velocity, and the dark period, were obtained. By comparing the acquired formation processes and the characteristic parameters with those in natural lightning flashes, the similarity between the NSLs in the simulation experiments and those in natural flashes was proved. Then, based on the local thermodynamic equation and the space charge estimation method, the required NSL formation conditions were deduced, and the space background electric field (Eb) was proposed as the primary parameter for NSL formation. Finally, the critical value of Eb required for the formation of NSLs in natural flashes was determined to be approximately 75 kV/m by extrapolation of the results of the simulation experiments.

Performance Evaluation for a Lightning Location System Based on Observations of Artificially Triggered Lightning and Natural Lightning Flashes

Abstract Performance evaluation for the lightning location system (LLS) of the power grid in Guangdong Province, China, was conducted based on observation data of the triggered lightning flashes obtained in Conghua, Guangzhou, during 2007–11 and natural lightning flashes to tall structures obtained in Guangzhou during 2009–11. The results show that the flash detection efficiency and stroke detection efficiency were about 94% (58/62) and 60% (97/162), respectively. The arithmetic mean and median values for location error were estimated to be about 710 and 489 m, respectively, when more than two reporting sensors were involved in the location retrieval (based on 87 samples). After eliminating one obviously abnormal sample, the absolute percentage errors of peak current estimation were within 0.4%–42%, with arithmetic mean and median values of about 16.3% and 19.1%, respectively (based on 21 samples).

Evaluation of U.S. National Lightning Detection Network performance characteristics using rocket-triggered lightning data acquired in 2004–2009

[1] We evaluated performance characteristics of the U.S. National Lightning Detection Network (NLDN) using rocket-triggered lightning data acquired in 2004-2009 at Camp Blanding, Florida. A total of 37 negative flashes that contained leader/return stroke sequences (a total of 139) were triggered during these years. For all the return strokes, locations of channel terminations on the ground were known exactly, and for 122 of them currents were measured directly using noninductive shunts. The NLDN recorded 105 Camp Blanding strokes in 34 flashes. The resultant flash and stroke detection efficiencies were 92% and 76%, respectively. The median absolute location error was 308 m. The median NLDN-estimated peak current error was -6.1%, while the median absolute value of current estimation error was 13%. Strokes in classical triggered flashes are similar to regular subsequent strokes (following previously formed channels) in natural lightning, and hence the results presented here are applicable only to regular negative subsequent strokes in natural lightning. The flash detection efficiency reported here is expected to be an underestimate of the true value for natural negative lightning flashes, since first strokes typically have larger peak currents than subsequent ones.

Attachment of Natural Lightning Flashes to Trees: Preliminary Statistical Characteristics

Attachment of Natural Lightning Flashes to Trees: Preliminary Statistical Characteristics

Correlation between Brightness and Channel Currents of Electrical Discharges

Channel brightness of 500 mm long electrical discharges were measured by analyzing a set of digitized images taken by 3 cameras placed symmetrically around a discharge gap at a radial distance of 200 cm from the axis of the spark. The sparks were generated between a steel rod and a plane electrode. The distribution of the brightness across the channel represented a Gaussian distribution. A linear correlation was seen between the channel brightness measured by different cameras looking at the same spark channel. No correlation was seen between the channel brightness and the channel depth (direction perpendicular to the camera plane). The measured peak current and the brightness of the main spark channel show a high degree of correlation (R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> =0.97). The sum of brightness of branches was equal to the brightness of the parent channel. One can use this result to calculate the relative distribution of branch currents in complex electrical discharges including natural lightning flashes. If the current in the parent channel is known, branch currents can be calculated by measuring the optical intensities using photographic techniques.

Return stroke peak current observations of negative natural and triggered lightning in Brazil

Observations of return stroke peak currents of negative natural and triggered lightning flashes in the Southeast region of Brazil are reviewed based on recent direct measurements. The natural flashes, both downward and upward, were captured in a 60 m tower in the Morro do Cachimbo Station. The triggered flashes were induced by the rocket-and-wire technique in the International Center for Triggered and Natural Lightning Studies, located in Cachoeira Paulista, Brazil, using both the classical and the altitude methods. Large variations in the peak current values were observed. The two data sets are compared to each other and to similar measurements done in other regions of the world, in an attempt to construct a general comprehensive picture of the peak current of different return strokes.

Measurement of the rear‐face temperature of metals struck by lightning long‐duration currents

AbstractThis paper presents an experimental investigation of the rear‐face temperature rise of different metals struck by lightning long‐duration currents on the front face. The experimental investigation simulates the heating effect of one current component of the natural lightning flashes that may strike in‐flight or landed aircraft and rockets; munitions and nuclear weapons; and storage/collection tanks and pipelines of the petroleum/gas industries. In the case of a lightning strike the rear‐face temperature rise must be less than certain limits to avoid catastrophic damage/accidents. A sophisticated high‐speed infrared camera, having an image frequency of 50 Hz and a temperature range up to 1200°C, is used to measure the rear‐face temperature profiles as a function of the coordinates and time. The results are interpreted in light of the optical observation of the arc motion, physical measurement of the spot diameter, and study of the energy balance at the cathode and anode spots. The results reveal that for constant charge transfer, lightning long‐duration currents having higher amplitudes give a higher rear‐face temperature rise and hence a higher risk of accidents. Both the maximum rear‐face core temperature and the front‐face spot diameter exponentially increase with the mean current amplitude. Moreover, the maximum rear‐face core temperature varies exponentially with the charge and linearly with the action integral. Current injection with positive polarity of the test electrode gives a lower rear‐face temperature for all metals used except for aluminium where the trend is reversed. Copyright © 2004 John Wiley &amp; Sons, Ltd.

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.

Propagation of lightning generated transient electromagnetic fields over finitely conducting ground

This paper elucidates the propagation effects on lightning generated electric fields. The mathematics of the problem is described and a simple procedure that can be used to predict the propagation effect is outlined together with its experimental confirmation. This procedure is used to analyse the propagation effects, over distances less than about 300 km, on the radiation fields of negative first return strokes, positive first return strokes and subsequent return strokes in triggered lightning flashes. From the results, empirical equations that can be used to correct for the propagation effects are extracted. The results show that the attenuation due to propagation effects of the initial peak amplitudes may differ in negative and positive return strokes and that the data from triggered lightning flashes should be applied with caution in correcting for the propagation effects of natural lightning flashes.