Natural Lightning Research Articles

Protection of Digital Telecom Exchanges Against Lightning Surges and Earth Faults

Low-power electronic devices used in digital telecom exchanges are vulnerable to surge voltages and currents primarily originating from natural lightning or due to the direct interactions between electric power and telecommunication lines, etc., causing the earth/ground potential rise, neutral potential rise, and faults in the system. The fault currents may flow directly to telecom lines or through the equipment to the customer's premises, causing adequate damage to the equipment and personnel safety. In wireline applications, analog or digital, central office, exchanges, and subscriber sides have to be protected. Decisive protection and protective methods have to be employed for proper functioning of the equipment under overvoltage/overcurrent conditions. Current investigation reports some interesting results obtained on the recently developed high-voltage high-current protection cards used in digital telecom exchanges. The performances of protection cards both for the ring wave and hybrid wave surges are evaluated and presented. The surge generators required for the investigation are developed and fabricated in house as per the relevant telecom standards.

The energy spectrum of X‐rays from rocket‐triggered lightning

Although the production of X-rays from natural and rocket-triggered lightning leaders have been studied in detail over the last 10 years, the energy spectrum of the X-rays has never been well measured because the X-rays are emitted in very short but intense bursts that result in pulse pileup in the detectors. The energy spectrum is important because it provides information about the source mechanism for producing the energetic runaway electrons and about the electric fields that they traverse. We have recently developed and operated the first spectrometer for the energetic radiation from lightning. The instrument is part of the Atmospheric Radiation Imagery and Spectroscopy (ARIS) project and will be referred to as ARIS-S (ARIS Spectrometer). It consists of seven 3′′ NaI(Tl)/photomultiplier tube scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than 1′′ of lead placed over the detector (all the detectors are in a 1/8′′ thick aluminum box). Using X-ray pulses preceding 48 return strokes in 8 rocket-triggered lightnings, we found that the spectrum of X-rays from leaders is too soft to be consistent with Relativistic Runaway Electron Avalanche. It has a power law dependence on the energies of the photons, and the power index, λ, is between 2.5 and 3.5. We present the details of the design of the instrument and the results of the analysis of the lightning data acquired during the summer of 2012.

Shipborne LF-VLF oceanic lightning observations and modeling

Approximately 90% of natural lightning occurs over land, but recent observations, using Global Lightning Detection (GLD360) geolocation peak current estimates and satellite optical data, suggested that cloud-to-ground flashes are on average stronger over the ocean. We present initial statistics from a novel experiment using a Low Frequency (LF) magnetic field receiver system installed aboard the National Oceanic Atmospheric Agency (NOAA) Ronald W. Brown research vessel that allowed the detection of impulsive radio emissions from deep-oceanic discharges at short distances. Thousands of LF waveforms were recorded, facilitating the comparison of oceanic waveforms to their land counterparts. A computationally efficient electromagnetic radiation model that accounts for propagation over lossy and curved ground is constructed and compared with previously published models. We include the effects of Earth curvature on LF ground wave propagation and quantify the effects of channel-base current risetime, channel-base current falltime, and return stroke speed on the radiated LF waveforms observed at a given distance. We compare simulation results to data and conclude that previously reported larger GLD360 peak current estimates over the ocean are unlikely to fully result from differences in channel-base current risetime, falltime, or return stroke speed between ocean and land flashes.

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.

The start of lightning: Evidence of bidirectional lightning initiation.

Lightning flashes are known to initiate in regions of strong electric fields inside thunderstorms, between layers of positively and negatively charged precipitation particles. For that reason, lightning inception is typically hidden from sight of camera systems used in research. Other technology such as lightning mapping systems based on radio waves can typically detect only some aspects of the lightning initiation process and subsequent development of positive and negative leaders. We report here a serendipitous recording of bidirectional lightning initiation in virgin air under the cloud base at ~11,000 images per second, and the differences in characteristics of opposite polarity leader sections during the earliest stages of the discharge. This case reveals natural lightning initiation, propagation and a return stroke as in negative cloud-to-ground flashes, upon connection to another lightning channel – without any masking by cloud.

Generator and Setup for Emulating Exposures of Biological Samples to Lightning Strokes.

