Lightning Return Stroke Channel Research Articles

Spectroscopic method to study the effect of lightning return stroke channel expansion on atmospheric pressure

Using the plasma radiation spectra of three natural cloud-to-ground multi-strike lightning discharges recorded by a slitless grating spectrograph, and based on the spectral wavelengths and relative intensities, combined with information about the lightning-synchronized ground electric field variations, we calculated the temperatures, line-charge densities, initial radii, and final radii of the lightning-strike channels. This was achieved by applying the theory of air plasma transport, lightning electrodynamics, the laws of conservation of energy and pressure. Subsequently, we determined the initial internal energies of the lightning plasma channels, the final internal energies, the work done by the return stroke channel expansion against atmospheric pressure, equivalent impedances, and the speed of charge movement. The results show that the temperature of the return stroke channel and the work done by the expansion of the return channel on the atmospheric pressure decrease with the development of the return stroke process. The final radius and the initial internal energy, as well as the initial peak electric field and the work done by the return stroke channel expansion on the atmospheric pressure, exhibit a strong linear relationship. The equivalent impedance and speed of charge movement show a nonlinear relationship with the work done by the return stroke channel expansion against atmospheric pressure. The changes in initial internal energy, final internal energy, and the expansion work of the return stroke channel against atmospheric pressure are essentially consistent and positively correlated. The initial peak electric field of the return channel increases, leading to enhanced spectral intensity. The return stroke channel temperature, initial radius, final radius, initial internal energy, final internal energy, and the work done by the return stroke channel expansion against atmospheric pressure all increase. Meanwhile, the equivalent impedance and speed of charge movement decrease.

Lightning radiometry in visible and infrared bands

Calibrated measurements of lightning optical emissions are critical for both quantifying the impacts of lightning in our atmosphere and devising detection instruments with sufficient dynamic range capable of yielding close to 100% detection efficiency. However, to date, there is only a limited number of investigations that have attempted to take such calibrated measurements. In this work, we report the power radiated by lightning in both visible and infrared bands, assuming isotropic emission, and accounting for atmospheric absorption. More precisely, we report peak radiated power and total radiated energy in the combined visible plus near-infrared range (VNIR, 0.34–1.1 μm), around the Hα line (652–667 nm), and for the 2–2.5 μm infrared band. The estimated peak power and total energy radiated by negative cloud-to-ground return strokes in the VNIR range is 130 MW and 20 kJ, respectively. Additionally, we detected peak radiated powers of 12 and 0.19 MW in the Hα and infrared bands, respectively. We cross-reference the optical data set with peak current reported by a lightning detection network. The resulting trend is that optical power emitted around the Hα line scales with peak return stroke current according to a power law with exponent equal to 1.25. This trend, which should be approximately true across the entire visible spectrum, can be attributed to the plasma negative differential resistance of the lightning return stroke channel. We conclude by discussing the challenges in performing calibrated measurements of lightning optical power in different bands and comparing the results with previously-collected data with different experimental setups, observation conditions, and calibration methods.

Wave Propagation Speed and Equivalent Impedance of Dielectric-Coated Conductor Representing Lightning Return-Stroke Channel

In electromagnetic models of lightning return stroke, dielectric coating of channel-representing conductor is used to adjust the current wave propagation speed. In this letter, we examine dependences of both the speed and channel equivalent impedance on the electric permittivity and magnetic permeability of such coating. With increasing the permeability, the propagation speed of current wave decreases and the equivalent impedance increases significantly. In contrast, these parameters are not much influenced by the increase of permittivity. The reason for the difference has been explained in terms of the transmission-line theory.

A New FDTD Model for Lightning Return Stroke Channel Above Lossy Ground and Its Validation With Rocket-Triggered Lightning Data

A New FDTD Model for Lightning Return Stroke Channel Above Lossy Ground and Its Validation With Rocket-Triggered Lightning Data

The initial radius of lightning return stroke channel and its relation with discharge intensity

The initial radius of the return stroke is one of the important parameters reflecting lightning discharge intensity. Based on spectral diagnosis and electrostatic approximation of the vertically descending leader model, the lightning channel temperature, line charge density, and electrostatic energy of seven dart leader–return stroke sequences were estimated. Finally, the initial radii of the return strokes were obtained. We analyzed the relationship between the initial radii and the corresponding discharge parameters. The initial radius is found to have a good linear correlation with each of the following quantities: the line charge density of the dart leader, the total intensity of ionic lines in the spectra, and the optical radius. It is also inversely proportional to the leader duration. These correlations confirm that the initial radius is closely related to discharge intensity, and that the charge stored in the leader channel directly determines the initial radius and discharge intensity of the return stroke.

