Leader Corona Research Articles

Hybrid leader corona virus herd optimizer with multilevel thresholding techniques for foreground and background image segmentation

In computer vision and image processing, an important role is given to image segmentation. As it can accurately extract and identify specific regions within an image. One of the key challenges that remain in this is achieving precise recognition and extraction of segmented regions. This can be solved by multilevel optimization techniques and this offers an effective solution. Recently, multilevel thresholding has emerged as an important technique for image segmentation as the separation of image pixels into various classes by the selection of optimal threshold values. However, when the number of thresholds increases, the computational complexity of multilevel thresholding also increases. To address this issue, various optimization algorithms are employed by the researchers. Thus, the Hybrid optimization techniques are integrated to enhance the overall efficacy of image segmentation processes. Thus, this research devised a Multithreshold-Hybrid Leader Coronavirus Herd Optimizer+kernel based Bayesian fuzzy clustering (Multithreshold-HLCHO+kernel-BFC) for the segmentation of foreground and background images. Here, the noise from the input image is removed using the Non-Local Means (NLM) filter and also the region of interest (ROI) extraction is done. The foreground and background image segmentation is done using the multilevel thresholding techniques, wherein threshold values are optimally generated utilizing HLCHO with multiobjectives like Renyi, Masi and Tsallic. Moreover, the integration of Hybrid Leader Based Optimization (HLBO) with Coronavirus Herd Immunity Optimizer (CHIO) forms the HLCHO. Also, the kernel-based BFC is employed for the segmentation of background and foreground images. Finally, the segmented output is achieved by fusing these two outputs using a fusion process. Additionally, Multithreshold-HLCHO+kernel-BFC acquired a maximum value of 0.891 for dice coefficient, 37.174 dB for PSNR, 0.918 for uniformity measure, and 0.297 for Mean Squared error (MSE).

On the mechanisms of lightning initiation and positive leader intergrowth

A model of a leader breakdown in air was developed, in which the channel is formed by heating the streamer trace in the leader corona. To determine the initial conditions, the model of a streamer breakdown was proposed. The minimal field intensity required to support the streamer breakdown was determined. The dependence of the leader's speed on the cloud potential was obtained. The lightnings are initiated by relativistic avalanches, the formation threshold field strength of which is below the threshold of the streamer discharge. By comparing the thresholds of relativistic and streamer breakdowns the heights for realization of the proposed initiation mechanism are specified.

Influence of rate of voltage rise on positive leader inception in long-air-gap discharge

Positive leader inception under different rates of voltage rise is a key element in physical modeling of dielectric breakdown of long air gaps and evolution of natural lightning. The leader inception phase comes first in long-air-gap discharge, including first corona, primary dark period, and second corona, which is also called “leader corona,” portending leader inception. The Les Renardières Group has stated that the primary dark period is determined by the electrode curvature, based on which no leader inception model currently in use considers the influence of the rate of the applied voltage. To determine whether that assertion is true, an experimental scheme is designed so that impulse voltages can be applied with a large range of rise rates. With an increasing rate of voltage rise, the test results clearly show the following: (i) the curves relating the injected charge and inception voltage of the first corona present regular differences; (ii) the duration of the primary dark period decreases exponentially; (iii) the leader inception voltage and the difference between the leader and first-corona inception voltages present decreasing trends. These findings indicate that the formation of the dark period is related to more than just the electrode curvature, which is just one cause but may not be the essential reason for the dark period. The present findings suggest that the rate at which the applied voltage rises impacts the physical process of the streamer-to-leader transition during the dark period.

ICCD Images of Leader Corona in a 4 m Rod-Plane Gap Discharge

As the featured discharge pattern and widely present in lightning discharge channels, leader corona system is crucial for continuous leader propagations in long air gap discharges and natural lightning, which had attracted numerous studies experimentally or theoretically. In the simulation of discharge process and lightning attachment calculations, the vertex angle of streamer zone ahead of leader channel is the crucial parameter, which could only be obtained by discharge experiments. As a result, morphological researches on leader corona system gave an intuitive insight on the streamer zone ahead of the leader channel and helped comprehensive theoretical modeling studies especially in the calculation of electric field. In order to obtain the accurate shape features and provide the calculation parameter for model modification, in this paper, experiments of 4-m long air gap discharges were carried out based on a four-frame PCO.dicam C4 ICCD camera. Clear images of leader corona were observed. Results showed that the corona zone was assumed as conical and the average angle at the vertex was 75.2° which was less than the first corona during the stable leader propagation stage. Influences of exposure time and leader branching phenomenon were investigated in this paper. It was concluded that the competitive relationship of the branched leaders shrank the vertex angle of the conical corona zone. Results in this paper could instruct the future theoretical model researches on leader corona system and help to modify the existing lightning attachment models like SLIM or leader process model.