We aimed to develop a system for controlled exposure of biological samples to conditions they experience when lightning strikes their habitats. We based the generator on a capacitor charged via a bridge rectifier and a dc-dc converter, and discharged via a relay, delivering arcs similar to natural lightning strokes in electric current waveform and similarly accompanied by acoustic shock waves. We coupled the generator to our exposure chamber described previously, measured electrical and acoustic properties of arc discharges delivered, and assessed their ability to inactivate bacterial spores. Submicrosecond discharges descended vertically from the conical emitting electrode across the air gap, entering the sample centrally and dissipating radially toward the ring-shaped receiving electrode. In contrast, longer discharges tended to short-circuit the electrodes. Recording at 341 000 FPS with Vision Research Phantom v2010 camera revealed that initial arc descent was still vertical, but became accompanied by arcs leaning increasingly sideways; after 8-12 μs, as the first of these arcs formed direct contact with the receiving electrode, it evolved into a channel of plasmified air and short-circuited the electrodes. We eliminated this artefact by incorporating an insulating cylinder concentrically between the electrodes, precluding short-circuiting between them. While bacterial spores are highly resistant to electric pulses delivered through direct contact, we showed that with arc discharges accompanied by an acoustic shock wave, spore inactivation is readily obtained. The presented system allows scientific investigation of effects of arc discharges on biological samples. This system will allow realistic experimental studies of lightning-triggered horizontal gene transfer and assessment of its role in evolution.

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.

The significance of median natural lightning current strokes on the energy handling capabilities of surge arresters employed in wind farms

The significance of the median natural lightning currents on the energy handling capabilities of surge arresters employed in wind farms is investigated. The Electromagnetic Transient Program Alternative Transient Program (EMTP-ATP) was used to simulate direct lightning strikes to the blade of an existing wind turbine in order to determine the current discharged through the surge arresters and hence the energy absorbed by the surge arresters located at different wind turbines. The values of the energy handling capabilities of surge arresters are generally published in manufacturers' datasheets. Yet, the impact of the subsequent lightning strokes is usually ignored. Also, the published energy handling capabilities are based solely on standard waveforms. This contribution uses two sets of natural lightning current parameters measured on instrumented towers at Mount San Salvatore and Morro do Cachimbo, to simulate lightning strikes to the wind turbine blade. The results show that the influence of the subsequent strokes can be significant for the current discharged through the surge arresters and the subsequent energy absorbed, especially at the stricken turbine.

Optical emission and peak electromagnetic power radiated by negative return strokes in rocket-triggered lightning

Calibrated measurements of the optical radiation produced by negative return strokes in rocket-triggered lightning (RTL) have been made in the visible and near infrared (VNIR) spectral region in correlation with currents measured at the channel base. Using a simple transmission-line model, the currents have been used to estimate the peak electromagnetic (EM) fields and Poynting power that are radiated in the time-domain (i.e. from about 1kHz to 3MHz). The results show that the optical power radiated by RTL at the time of the peak current has a mean and standard deviation of 130±120MW, a value that is only about 5% of the Poynting power that is radiated into the upper half-space at that time. These results are in good agreement with similar measurements made on the subsequent return strokes in natural lightning that remain in a pre-existing channel. Our methods and assumptions are similar to those of (Guo and Krider, 1983; Krider and Guo, 1983; Quick and Krider, 2013).

Time domain simulations of preliminary breakdown pulses in natural lightning.

Lightning discharge is a complicated process with relevant physical scales spanning many orders of magnitude. In an effort to understand the electrodynamics of lightning and connect physical properties of the channel to observed behavior, we construct a simulation of charge and current flow on a narrow conducting channel embedded in three-dimensional space with the time domain electric field integral equation, the method of moments, and the thin-wire approximation. The method includes approximate treatment of resistance evolution due to lightning channel heating and the corona sheath of charge surrounding the lightning channel. Focusing our attention on preliminary breakdown in natural lightning by simulating stepwise channel extension with a simplified geometry, our simulation reproduces the broad features observed in data collected with the Huntsville Alabama Marx Meter Array. Some deviations in pulse shape details are evident, suggesting future work focusing on the detailed properties of the stepping mechanism.Key PointsPreliminary breakdown pulses can be reproduced by simulated channel extension Channel heating and corona sheath formation are crucial to proper pulse shape Extension processes and channel orientation significantly affect observations

Seasonality of fire weather strongly influences fire regimes in South Florida savanna-grassland landscapes.