Diagnosis of lightning return stroke channel temperature according to different band spectra

The spectral data of a lightning discharge process are captured by a high-speed slit-less grating spectrograph. Based on the plasma spectrum theory, the lightning return stroke channel temperature is estimated according to the spectral information of the different bands. The results show that the average temperatures of the lightning return stroke channel estimated by the Boltzmann plot method are 43270, 17660 and 17730 K, respectively, when the spectral line group of the single ionized nitrogen atom (NII), neutral oxygen atom (OI) and neutral nitrogen atom (NI) are selected respectively. The average temperature of the lightning return stroke channel is 24770 K, which is estimated based on the Saha-Boltzmann plot method by using NII and NI spectral line groups. Based on the lightning channel corona sheath model and the spectral radiation theory, it can be inferred that the temperature obtained by using only NII spectral line group should be the temperature of the lightning return stroke channel core, and the temperature obtained by using only NI or OI spectral line group should be the temperature of the corona sheath around the lightning return stroke channel core. Using both NII and NI spectral line groups, the obtained temperature should be the average temperature of the entire channel section (including the channel core and corona sheath) in the exposure time.

Effects of Atmospheric Attenuation on the Lightning Spectrum

AbstractSpectrum carries important information reflecting atomic and molecular processes inside a light source. Accurate spectral diagnosis is vital in revealing the microscopic physical mechanism of the lightning discharge process. Spectral correction was applied considering the influence of atmospheric attenuation, grating efficiency, and camera response on the observed spectrum, thereby solving the problems of atmospheric attenuation in long‐distance lightning spectrum observations. Based on the restored spectrum, the temperature of the lightning return stroke channel was calculated by the ionic and atomic lines respectively. The result showed that corrected temperature at the initial stage of the return stroke, calculated by the ionic line, could reach up to 40,000 K, which was about 10,000 K higher than the values directly obtained from the observed spectrum. Atmospheric attenuation of the atomic spectral line in the near‐infrared band is relatively weak; therefore, atmospheric attenuation was inferred to have a relatively less effect on the channel temperature that was calculated by the atomic spectral lines. This work provided the attenuation ratio of the characteristic lines in lightning spectra with distance, and can used for more precisely quantitative investigation on the physical characteristics of the lightning process. It also has application value for improving the spectral diagnostic techniques on celestial body and other natural luminous process.

Conductivity characteristics and corona sheath radius of lightning return stroke channel

The characteristics of lightning corona sheath are of common concern in the lightning physics and lightning protection research. Using the spectra of two natural cloud-to-ground lightning including multiple return strokes recorded by a slit-less spectrograph, we investigated the electrical conductivity distribution along the radial direction of the channel. The results indicated that a high conductivity channel of above 104 S/m existed within an average radius of around 1.8 cm and corresponded to the temperature of around 20,000 K. This finding reflects the characteristics of the current-carrying channel. The electrical conductivity decayed steeply to about 500 S/m with increasing radius and then decayed slowly. Based on these results, it was estimated that the corona sheath radii of the return stroke channel were around 3.34–19.10 cm. The corona sheath radius was positively correlated with the leader charge quantity and the apparent diameter of the corresponding leader channel. A wide corona-channel radius corresponds to an intense discharge.

On the Use of the Antenna Theory Model of Lightning Return Stroke With Distributed Current Source for EMC Analysis

A full-wave solution method of Maxwell's equations is adopted for the electromagnetic analysis of the lightning return stroke channel. The method is based on a numerical solution of the electric field integral equation by the method of moments. The lightning channel is modeled as a wire monopole antenna with current sources distributed along its length. The method makes it possible to consider phase shift and amplitude decay between consecutive current sources in the frequency domain. The implementation of any engineering models of lightning in the integral form of Maxwell's equations is thus realizable. Special attention is devoted to the consequences of a limited channel length. Indeed, taking into account a finite channel length generates undesirable reflections from the top of the channel, which limits the use of the proposed method for electromagnetic compatibility analysis. Procedures are proposed to mitigate the effects of a limited channel length on the obtained results. Transient and frequency analyses of various lightning related problems demonstrate the applicability of the method to the analysis of complex electromagnetic problems.