General processes responsible for the space leader birth in streamer coronas of negative leaders

Local section of one streamer in the streamer coronas ahead of negative leaders of long laboratory sparks can develop into ‘space leaders’, called so because of their high conductivity and initially being unconnected to the main negative leader. Available data of experimental research do not allow reliably unveiling a mechanism causal to the space leader birth. We, here, discuss a hypothetical but general mechanism based, mostly, on thermodynamics. The plasma-chemistry processes in one of the streamers of the negative leader corona and Joule heating of the gas within the streamer channel are casual to a birth of a conducting area in this streamer at the strength of the ambient electric field less than the selfbreakdown magnitude of 30 kV cm−1. Two conclusions follow from the analysis. First, sufficiently high ambient field strength E 0 is required to heat the gas to a temperature sufficiently high for a birth of the conducting area during the observed time t obs. Second, the current of one streamer does provide sufficient heating; for this, the being heated area is to be fed by a multitude of the streamer branches. We limit ourselves to considering processes responsible for a formation of hot conducting areas in the negative corona. We assume that the birth of the heated area in the streamer, start new streamers from its ends and further heating of this area occurs in some local domain where the field strength exceeds the mean magnitude 11 kV cm−1 in the leader corona. We, proceeding from the experimentally observed time t obs of 1 show that the minimal magnitude of the E 0 of 20 kV cm−1 in the being heated area is required to ensure the gas temperature of 2000 K fitting the t obs. To illustrate our conclusions, we, using simplified models, carry out simulations treating selfconsistently the evolution of the electric field and the associated plasma chemistry.

Heating of a Local Region of a Branching Streamer as a Starting Point of a Space Leader and a Negative-Leader Step

Localized plasma formations called space leaders are observed in streamer coronas of negative leaders of long laboratory sparks. The main leader completes a step when the space leader comes into contact with the head of the main leader. In the present work, we discuss the creation mechanism of local plasma formations (henceforth, generally, referred to as the hot spots) that are able to initiate a space leader. It is assumed that spontaneous increase in the conductivity in a local region of one of the streamers of the main leader corona initiates two secondary coronas from the ends of this region. The corona current warms up the region, leading to formation of the hot spot, provided that the magnitude of the field in the heated region is sufficiently high. Finally, hot spots grow into the space leader with further increase in the temperature and conductivity. The necessary condition for achieving the temperature of ≈2000 K in the hot spot within the observation time of tobs ≤ 1 µs is the magnitude of the ambient electric field strength E0 = 20 kV cm–1 that is almost twice higher than the average magnitude of the electric field strength of ≈11 kV cm–1 in a negative corona.

Some additional materials illustrating space stem precursor formation process at the negative lightning leader corona periphery

Some additional materials illustrating space stem precursor formation process at the negative lightning leader corona periphery

Morphological characteristics of streamer region for long air gap positive discharge

Knowledge of leader corona is the basis for long spark discharge modelling, which is critical to understand the underlying physics of lightning flashes and dielectric breakdown of long air gap widely utilized in power systems. This paper presents an accurate observation of the leader corona. The morphology of the leader corona region during the discharge was obtained using a high-speed camera and an image-enhanced charge-coupled device camera. Statistical results showed that the morphology of the streamer region varied at different discharge sub-phases. The first corona was a conical region with the average apex angle of 118.91°; the leader corona region behaved as a cone with an average vertex angle of 75.87° during the leader propagation process; the streamer area in the final jump was a combination of a cylinder and a cone with the average vertex angle of 82.61°. Noting that the obtained cone apex angle of the leader corona during the leader propagation phase was significantly smaller than that in photographical reports, and the observation results were related to the selected temporal resolution of the optical image apparatus. The discussion on the effect of the single-frame exposure time on observation results found that the average vertex angle of the conical streamer region gradually decreased with the decrease of the single-frame exposure time, and eventually tended to a stable value of about 75.96°. For comparison purposes, discharge experiments under two gap-distances demonstrated that on the premise of stable leader, the statistical distribution characteristics of the leader corona vertex angle were consistent, indicating the universal applicability to the simulation researches. Finally, it is found that the distortion of the electric field caused by the space charge explained the variation of the streamer area during the discharge process.

