Electromagnetic Fields Associated With the M‐Component Mode of Charge Transfer

In upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M-component or leader/return-stroke like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC-carrying channel at relatively short distance from the tower top, resulting in the so-called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M-component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Säntis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M-component charge transfer mode (including classical M-component pulses and M-component-type pulses superimposed on the initial continuous current) is analyzed, and compared with its effect on the fields associated with the return-stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2D FDTD (2-Dimentional Finite-Difference Time-Domain) method, in which the M-component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al. 1995 model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Säntis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for ‘classical’ M-component and M-component-type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M-component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster travelling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M-component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component which mainly depends on the overall transferred charge. The contribution of the radiation component to the M-component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of the current pulses. However, the results indicate a high variability of the propagation factors as a function of the branch-to-channel junction point height. For junction point heights of about 1 km, the propagation factors reach a value of about 1.6 for the E-field and 1.9 for the H-field. For a junction height greater than 6 km, the E-field factor becomes slightly lower than 1. The obtained results are consistent with the findings of Li et al. (2016b) in which an electric field propagation factor of 1.8 was inferred for return strokes and mixed mode pulses, considering that junction points lower than 1 km or so would result in a mixed-mode of charge transfer, in which a downward leader/return-stroke like process is believed to take place. It is also found that the field enhancement (propagation factor) for return stroke mode is higher for larger ground conductivities. Furthermore, the enhancement effect tends to decrease with increasing current risetime, except for very short risetimes (less than 2.5 s or so) for which the tendency reverses. Finally, model-predicted fields associated with different charge transfer modes, namely return stroke, mixed mode, classical M-component, and M-component-type ICC pulse are compared with experimental observations at the Säntis tower. It is found that the vertical electric field waveforms computed considering the mountainous terrain are in very good agreement with the observed data. The adopted parameters of the models that provide the best match with the measured field waveforms were consistent with observations. The values for the current decay height constant adopted in the return stroke and mixed mode models (1.0 km for the return stroke and 0.8 km for the mixed-mode pulse) are lower than the value of 2.0 km typically used in the literature.

Electromagnetic Environment Associated with Lightning Strikes to Tall Strike Objects