Using Saha‐Boltzmann Plot to Diagnose Lightning Return Stroke Channel Temperature

AbstractBased on the high time‐resolved spectra of a cloud‐to‐ground lightning flash with seven return strokes captured by a high‐speed slitless spectrograph, this work was the first to use the Saha‐Boltzmann plot method for the determination of the lightning return stroke channel temperature. The channel temperatures of the seven return strokes ranged from 21370 to 26500 K. Compared with the traditional Boltzmann plot method, the Saha‐Boltzmann plot method can provide more reliable temperature estimation for lightning channel. Analysis of several strong return strokes along the channel showed that the temperature changed little and there was no obvious tendency with increasing height. In addition, it was found that the channel temperature in the late stage of a return stroke was only about 3000–4000 K lower than in the initial stage. This means the channel remains >20000 K during nearly the entire period of the return stroke. This sustained high temperature could be the main cause of lightning‐related disasters.

An Improved Antenna Theory Model of Lightning Return Stroke Using a Distributed Current Source–Part I: Theory and Implementation

An improved antenna theory model (AT) approach in the frequency domain is presented for the electromagnetic analysis of the lightning return stroke channel. The lightning channel is modeled as a wire monopole antenna. Unlike the conventional AT models, which employ a voltage or current source localized at the channel base, the new model considers a distributed current source along the antenna. The implementation of appropriate phase shifts between consecutive current sources in the frequency domain accounts for the current wave propagation speed along the channel. Contrary to conventional AT models, the implemented methodology permits the establishment of an accurate propagation speed along the channel without the necessity to adjust the electrical parameters of the elements representing the channel. A solution of the electric field integral equation by the method of moments in the frequency domain is adopted for modeling the lightning channel and the nearby structures. The presence of multilayer soil is accounted by using Sommerfeld integrals. The implemented methodology enables an efficient analysis of lightning channel in the presence of a complex electromagnetic environment.

The radius and temperature distribution along radial direction of lightning plasma channel

Based on various radiation components in the spectra of lightning return stroke processes, the temperature of the lightning channel is diagnosed by different methods. Combining with the synchronized electric field data and optical pictures of the lightning channel, the radii of the core current channel and the luminous channel are estimated. On the basis of the above mentioned facts, according to the heat conduction equation, the channel temperature distribution along the radial direction of the lightning return stroke channel is also achieved. The results from six lightning return strokes show that for the core current channel and luminous channel, the radii are in the range of 0.66–1.01 cm and 5.03–7.96 cm, respectively, and the corresponding channel temperatures are 27 300–30 300 K and 3300–5800 K, respectively. Within an average radius of around 1.76 cm, channel temperatures are above 20 000 K and fall sharply along the radial direction while gradually decline outside this range. The radii of the glowing edge with a temperature of around 500 K are first estimated, around 6.94–13.0 cm.

Relationship Between the Electromagnetic Wave Energy Coupled by Overhead Lines and the Radiation Source Current Explored in the Laboratory

AbstractIn order to solve the damage and interference problems to the electronic devices, which are induced by overvoltage excited by the coupling process between lightning electromagnetic wave and overhead lines, the lightning channel is set to be equivalent to a radiant wire antenna. Based on the integration model of lightning return stroke channel, transmission line, and ground, we take advantage of the derived formula gotten from the transmission line model. By combing the theoretical and experimental methods, we conduct a comparative analysis on the coupling process between natural/simulated lightning and overhead line. Besides, we also calculate the amplitude and energy of overvoltage, which is caused by the coupling process between lightning electromagnetic wave and overhead lines. Upon these experimental results, we can draw several conclusions as follows: when the amplitude of the lightning current in the channel is between 5 kA and 41 kA, it takes on an excellent linear relation between the amplitude of overvoltage and the magnitude of the lightning current, the relation between coupling energy and magnitude of the lightning current takes on an exponential trend. When lightning wave transmits on the transmission lines, the high‐order mode will be excited. Through analysis on the high‐order mode's characteristics, we find that the theoretical analysis is consistent with the experimental results, which has a certain reference value to the protection on overhead lines.

3D-FDTD Computation of Lightning Return Stroke Current and Associated Electromagnetic Field Using Electromagnetic Models

This paper presents a new approach for the calculation of the lightning channel current distribution and the associated electromagnetic field components in the close vicinity of a vertical lightning return stroke channel. This approach is based on the use of lightning return stroke electromagnetic models combined to the 3D-FDTD method. The novelty of this approach is the use of Taflove formulation in order to calculate both the electric and magnetic fields and their densities obtaining then the lightning channel return stroke current space-time distribution. The employed electromagnetic models include a vertical resistive wire having an additional series distributed inductance and a wire surrounded by a dielectric medium (other than the air) that occupies the half space above the ground. The obtained results are compared to others taken by the specialized literature notably measured data. It appears that the used approach yield reasonably accurate results of close electromagnetic fields and lightning return stroke current spatiotemporal distribution.

