Lightning Electromagnetics. Volume 1: Return stroke modelling and electromagnetic radiation (2nd Edition)
2: Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
3: School of Engineering and Management Vaud, HES-SO University of Applied Sciences and Arts Western Switzerland, Switzerland
Lightning is important for all scientists and engineers involved with electric installations. It is gaining further relevance since climate warming is causing an increase in lightning strikes, and since the rising numbers of renewable power generators, the electricity grid, and charging infrastructure are susceptible to lightning damage. This is the second edition to this comprehensive work.
Both volumes have been thoroughly revised and updated for this second edition. Volume 1 treats lightning return stroke modelling and lightning electromagnetic radiation, and Volume 2 addresses electrical processes and effects. Chapter coverage includes various models and simulations of lightning strokes, measurements of lightning-generated EM fields, HF, VHF and microwave radiation, and lightning location systems; atmospheric discharge processes, lightning strikes to grounded structures and towers, EM field propagation, interaction with cables, effects on power transmission and distribution systems, effects in the ionosphere, mesosphere and magnetosphere, as well as NO x generation and climate effects. The volumes provide the rules and procedures to combine the readers' understanding with a model of every lightning-related electromagnetic process, and their effects and interactions. Readers obtain first-hand experience through simulations of the EM field of thunderclouds and lightning flashes and their effects.
These volumes are a valuable resource for researchers and engineers in the areas of electrical engineering and physics involved in the fields of electromagnetic compatibility, lightning protection, renewable energy systems, smart grids, and lightning physics, as well as for professionals from telecommunication companies and manufacturers of power equipment, and advanced students.
Inspec keywords: lightning; ionosphere; clouds; atmospheric techniques; thunderstorms
Other keywords: radiowave propagation; electromagnetic fields; corona; atmospheric electricity; thunderstorms; ionosphere; clouds; lightning; finite difference time-domain analysis; current distribution
Subjects: Cloud physics; Ionospheric electric fields and currents; Instrumentation and techniques for geophysical, hydrospheric and lower atmosphere research; Atmospheric electricity; Atmospheric storms
- Book DOI: 10.1049/PBPO127F
- Chapter DOI: 10.1049/PBPO127F
- ISBN: 9781785615399
- e-ISBN: 9781785615405
- Page count: 471
- Format: PDF
-
Front Matter
- + Show details - Hide details
-
p.
(1)
-
1 Basic electromagnetic theory - a summary
- + Show details - Hide details
-
p.
1
–49
(49)
The goal of this chapter is to provide a summary of the basic concepts of electromagnetic theory as a complement to the subject matter, most of which is related to electromagnetism, discussed in this book. The chapter covers only the concepts which are necessary to understand the electromagnetics of lightning flashes. A detailed description of the electromagnetic theory can be found in [1-3].
-
2 Application of electromagnetic fields of accelerating charges to obtain the electromagnetic fields of engineering return stroke models
- + Show details - Hide details
-
p.
51
–63
(13)
In this chapter, we have described how to calculate the electromagnetic fields generated by return strokes simulated by different types of engineering return stroke models, namely CP, CG, and CD using accelerating charge equations. The same total fields of course can also be obtained using dipole technique. Acceleration charge technique has several advantages over the dipole technique. First, note that there is no need to perform an integration of the current derivative over the length of the channel. Second, in the dipole technique in order to obtain the charge one has to integrate the total current at any given level which in turn has to be obtained numerically by using model parameters. However, in the charge acceleration equation technique, the charge necessary for the static field in most cases can be obtained analytically and thus one can skip additional numerical integrations in mathematical routines. Thus, there are many cases where the charge acceleration equations provide results that can be realized with less computations. Furthermore, the charge acceleration technique provides a direct connection between the field components and the physical process that leads to the electric field.
-
3 Basic features of engineering return stroke models
- + Show details - Hide details
-
p.
65
–82
(18)
In this chapter, we have presented the basic principles underlying engineering return stroke models together with the information necessary to use available return stroke models to evaluate the spatial and temporal variation of the return stroke current and to use that information to calculate the electromagnetic fields generated by return strokes.