Seyed Abbas Mosaddeghi

The study of the lightning interaction with tall strike objects has attracted considerable attention of lightning researchers lately. Many lightning measurements including current and associated electromagnetic fields were recently made all over the globe namely in Russia, South Africa, Germany, Brazil, Japan, and Austria. It is a novel area of studies, and the resolution of associated questions will have an impact upon many lightning-related applications such as lightning protection and the determination of lightning parameters from remote field measurements. The main objective of the thesis is to carry out further theoretical investigations and experimental measurements to understand and elucidate recently raised questions on the characteristics of lightning return-strokes to tall structures and their associated electromagnetic radiation. Chapter 2 presents a review on recent progress in the modeling of lightning strikes to tall towers and associated experimental data obtained during the last decade or so. Two types of return stroke models namely the Engineering Models, and the Electromagnetic or Antenna-Theory (AT) models, extended to take into account the presence of a tall strike object are discussed. The Chapter contains also a description of the computational methods for the evaluation of electromagnetic fields generated by a lightning strike to a tall structure, as well as an overview of available data on lightning current and associated electromagnetic fields. The chapter finally highlights some important questions raised by different research groups in the past few years which call for further investigations. These questions are as follows: No systematic theoretical analysis nor experimental data are available for electromagnetic fields in the immediate vicinity of a tall structure struck by lightning. The characterization of nearby electromagnetic fields is particularly important in the analysis of the interaction to nearby electrical and electronics systems. Why do lightning return stroke models not reproduce the far-field zero crossing associated with lightning to tall structures? How should these models be revised to be able to reproduce such an effect? How should the engineering models be revised in order to remove the associated current discontinuity at the return stroke wavefront? It is well-known that the measurements of electromagnetic fields from lightning are affected by the presence of nearby buildings and metallic structures. However, no systematic and quantitative analysis of such an effect is presently available in the literature. The work presented in this thesis addresses all of the above questions. The main original contributions of this thesis, consisting of both theoretical and experimental work, are presented in Chapters 3 through 6. Chapter 3 is devoted to a theoretical description of the signature of electric and magnetic fields at very close distance associated with lightning strikes to a tower. It is shown that the electric field generated by a lightning return stroke to a tall structure can change polarity at very close distance range. This change in the polarity seems to be a specific signature of the very close vertical electric field. A simple equation is derived which provides an estimate of the critical distance below which such an inversion of polarity might occur. It is also shown that the inversion of polarity depends on the value of the reflection coefficient at the base of the tower and disappears for reflection coefficients close to 1. On the other hand, other parameters such as the return stroke speed, the reflection coefficient at the top of the strike object, and the adopted return stroke model seem not to have an impact on the inversion of polarity. Simulation results also showed that the electric field peak at distances beyond the height of the tower or so exhibits the typical 1/r dependence. At closer distances, however, the E-field peak features a saturation, due to the so-called tower shadowing effect. This shadowing effect results in a substantial decrease of the nearby electric field. On the other hand, the magnetic field peak varies inversely proportional to the horizontal distance and does not depend significantly on the presence of an elevated strike object. Chapter 4 introduces an improved version of the engineering models for return-strokes to tall structures which accounts for (1) the presence of possible reflections at the return stroke wavefront, and, (2) a return stroke initiation above the structure due to an upward connecting leader. We also propose an elegant iterative solution that can be easily implemented into computer simulation programs to take into account in a straightforward way multiple reflections occurring at the discontinuities at the tower ends and at the return stroke wavefront. Simulation results for the magnetic fields are compared with experimental waveforms associated with lightning strikes to the CN Tower (553 m). It is shown that taking into account the reflections at the return-stroke wavefront results in better reproducing the fine structure of the magnetic field waveforms. Chapter 5 presents and discusses obtained measurements of electric (vertical and radial) and magnetic fields from leaders and return strokes associated with lightning strikes to the Gaisberg tower in Austria obtained in 2007 and 2008. The data include simultaneous records of vertical and radial electric fields, which were obtained for the first time at such close distances. It is found that the vertical and radial electric field waveforms appear as asymmetrical V-shaped pulses. For the vertical electric field, the initial, relatively slow, negative electric field change is due to the downward leader and the following fast positive field change is due to the upward return stroke phase of the lightning discharge. For the horizontal electric fields, however, the bottom of the V is not associated with the transition from the leader to the return stroke. The horizontal field change due to the return stroke is characterized by a short negative pulse of the order of one microsecond or so, starting with a fast negative excursion followed by a positive one. In addition, an analytical expression for the radial electric field, assuming a uniform charge distribution along the leader with constant speed is derived. It is also shown that the return-stroke vertical electric field changes appear to be significantly smaller than similar measurements obtained using triggered lightning. This finding confirms the shadowing effect of the tower predicted by the theoretical analysis of Chapter 3, which results in a significant decrease of the electric field at distances of about the height of the tower or less. Finally, the ability of two different models for the return stroke in reproducing measured vertical and horizontal electric fields is tested using the obtained measured data. The considered models are (1) the engineering MTLE (Modified Transmission Line with Exponential Decay) model, and (2) the electromagnetic model implemented using the Numerical Electromagnetics Code NEC-4. It is shown that both models predict electric field waveforms which are in reasonable agreement with measured waveforms. In general, the predicted fields by the electromagnetic model appear to be in better agreement with measured data, because of the direct use of the measured current waveform as an input and the more accurate representation of the tower. Chapter 6 reports on the effect of nearby buildings on electromagnetic fields from lightning. Indeed, sensors used for the measurement of lightning electric and magnetic fields are often placed close to or on top of buildings or other structures. Metallic beams and other conducting parts in those structures may cause enhancement or attenuation effects on the measured fields. Experimental waveforms radiated from distant natural lightning recorded during the summers of 2006 and 2007 are presented. Electric and magnetic field waveforms were measured simultaneously on the roof of a building and on the ground at different distances away from it. The results suggest that the measured electric field on the roof of the building could be enhanced by a factor of 1.7 to 1.9, whereas the electric fields on the ground experienced a significant reduction by a factor ranging from 5 to 20. Also, it is shown that for a sensor located on the ground close to a building, the magnetic field component perpendicular to the building can experience significant attenuation, presumably due to the effect of the induced currents in the building. The magnetic field on the roof of the building seems not to be significantly affected by the building. Simulations using the Numerical Electromagnetic Code (NEC-4) were also carried out in which the building was represented using a simple wire-grid model. The simulation results support in essence the findings of the experimental analysis, despite quantitative differences which are ascribed, at least in part, to the oversimplified model of the building.