Fire seasonality, an important characteristic of fire regimes, commonly is delineated using seasons based on single weather variables (rainfall or temperature). We used nonparametric cluster analyses of a 17-year (1993–2009) data set of weather variables that influence likelihoods and spread of fires (relative humidity, air temperature, solar radiation, wind speed, soil moisture) to explore seasonality of fire in pine savanna-grassland landscapes at the Avon Park Air Force Range in southern Florida. A four-variable, three-season model explained more variation within fire weather variables than models with more seasons. The three-season model also delineated intra-annual timing of fire more accurately than a conventional rainfall-based two-season model. Two seasons coincided roughly with dry and wet seasons based on rainfall. The third season, which we labeled the fire season, occurred between dry and wet seasons and was characterized by fire-promoting conditions present annually: drought, intense solar radiation, low humidity, and warm air temperatures. Fine fuels consisting of variable combinations of pyrogenic pine needles, abundant C4 grasses, and flammable shrubs, coupled with low soil moisture, and lightning ignitions early in the fire season facilitate natural landscape-scale wildfires that burn uplands and across wetlands. We related our three season model to fires with different ignition sources (lightning, military missions, and prescribed fires) over a 13-year period with fire records (1997–2009). Largest wildfires originate from lightning and military ignitions that occur within the early fire season substantially prior to the peak of lightning strikes in the wet season. Prescribed ignitions, in contrast, largely occur outside the fire season. Our delineation of a pronounced fire season provides insight into the extent to which different human-derived fire regimes mimic lightning fire regimes. Delineation of a fire season associated with timing of natural lightning ignitions should be useful as a basis for ecological fire management of humid savanna-grassland landscapes worldwide.

Mass spectrometry of atmospheric-pressure ball plasmoids

Ball lightning is a naturally occurring atmospheric event that has perplexed researchers for centuries, and there is to date no complete explanation (chemical, physical, or otherwise) as to why ball lightning behaves the way that it does. There has been considerable effort to try to both produce and measure the properties of ball lightning type discharges over recent years, and this collected work has begun to reveal some interesting physical and chemical phenomena. We are able to produce water-based plasma ball discharges using high-voltage equipment, and these self-contained plasmoids are considered to be similar to natural ball lightning. In this article we present the first mass spectrometric analysis of water-based ambient ball plasmoids. Using an extremely simple sampling technique, we were able to detect several chemical species within the interior of the plasmoid. Several molecules that are common to plasmas generated in air were observed in the mass spectra, such as [NO2]+ and [NO3]+. More interestingly, we observed the protonated water clusters [(H2O)2H]+ and [(H2O)3H]+, ammonia (NH3) as a component of a copper cluster, and several anions. Furthermore, many species observed in the mass spectra are in the form of hydrated clusters.

Rocket‐triggered lightning propagation paths relative to preceding natural lightning activity and inferred cloud charge

AbstractLightning Mapping Array (LMA) data are used to compare the propagation paths of seven rocket‐triggered lightning flashes to the inferred charge structure of the thunderstorms in which they were triggered. This is the first LMA study of Florida thunderstorm charge structure. Three sequentially (within 16 min) triggered lightning flashes, whose initial stages were the subject of Hill et al. (2013), are reexamined by comparing the complete flashes to the preceding natural lightning to demonstrate that the three rocket‐triggered flashes propagated through an inferred negative charge region that decreased from about 6.8 to about 4.4 km altitude as the thunderstorm dissipated. Two other flashes were also sequentially triggered (within 9 min) in a thunderstorm that contained a convectively intense region ahead of a stratiform region, with similar observed results. Finally, two unique cases of triggered lightning flashes are presented. In the first case, the in‐cloud portion of the triggered lightning flash, after ascending to and turning horizontal at 5.3 km altitude, just above the 0°C level, was observed to very clearly resemble the geometry of the in‐cloud portion of the preceding natural lightning discharges. In the second case, a flash was triggered relatively early in the storm's lifecycle that did not turn horizontal near the 0°C level, as is usually the case for triggered lightning in dissipating storms, but ascended to nearly 7.5 km altitude before exhibiting extensive horizontal branching.

Charge on luminous bodies resembling natural ball lightning produced via electrical arcs through lump silicon.

A phenomenon resembling natural ball lightning can be produced via electrical arcing through silicon. We use lump silicon instead of silicon wafers to achieve higher production rates and larger, longer-lived luminous balls than previously reported. The luminous balls consist of a silicon core surrounded by a porous network of loosely bound silicon dioxide nanoparticles. We find that the balls carry a small net charge on the order of 10(-12) C and propose that the nanoparticles are electrostatically bound to the core due to this charge.

A negative cloud-to-ground flash showing a number of new and rarely observed features

An unusual natural lightning flash containing two branched negative strokes to ground was recorded at the Lightning Observatory in Gainesville, Florida, on 8 June 2013. The flash was apparently a bolt from the blue, whose first-stroke leader emerged from the side of the cloud at a height of about 3.5 km above ground level. The first leader showed profuse branching and what appears to be corona-like formations with spatial extent of 100 to 200 m (probably an intensified portion of radial corona sheath) at the upper part of the channel. Leader branching process facilitated by two simultaneous space stems was observed. The corresponding step lengths were estimated to be 14 and 15 m. The first-stroke attachment process involved a streamer zone about 50 m in length. One of the second-leader branches appeared to abruptly change its direction at the beginning of return stroke process.