Effect of Shield Wires on the Lightning-Induced Currents on Buried Cables

The effect of a shield wire, which is a bare conductor buried above an insulated metallic cable, on lightning-induced currents on the cable has been studied using the three-dimensional (3-D) finite-difference time-domain (FDTD) method. In the 3-D FDTD model, a 1-km-long insulated cable is buried at depth 0.6 m in 1850-Ω·m ground, and a 1-km-long shield wire is installed 0.3 m above the cable (at the depth 0.3 m from the ground surface). A 1-km-long vertical lightning return-stroke channel, attached to a 30-m-high tower or flat ground, is located 25 m away from the cable and the shield wire. The validity of this model has been shown by comparing the FDTD-computed lightning-associated currents on the cable and the shield wire with the corresponding measured currents associated with rocket-triggered lightning. The FDTD-computed results show that the presence of the shield wire reduces the induced current on the cable and the voltage across the outer insulation layer of the cable, as expected, but the series resistance of the shield wire has a negligible effect on this reduction. The presence of 30-m-high strike object slightly reduces the cable current. The ground permittivity and the return-stroke speed have negligible effects on the cable current, but the higher the ground resistivity, the higher is the cable current. The increase of bonding points between the cable sheath and the shield wire increases the cable current, but decreases the voltage across the outer insulation layer of the cable.

Temperature distribution and evolution characteristic in lightning return stroke channel

According to the time-resolved spectra of four lightning return strokes, the temperatures of arc core channel and the peripheral optical channel surrounding the arc core are investigated by different methods; the temperature distribution along the radial direction of channel on the peak current stage is discussed. The results show that a temperature gradient is formed along the radial direction of channel during the discharge process. With the increasing of the radius, the temperature decreases gradually. The temperature of arc core channel is about 4000–5000K higher than that of the peripheral optical channel. The time evolution of channel temperature shows that the falling of the temperature is very slow compared with the decreasing of the current after their peak values. After the peak current, the channel temperature is still maintained at around 20,000K up to 200–400μs. The heat effect resulting from such a long-time high temperature is the main source of most direct lightning disasters.

The heat transfer characteristics of lightning return stroke channel

Based on the time-resolved spectra of lightning return stroke processes, the evolutional characteristics of thermal conductivity and thermal diffusivity of the discharge channels are discussed. The distribution of temperature along the radial direction of channels at the peak current stage of return stroke is also investigated, and then the heat transferring characteristics along radial direction of the channels are analyzed. The results show that a temperature gradient along radial direction of lightning channel is formed due to the outward heat transfer. The closer the distance is to the current core channel, the greater the temperature gradient is and the more heat is transferred along the radial direction of the channel. The heat transferring in per unit length of the channel and per unit time is in the order of 104J/m⋅s at the initial moment of lightning return stroke. After the peak current, the channel temperature decreases slowly and the heat transport coefficients vary as a monotonically decreasing function.

利用近红外波段光谱研究闪电回击通道温度的演化特性

利用近红外波段光谱研究闪电回击通道温度的演化特性

Numerical study on Rayleigh-Taylor instabilities in the lightning return stroke

The Rayleigh-Taylor (R-T) instabilities are important hydrodynamics and magnetohydrodynamics (MHD) phenomena that are found in systems in high energy density physics and normal fluids. The formation and evolution of the R-T instability at channel boundary during back-flow of the lightning return stroke are analyzed using the linear perturbation theory and normal mode analysis methods, and the linear growth rate of the R-T instability in typical condition for lightning return stroke channel is obtained. Then, the R-T instability phenomena of lightning return stroke are simulated using a two-dimensional Eulerian finite volumes resistive radiation MHD code. The numerical results show that the evolution characteristics of the R-T instability in the early stage of back-flow are consistent with theoretical predictions obtained by linear analysis. The simulation also yields more evolution characteristics for the R-T instability beyond the linear theory. The results of this work apply to some observed features of the return stroke channel and further advance previous theoretical and experimental work.

A numerical study on bow shocks around the lightning return stroke channel

Bow shock structures are important to various hydrodynamics and magnetohydrodynamics (MHD) phenomena in geophysics and astrophysics. The formation and propagation of bow shocks around the lightning return stroke channel are investigated based on the self-similar motion theory and simulated with a two-dimensional Eulerian finite volume resistive radiation MHD code. In this framework, as verification of theoretical models, the evolving structures of many quantities, such as the plasma density, temperature, pressure, shock velocity, and magnetic field, can be obtained, which present all the characteristics of bow shocks in the lightning return stroke processes. The evolution characteristics and the configuration of the curved return stroke channels, e.g., the non-ideal effects and the scaling laws, are discussed in detail. The results may have applications for some observed features of the return stroke channels and other phenomena in the lightning discharge plasmas.