EPFL2011

Electromagnetic radiation from lightning return strokes to tall structures

Davide Pavanello

The study of the interaction of lightning electromagnetic fields with electrical systems and the design of appropriate protection strategies are generally based on statistical distributions of the lightning current measured at the channel base using either instrumented towers or artificial initiation of lightning using rockets. Recent studies based both on numerical modeling and experimental observations have shown that the presence of the structure struck by (or used to initiate) lightning does affect the current measurement in a way depending upon the geometry of the structure itself, compromising therefore the reliability of the statistics adopted so far for lightning data. The aim of this thesis is to provide new elements (from both theoretical and experimental investigations) to improve the understanding of the electromagnetic consequences of the impact of lightning return strokes to tall structures. Chapter 2 introduces to the phenomenology of cloud-to-ground lightning and the importance of lightning return-stroke modeling. Among the different classes of return-stroke models existing in the literature, the attention is focused in this thesis on the so-called engineering models, which allow describing the current distribution along the channel as a function of the current at the channel base and the return-stroke speed, two quantities for which data can be obtained experimentally. After presenting a review of five engineering return-stroke models describing lightning strikes to ground, the extension of the engineering models to take into account the presence of an elevated strike object is presented and discussed. The original contributions of this thesis, consisting of both theoretical and experimental works, are presented in Chapters 3 through 6. Chapter 3 is devoted to the computation of the electromagnetic field produced by lightning return strokes to elevated strike objects, using the extension of the engineering models to include an elevated strike object presented in the previous chapter. It is shown, for the first time, that the current distribution associated with these extended models exhibits a discontinuity at the return-stroke wavefront which (although not physically conceivable) needs to be taken into account by an additional term in the equations for the electromagnetic field, the so-called "turn-on" term. A general analytical formula describing the "turn-on" term associated with this discontinuity for various engineering models is derived and simulation results illustrating the effect of the "turn-on" term on the radiated electric and magnetic fields are also presented. In the second part of the chapter, dedicated to the investigation of the propagation effects on lightning electromagnetic field traveling along a finitely-conducting ground, the commonly-used assumption of an idealized perfectly-conducting ground is relaxed in order to analyze, for the first time, how the electromagnetic field radiated by a tower-initiated strike is affected while propagating along a soil characterized by a finite conductivity. The results showed that the attenuation of the initial peak of the field radiated by a tower-initiated strike, resulting from the propagation over finitely conducting ground, depends strongly on the risetime of the current, the tower height and the ground conductivity and is, in general, much more important than the attenuation experienced, while propagating along the same finite ground, by the field produced by ground-initiated strikes. Chapter 4 presents a comparison among the predictions obtained using the five extended engineering return-stroke models for lightning strikes to tall structures described in Chapter 2. The spatial-temporal current profiles along the tower-channel axis predicted by the engineering models, as well as the respective predictions for the radiated electric and magnetic fields, calculated at different distances, are compared and discussed. It is shown that the computed electromagnetic fields associated with a strike to a tall tower are generally less model-dependent than those corresponding to a strike to ground, especially as far as the first-peak value is concerned, which is nearly model-insensitive in case of tall-tower strikes. A theoretical analysis is performed in the last part of the chapter with the aim to provide, for the same five engineering models extended to take into account the presence of the tower, expressions relating the return-stroke current and the associated distant radiated electric and magnetic fields. It is demonstrated, in addition, that only one model among the five presented is characterized by simple analytical formulas relating current-peak and far-field peak values, which (being the electromagnetic field peak value nearly independent of the adopted model) become general expressions applicable for any engineering return-stroke model in case of tower-initiated lightning. It was also shown that the peak amplitude of the electromagnetic field radiated by a lightning strike to a tall structure is relatively insensitive to both the values of the top reflection coefficient and the return-stroke speed. This latter result is important, in particular, because, unlike ground-initiated strikes, for which the far-field peak is strongly dependent on the return-stroke speed, far field peaks associated with strikes to tall structures are little sensitive to the return stroke speed. Since in most practical cases the value of the return-stroke speed is unknown, this interesting result suggests a possible calibration procedure for lightning detection systems by means of direct measurement of lightning currents on instrumented towers. Chapter 5 reports on the simultaneous measurements of the return-stroke current and of the electric and magnetic fields at three distances associated with lightning strikes to the Toronto CN Tower (553 m) that have been carried out during the summer of 2005. This is the first time ever that simultaneous records of lightning current and associated electric and magnetic fields at three distances have been obtained. Two propagation paths for the electromagnetic field to the first and to the second field measurement stations (located, respectively, 2.0 km and 16.8 km away from the CN Tower) were along the soil and through the Toronto city, whereas for the third location (50.9 km away) the propagation path was nearly entirely across the fresh water of Lake Ontario. It is shown that the waveforms of the electric and magnetic fields at 16.8 km and 50.9 km exhibit a first zero-crossing about 5 microseconds after the onset of the return-stroke, which is part of a narrow undershoot and which may be attributed to the transient processes along the tower. Effects of propagation (decrease of field amplitude and increase of its risetime) could also be observed in experimental records. It is shown that the fields at 50.9 km are less affected by such attenuation, compared to those at 16.8 km, presumably because the path of propagation was mostly across Lake Ontario. The measured waveforms are compared with the theoretical predictions obtained using five engineering return-stroke models, extended to include the presence of the strike object, finding a reasonable agreement for the magnetic field waveforms at the three considered distances. The overall agreement between the theoretically predicted and the experimentally observed field-peak-to-current-peak ratio is reasonable, although the theoretical expression appears to underestimate the experimentally measured ratio (by about 25 %). This may be due, at least in part, to the enhancement effect of the buildings on which the field measurement antennas were installed. Finally, the directly-measured lightning currents at the tower were correlated and compared with the current-peak estimations provided by the US National Lightning Detection Network (NLDN). It is shown that the NLDN-inferred values overestimate the actual current peaks because the presence of the tall struck object produces an enhanced radiated field at far distances (with respect to strikes to flat ground), which is not included in the algorithm used to infer lightning current peaks from remote field measurements. It is shown in this thesis that correcting the NLDN estimates using the correction factor introduced by the tower results in an excellent estimation of lightning current peaks. This is an important conclusion of this study showing that the estimation of lightning peak currents for tall towers can be greatly improved by considering the tower correction factor. Chapter 6 is devoted to the measurement of electromagnetic fields radiated by lightning. In its first part, the need for guidelines for reporting lightning data obtained experimentally is emphasized. The second part of the chapter presents the design, the construction and preliminary tests of a low-cost, multi-channel lightning field measuring system for the simultaneous measurement of three components of the electromagnetic field radiated by lightning. The proposed system uses one single optical link for the transmission of the three signals, appropriately digitized and multiplexed, lowering considerably the overall cost of the system itself.