A stepped leader model for lightning including charge distribution in branched channels

The stepped leader process in negative cloud-to-ground lightning plays a vital role in lightning protection analysis. As lightning discharge usually presents significant branched or tortuous channels, the charge distribution along the branched channels and the stochastic feature of stepped leader propagation were investigated in this paper. The charge density along the leader channel and the charge in the leader tip for each lightning branch were approximated by introducing branch correlation coefficients. In combination with geometric characteristics of natural lightning discharge, a stochastic stepped leader propagation model was presented based on the fractal theory. By comparing simulation results with the statistics of natural lightning discharges, it was found that the fractal dimension of lightning trajectory in simulation was in the range of that observed in nature and the calculation results of electric field at ground level were in good agreement with the measurements of a negative flash, which shows the validity of this proposed model. Furthermore, a new equation to estimate the lightning striking distance to flat ground was suggested based on the present model. The striking distance obtained by this new equation is smaller than the value estimated by previous equations, which indicates that the traditional equations may somewhat overestimate the attractive effect of the ground.

Identification of striking distance considering the velocity ratio and competition among the leaders

The striking distance is an important parameter to design a reliable lightning protection system. To protect an object against direct strikes, an improved leader progression model combined fractal geometry and the leader initiation criteria proposed by M. Becerra is introduced to determine the striking distance on a given structure. Both the deterministic and stochastic features of the lightning leader are considered and more detailed three-dimensional numerical simulation of the leader discharge is achieved due to the application of extended grid. Some of the basic methods and strategies are outlined and the influence on protection zone caused by leader velocity ratio is analyzed. Moreover, a comparative analysis of the results obtained through the improved model, the rolling sphere method and the Cooray model is performed. The results indicate that the presented method is closer to the natural lightning and the selection of the breakdown point is actually a complex competitive process among numerous upward leaders.

Lightning activity following the return stroke

Natural lightning is both frequent and variable and thus a good subject for statistical studies. A typical negative cloud-to-ground (CG) flash consists of multiple individual return strokes. The spatial and temporal distributions of various lightning events throughout the discharge provide a surrogate look inside the CG flash and offer insight into the underlying physical processes. In this study, we combine 8years of National Lightning Detection NetworkTM (NLDN) and North Alabama Lightning Mapping Array (NALMA) data to compute the spatial and temporal distributions of (i) subsequent NLDN-reported return strokes and (ii) LMA-reported sources around NLDN-reported CG strokes. Subsequent strokes are separated into those with the same contact point as the first stroke and those flowing along new lightning channels. Statistically, the distribution of strokes along new channels evolves deterministically, with ∼200 km/s propagation speed from the original channel, comparable to the speed of a stepped leader. This suggests that the −CG subsequent strokes forming new channels may be directly linked to the initial one by a propagating leader inside the cloud. We present LMA case studies and a multiyear analysis of NLDN-LMA data that support this behavior. Our results are supported by ground-truth measurements and video recordings from previous field studies.

Lightning Observatory in Gainesville (LOG), Florida: A review of recent results

A review of several recent studies conducted at the Lightning Observatory in Gainesville (LOG), Florida, is given, including (a) evaluation of field-to-current conversion equations, (b) lower positive charge in the cloud and lightning type, (c) positive lightning, (d) compact intracloud lightning discharges, (e) lightning interaction with the ionosphere, and (f) X-rays produced by first and subsequent strokes in natural lightning. The results serve to improve our understanding of the physics of lightning with important implications for lightning detection and lightning protection areas.

Registration of X‐rays at 2500 m altitude in association with lightning flashes and thunderstorms

AbstractElectric fields and high‐energy radiation of natural lightning measured at close range from a mountaintop tower are discussed. In none of the 12 negative cloud‐to‐ground upward flashes were X‐rays observed. Also no energetic radiation was found in one negative upward leader at close range (20 m). In the first of two consecutive negative cloud‐to‐ground flashes, X‐rays were detected during the last ~1.75 ms of the leader. During the time of energetic radiation in the flash an intense burst of intracloud VHF sources was located by the interferometers. The X‐ray production is attributed to the high electric field runaway electron mechanism during leader stepping. Even though the second flash struck closer than the previous one, no X‐rays were detected. The absence of energetic radiation is attributed to being outside of the beam of X‐ray photons from the leader tip or to the stepping process not allowing sufficiently intense electric fields ahead of the leader tip. High‐speed video of downward negative leaders at the time when X‐rays are commonly detected on the ground revealed the increase of speed and luminosity of the leader. Both phenomena allow higher electric fields at the leader front favoring energetic radiation. Background radiation was also measured during thunderstorms. The count rate of a particular day is presented and discussed. The increases in the radiation count rate are more coincident with radar reflectivity levels above ~30 dBZ than with the total lightning activity close to the site. The increases of dose are attributed to radon daughter‐ion precipitation.