It is important to note here that any new return stroke model that is introduced into the scientific literature should be able to present a new way of studying the return stroke process. On the other hand, the model parameters should be considered as information that should or could be changed when more experimental data becomes available concerning the return stroke process. Unfortunately, some scientists give more emphasis to the model parameters and by doing so losses the important message that a model builder is trying to convey to the scientific establishment. This wrong way of looking at the models also leads to the creation of "new models" by changing one or two parameters of an existing model.
-
4 Electromagnetic models of lightning return strokes
- + Show details - Hide details
-
p.
83
–136
(54)
In this chapter, electromagnetic (full-wave) models of the lightning return stroke, which have been used in lightning electromagnetic field and surge simulations, are reviewed and evaluated. In this class of models, using a numerical technique such as the method of moments or the finite-difference time-domain method, Maxwell's equations are solved to yield the distribution of current along a vertical wire that represents the lightning return-stroke channel. Lightning models are needed for specifying the source in studying lightning interaction with various systems and with the environment. Here, it is shown that a current wave necessarily suffers distortion as it propagates upward along a vertical non-zero-thickness wire above perfectly conducting ground excited at its bottom by a lumped source, even if the wire has no ohmic losses, which is a distinctive feature of this class of models. Electromagnetic models proposed to date are classified into six types depending on lightning channel representation. Channel-current distributions and resultant electromagnetic fields calculated for these channel representations are presented. Further, methods of excitation, representative numerical procedures for solving Maxwell's equations, and applications of lightning return-stroke electromagnetic models are reviewed.
-
5 Antenna models of lightning return-stroke: an integral approach based on the method of moments
- + Show details - Hide details
-
p.
137
–236
(100)
In this chapter, we have presented the antenna theory-based electromagnetic models of the lightning return stroke where the lightning return-stroke channel (RSC) is considered as a monopole wire antenna above a conducting ground. We have also described the time- and frequency-domain solutions of the governing electric field integral equation (EFIE), utilizing the method of moments. The EFIE is used to determine the current distribution along the channel from which remote electromagnetic fields are readily computed.
In the original antenna theory (AT) model in the time domain, the lightning RSC is represented by a lossy vertical monopole antenna, which is fed at its lower end by a voltage source. The voltage waveform is specified on the basis of the assumed input current of the antenna and the antenna resistance per unit length. There are only two adjustable parameters in this model, namely, the wave propagation speed for a nonresistive channel and the value of the distributed channel resistance. Once these two parameters are specified, the spatial and temporal distributions of the current along the channel are found by solving the governing EFIE, using the method of moments. The time-domain AT model has been compared with other lightning return-stroke models in terms of current and line charge density distributions along the channel, and the predicted remote electromagnetic fields. The primary features of the time-domain AT model are as follows: (1) the current amplitude decreases and current rise time increases as the current wave propagates along the channel, in agreement with optical observations, (2) the current wave propagates along the channel at a speed lower than the speed of light due to corona effects and ohmic losses in the channel, and (3) the model-predicted electric and magnetic fields are reasonably consistent with typically measured fields.
-
6 Transmission line models of the lightning return stroke
- + Show details - Hide details
-
p.
237
–274
(38)
This chapter discusses the modeling of the return stroke channel as a transmission line. A brief review of the pertinent literature indicates that the transmission line models of the return stroke can be roughly classified as either discharge-type models, which assume the return stroke to correspond to the discharge of a previously charged transmission line to ground through a closing switch, lumped-excitation models, which assume the return stroke to correspond to an initially neutral transmission line that is fed by a lumped voltage or current source at one of its terminations, or finally as models that use transmission-line or distributed-circuit theory to infer relevant lightning properties.