EPFL2007

Modeling of Different Charge Transfer Modes in Upward Flashes Constrained by Simultaneously Measured Currents and Fields

Mohammad Azadifar, Lixia He, Qi Li, Mario Paolone, Davide Pavanello, Marcos Rubinstein

The purpose of the paper is to investigate charge transfer modes in upward lightning flashes by means of numerical simulation constrained by concurrent observations of electromagnetic fields and currents. In particular, we focus on different types of pulses occurring in upward negative flashes. The MTLE return stroke model is used to compute the electric fields associated with return strokes and mixed-mode pulses, while the M-component model of Rakov et al. (1995) is used to compute electric fields associated with M-components and M-component-type ICC pulses. The simulation results are constrained by experimental data consisting of simultaneous records of lightning currents and electric fields associated with upward flashes at the Santis tower. The inferred velocities for M-component and M-component-type ICC pulses range from 2.0x10(7) m/s to 9.0x10(7) m/s, and the corresponding junction point heights range from 1.0 km to 2.0 km. The inferred pulse velocities for return strokes and mixed-mode pulses range from 1.3x10(8) m/s to 1.65x10(8) m/s. The inferred current attenuation constants of the MTLE model obtained in this study range from 0.3 km to 0.8 km, lower than the value of 2.0 km suggested in previous studies. The obtained results confirm the similarity of mixed-mode charge transfer to ground with return strokes on the one hand, and of the M-component-type ICC with classical M-components mode of charge transfer on the other hand.