The Spatial Evolution of Upward Positive Stepped Leaders Initiated From a 356‐m‐Tall Tower in Southern China

AbstractTwo initial upward positive leaders (UPLs) in upward negative flashes initiated from a 356‐m‐tall tower were observed with a high‐speed camera, an electric field sensor, and a magnetic field sensor. Although the waveforms of electric fields measured were saturated, both camera images and magnetic fields showed that the two UPLs had obvious stepwise characteristics in their upward moving stage. The magnetic fields associated with the UPLs were characterized by a series of fast‐changing bipolar impulses superimposed on a slowly increasing continuous component as the leaders moved upward, which were well corresponding to the light intensity changes of their high‐speed camera images. The 2‐D step length and step extension speed of the two leaders were estimated in the range of 0.5–3 m and 1.1–9.4 × 105 m/s, respectively. The radius of the leader luminous channel as a function of time and height was also estimated. The channel radius showed first a stable increasing trend in the leader upward moving stage and then 4–5 times of shrinking and expanding processes after the leader connected the cloud. The channel expanding and shrinking speeds were in the range of 2–9 × 104 m/s. Particularly, the channel radius showed an obvious increasing trend with the increase of the height all the time, which was in the range of 2.8–3.5 m when around the tower tip and of 5.6–8.4 m when at 24 m high above the tower tip. Such a feature of the channel radius is in good agreement with the concept of leader corona sheath in literature.

A macroscopic physical model for self-initiated upward leaders from tall grounded objects and its application

This paper presents a macroscopic physical model that can simulate an upward leader initiated from a tall grounded object under thunderclouds. Based on a tri-layer leader channel structure and the energy conservation law, a new equation for estimating the upward leader propagation speed is proposed. Equations for modeling other physical parameters, such as the leader line charge density, leader core radius, leader corona sheath radius, leader current, leader electric field and leader conductance, are also proposed. Besides, a set of initiation and survival criteria for a steady self-initiated upward leader from a tall grounded object is suggested. Based on the suggested criteria and the proposed model, the critical corona and charge amount as well as the minimum height for successful initiation of an upward positive leader (UPL) from a tall grounded object are evaluated and discussed. The model is then used to investigate the general properties of UPLs self-initiated from tall grounded objects with and without the effect of corona space charge layer near the ground under different thunderstorm conditions. The modelling results can well explain the leader properties observed in literature. The model is further tested with two set of experiment data and very promising results are obtained.

A 3-D Self-Organized Leader Propagation Model and Its Engineering Approximation for Lightning Protection Analysis

This paper presents a 3-D self-organized model of a downward stepped leader in negative cloud-to-ground lightning. In the model, the characteristic features of stepped leaders are generalized, so that parameters, such as the charge density along with the leader channel, leader corona sheath radius, leader step length, step time interval, and step advance speed, are calculated. Stepwise growth of the 3-D leader channel is developed stochastically. Parameters predicted by the model are qualitatively compatible with observation results, while the engineering approximation of the model for convenient lightning protection analysis is derived thereafter. The model is operated under various conditions at different leader initiation heights and electric potentials. Based on statistics of the simulation results, analytical relationships between the height and potential of leader initiation in the cloud and the charge distribution along the leader channel are approximated. Moreover, the lightning striking distance to flat ground is defined and calculated. Depending on the present model, the striking distance is found to be well associated with the charge density (or electrical potential) of the leader tip near to ground rather than the total charge in the leader channel. Based on appropriate physical understanding, the striking distance is connected to the return stroke peak current.

The NASA Lightning Nitrogen Oxides Model (LNOM): Application to air quality modeling

Recent improvements to the NASA Marshall Space Flight Center Lightning Nitrogen Oxides Model (LNOM) and its application to the Community Multiscale Air Quality (CMAQ) modeling system are discussed. The LNOM analyzes Lightning Mapping Array (LMA) and National Lightning Detection Network™ (NLDN) data to estimate the raw (i.e., unmixed and otherwise environmentally unmodified) vertical profile of lightning NOx (=NO+NO2) production. The latest LNOM estimates of mean lightning channel length, the lightning 10-m segment altitude distribution, and the vertical profile of lightning NOx production are obtained. The primary improvement to the LNOM is the inclusion of non-return stroke lightning NOx production due to: hot core stepped and dart leaders, stepped leader corona sheath, K-changes, continuing currents, and M-components. The impact of including LNOM-estimates of lightning NOx for an August 2006 run of CMAQ is discussed. An estimate of global annual lightning NOx production is also provided using the NASA satellite global lightning climatology.