A discussion is presented on the calculation of the per-unit-length parameters necessary for simulating the return stroke channel as a transmission line. It is shown that if a transmission line is intended to represent the lightning channel, it must be nonuniform (in order to accommodate the variation of the channel parameters with position) and nonlinear (in order to accommodate the temporal variation of the channel resistance as a function of the channel current and the gradual neutralization of the corona sheath surrounding the channel core). Engineering equations are presented for estimating the per-unit-length parameters associated with a vertical transmission line in the presence of nonlinear losses and corona. The proposed equations are suitable to computer simulation and can be easily implemented using a first-order FDTD scheme.
Computed results show that the modeling of the lightning channel as a lossy nonuniform transmission line in the presence of corona is able to reproduce the most important characteristics of subsequent return strokes of negative lightning, including the reduction in amplitude and increase in front time of the lightning current with increasing height, realistic return stroke speeds, realistic speed profiles, and, if a suitable set of parameters is selected, most signatures typically observed in measured electromagnetic fields. Besides confirming the consistency of modeling the return stroke channel using transmission line theory, the obtained results suggest that the consideration of relevant lightning properties let model predictions closer to measured data. It can be expected, therefore, that model predictions are likely to consistently improve if better models become available for representing the various physical processes in the lightning discharge. A transmission line model of the return stroke is flexible enough to accommodate virtually any model improvement provided it can be written, analytically or numerically, in the form of per-unit-length parameters.
-
7 Measurements of lightning-generated electromagnetic fields
- + Show details - Hide details
-
p.
275
–293
(19)
Lightning-generated electromagnetic fields consist of vertical and horizontal components. The horizontal field consists of two components depending on the alignment of the antenna. The vertical component of the electric field generated by lightning flashes can be measured either using a field mill [1] or using a flat plate (or a vertical whip) antenna [2,3]; each method has its advantages and disadvantages. The electric field's three components can be measured using specially adapted spherical antennas [4].
The conventional method used to measure the magnetic field is the crossed loop antenna [5-7], but in some studies, magnetometers have been used to measure this field component. The frequency spectrum of the electromagnetic fields generated by lightning flashes can be estimated by either Fourier transforming the broadband signal or using antenna systems tuned to the desired frequencies [8-13].
A brief description of the theory pertinent to these measuring techniques is given in this chapter. Some parts of this chapter are adapted from Refs. [14,15].
-
8 HF and VHF electromagnetic radiation from lightning
- + Show details - Hide details
-
p.
295
–315
(21)
HF-VHF emission is extremely useful in developing prediction mechanisms for extreme weather events such as severe storms and supercells, flash floods, and hurricanes, as many such events are directly correlated to cloud electrification activities. However, to utilize the RF signals for weather forecasting, two important requirements should be fulfilled; the emitted RF from a discharge event should be strong enough for remote sensing (highly advantageous if they reach satellite altitudes) and the RF emission should be correlated with a known event of atmospheric dynamics. Once these two conditions are satisfied the information could be incorporated with smart algorithms to make precise forecasting and at least nowcasting. In this respect, TIPPs and narrow bipolar pulses could make a significant impact on weather forecasting methodologies in future. There are several other lightning activities of which the broadband existence is known but the HF-VHF emission behaviour is yet to be explored.
-
9 Microwave radiation generated by lightning
- + Show details - Hide details
-
p.
317
–335
(19)
This chapter has presented a comprehensive review on the previous and current studies of microwave and VHF electromagnetic radiations generated by lightning flashes. Since the first experimental work done by Brook and Kitagawa in 1964, more research works have been conducted at various geographical regions to understand the underlying mechanism behind the emissions of VHF and microwave radiations by lightning flashes. Based on the findings of previous studies, most likely the microwave and VHF radiations have been generated by two different processes. The VHF radiation is generated by propagating streamers while microwave radiation is generated by electron avalanches and corona discharges.
-
10 The Schumann resonances
- + Show details - Hide details
-
p.
337
–364
(28)
Being a global phenomenon, SRs have numerous applications in lightning research. Background SR records can serve as a convenient and a low-cost tool for global lightning activity monitoring. The SR can provide a global geoelectric index for monitoring climate changes. It provides one of the few tools that through variations in global lightning activity, can provide continuous and long-term monitoring of such important global climate change parameters as tropical land surface temperature and tropical UTWV.
SR transients (Q-bursts) can be used to geolocate intense lightning strikes anywhere on the planet. These large-amplitude pulses are apparently related to the occurrence of sprites and elves above thunderstorms, and therefore TLEs can be studied using SR observations. An additional application of SR is extraterrestrial lightning research. SRs may be used to detect and, if necessary, monitor lightning activity on the planets and moons of the Solar System.
There are still many open questions in SR research: importance of the day-night variation in the ionosphere conductivity profile [178,179], influence of the latitudinal changes in the Earth magnetic field, impacts of cosmic, solar and geomagnetic disturbances [180,181], polar cap absorption, accuracy of source geolocation and the determination of the spatial lightning distribution from the background records [43,89,182]. Despite these open problems, SR is one of the most promising tools in a variety of fields related to lightning electromagnetics.
In summary, the knowledge of the Schumann resonances in the atmosphere, their origins, propagation, variability and changes, may have implications for monitoring global climate changes, upper atmospheric TLEs, for space missions to other planets, for space weather research, and possibly even in the field of medicine and agriculture.
-
11 High energetic radiation from thunderstorms and lightning
- + Show details - Hide details
-
p.
365
–395
(31)
Great progress has been made in the last 20 years regarding the measurement of the energetic radiation from thunderclouds and lightning and developing theory and models to explain these emissions, and with the recent launch of the Atmosphere-Space Interactions Monitor (ASIM) onboard the international space station continued progress is expected in the coming years. To date, four basic mechanisms have been used to describe the production of runaway electrons and the resulting energetic radiation: Wilson runaway electrons; RREA, relativistic feedback, and cold runaway. Although all four share some features and some underlying physics, their behavior and the regimes of applicability are sufficiently different, which is useful in treating them independently when describing the production of energetic radiation in our atmosphere. Discharges involving runaway electrons also behave very differently from conventional discharges that involve only low-energy electrons. Because runaway electrons may be involved in thundercloud and lightning processes, understanding these interesting and novel categories of electrical discharges is an important part of atmospheric physics.
-
12 Excitation of visual sensory experiences by electromagnetic fields of lightning
- + Show details - Hide details
-
p.
397
–414
(18)
Lightning flashes can interact with a human in six different ways [1-3]. These are through direct strikes, side flashes, ground currents, shock waves, connecting leaders, and electromagnetic fields. A direct strike results when the lightning channel terminates on the person's body. In this case, there is a direct current injection from the lightning flash into the body as a result of which the person generally may experience cardiac and/or respiratory arrests; about 20% of the victims die as a result. A side flash happens when the human is located close to an object struck by lightning. The potential gradient created by the current flow along the object may give rise to a discharge between the object and the human. A portion of the lightning current will flow along this discharge path and pass through the body. Depending on the path along which the current travels through the body, the injuries could be similar to that of a direct strike. Someone standing close to the point at which lightning strikes could be injured by ground currents as the current flowing through the ground short-circuits its path by passing into the body from one leg and flowing out from the other. Since the current path is not directly through the brain or the heart, the injuries tend to be less severe than those caused by a direct strike. Injuries can also arise from shock waves created by the lightning channel. During a lightning strike, the channel temperature can rise to about 25,000 K and the channel pressure can increase to several atmospheres. The rapid expansion of heated air creates a shock wave that can injure a person located close to a lightning strike. Someone standing in the vicinity of a lightning strike could also be injured by an aborted connecting leader. Connecting leaders are initiated from grounded objects, including humans, under the influence of the electric field generated by the down-coming stepped leader. The resultant electric field is concentrated on sharp points and on the tips of grounded objects, increasing the electric field at these points to several times the background electric field. In the case of a person standing, the maximum field enhancement would take place on the top of the head. The electric field continues to grow as the stepped leader approaches the ground, and when the electric field at the tip of an object exceeds a critical value, a corona discharge (known in the popular literature as St. Elmo's fire) is initiated from it. This corona discharge continues to grow as the stepped leader approaches the ground and when the charge associated with it reaches a critical level, a thermalized electrical discharge travelling from the tip towards the down-coming stepped leader is formed. Several objects, including humans, in the vicinity of a lightning channel may launch connecting leaders, but only one of them will succeed in joining with the stepped leader. The object that initiated the 'successful' connecting leader will receive a direct strike. As the electric field collapses during the lightning strike, the connecting leaders issued by other objects will be aborted, but the current generated by these aborted leaders could be large enough to cause injury especially if it happened to be created from a person's head. When the person is located at a sufficiently large distance where the electric field is not large enough to launch a connecting leader, a corona discharge may still be issued from the person's head.