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse-II: modeling

We have used the Bondio-Gallimberti model of positive leader propagation to simulate laboratory experiments of laser triggering and guiding of upward leaders initiated from a ground rod. The model proves to be capable of reproducing all the important features of laser-guided leader propagation that have been observed experimentally. The leader guiding effect of the laser-created plasma channel is taken into account in the model by adjusting the value of the charge per unit length of the leader, which has been measured in the laboratory to be lower for a laser-guided leader than for an ordinary one. The charge per unit length of the leader is related in the model to the critical temperature at which the air in the transition region at the leader tip must be heated to be conductive enough to become a new leader portion. For an ordinary leader, this critical temperature is 1500 K, at which the electrons all detach from the negative ions in the leader corona, increasing the air conductivity. We give the interpretation that in the case of the laser-guided leaders, because of the relatively high density of negative ions per unit length in the laser-ionized channel, the right conditions of conductivity can be met in the transition region without the electrons being all detached from the ions, allowing a reduction of the critical temperature and of the charge per unit length.

A simplified model for the simulation of positive-spark development in long air gaps

This paper presents a new modelling of positive-discharge development, based on new simplifying assumptions. It presents these assumptions, which are deduced from the existing theoretical models and concern mainly the coupling between the leader corona and the corona region. This model has been validated through the comparison of simulations with experimental results in the reference configuration of a rod - plane gap subjected to various positive impulse voltages. It has also been used to simulate the discharge behaviour with perturbations of the applied potential form.

Theoretical modelling of the development of the positive spark in long gaps

The main purpose of this paper is to present a physical model of the positive discharge in long air gaps. A large number of previous experimental and theoretical studies led to the identification of the different successive phases of the spark development: formation and propagation of first corona streamers, inception of the conductive stem at the electrode tip, formation and development of second corona (or 'leader corona') from the stem, and, eventually, the propagation of the leader and leader corona system until the final jump preceding the arc onset. Details of the specific modelling of each phase is presented, using the classical equations for conservation of mass, momentum and energy for each particle species. These basic equations are simplified according to the dominant electrostatic, hydrodynamic or thermodynamic processes involved in each step of the spark development. The resulting models for simulation of the corona and leader phases are coupled with an analytical calculation of the electric field due to the electrodes, the leader channel and the space charge injected into the gap. The different phase simulation models are expressed with a homogeneous simplification level and then linked sequentially into a complete model, which performs the step-by-step simulation of all the successive discharges phases until the final jump. The model described here is self-consistent since the only input data are the electrode geometry and the applied potential wave-shape. A good agreement between computed and experimental results has been obtained in various test configurations; the model has been also used to simulate the discharge behaviour with perturbations of the applied potential wave and permits the analysis of the conditions for stable propagation of the positive leader. It is shown that some parameters of practical interest, as the 50% breakdown voltage or the time to breakdown can be derived from the proposed model.

HF radiation at 3 MHz associated with positive and negative return strokes

HF radiation at 3 MHz generated by positive and negative return strokes is presented along with the associated broadband electric fields. The mean duration of the HF associated with negative return strokes was about 140 μs, with minimum and maximum values of 60 and 600 μs, respectively. The durations of the HF associated with the positive return strokes ranged from 5 to 25 ms which is much longer than the duration of HF radiation associated with negative return strokes. It is suggested that at least part of the HF radiation emitted by the return stroke is generated in the collapse of the leader corona sheath during the return stroke.

Corona envelopes and lightning currents

Laboratory results on coronas and considerations of fundamental electrostatics are used to study the corona envelope around the lightning channel. Calculations predict that immediately after a return stroke, a corona current of the order 2 kA flows for less than a millisecond, that between steps of a stepped leader corona draws a current of about 3 kA, and that at all other times the current governed by the velocity of radial charge motion is less than 1 A. Observed currents in all phases of the lightning discharge are accounted for by longitudinal channel extension, not radial expansion. A speculative picture of the roles of the corona envelope and of channel extension in discrete strokes of lightning is presented.