The electric current passing through the body during a direct strike or during the launch of a connecting leader can be large enough to cause either cardiac arrest or a respiratory arrest, or both. If, however, someone is located at a distance large enough to ensure that they will not be affected by direct strikes, connecting leaders or ground currents, the person can still be affected by the lightning-generated electromagnetic fields and the corona currents generated from the body.
In this chapter, we will consider the possible interactions, either direct or indirect, of the lightning-generated electromagnetic fields with the brain or the visual system of humans to induce visual sensations. Some of these visual sensations are known as phosphenes in the medical literature. Since some of these visual sensations could be misinterpreted as ball lightning, this subject is of interest for lightning researchers due to the still unsolved problem of the origin of ball lightning.
-
13 Lightning location systems
- + Show details - Hide details
-
p.
415
–436
(22)
In the early days, the risk of lightning strikes was described by the average number of thunderstorm days or thunderstorm hours, where a thunderstorm day is defined as an "Observational day during which thunder is heard at the station" [1]. On the basis of long-term records of thunderstorm days by the meteorological services, maps showing the so-called isoceraunic level were produced for the individual countries, from which the regional thunderstorm hazard could be obtained [2]. The first attempts to locate lightning discharges date back to the 1920s. W. Watt in [3] describes the considerations and experimental observations made at that time, which made it possible to identify thunderstorms and lightning discharges as the main cause of the observed electromagnetic disturbances in the early days of long-range radio communication.
In the beginning, the problem of electromagnetic disturbances in radio communication was the driving force for research activities related to lightning electromagnetic fields and their source location. Only since the middle of the 1980s lightning detection has been applied in the field of thunderstorm observation for meteorological applications, in the field of lightning risk assessment and in fault analysis for power utilities. These new applications also placed significantly higher demands on the performance of the lightning location systems (LLS), especially with regard to detection efficiency and location accuracy. Lightning detection has thus evolved from a real-time thunderstorm observation tool to the most precise detection of each individual electrical discharge in lightning both in cloud-to-ground (CG) flashes and discharges within the thundercloud, often referred to as intracloud lightning (IC).
In a typical thunderstorm, the number of IC discharges exceeds the number of CG discharges many times with an average ratio of IC/CG discharges being in the range of 5-10, but this ratio can be much higher in individual thunderstorms. As CG lightning poses the greatest danger to humans and property, at the beginning of the development of LLS, the focus was on the best possible and reliable detection of CG lightning. Only in recent years, the trend is increasingly moving towards the detection of total lightning. New lightning data have recently become available through the detection of lightning discharges by the Geostationary Lightning Mapper (GLM) on the satellites GOES West and GOES East [4]. Compared to land-based LLS, optical detection from space has the advantage that lightning activity is monitored over continents and oceans with approximately the same detection quality. The main limitation of lightning detection from space is the lack of differentiation between CG and IC discharges and the comparatively limited detection accuracy due to the spatial resolution of the optical sensor in the range of 4-5 km looking to earth surface from a distance of about 36,000 km. In this chapter, we will focus on ground-based systems only which are employing Magnetic Direction Finding (MDF) and/or Time of Arrival (TOA) technique, as data from these systems are used for many applications from lightning risk management to severe storm forecast.
-
Back Matter
- + Show details - Hide details
-
p.
(1)