Basis of Lightning Discharge Phenomena
Lightning Discharge: the Initial Development, Experimental Investigation of Microstructure and RF Image of a Thunderstorm Cloud, Laboratory and Computer Modeling
Tech Area / Field
- PHY-PLS/Plasma Physics/Physics
- OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences
3 Approved without Funding
Russian Academy of Sciences / Institute of Applied Physics, Russia, N. Novgorod reg., N. Novgorod
- Scientific Research Radiophysical Institute, Russia, N. Novgorod reg., N. Novgorod
- University of Electro-Communications / Department of Electronic Engineering, Japan, Tokyo\nObirin University, Japan, Tokyo\nNiagara University, USA, New York\nUniversity of Mississippi, USA, OH, Oxford\nUniversity of Florida / Department of Electrical and Computer Engineering, USA, FL, Gainesville\nJAXA / Earth Observation Research Center (EORC), Japan, Tokyo
Project summaryLightning is one of the most impressive and dangerous atmospheric phenomena, attracting great attention of people and scientists for many years. Although there is a large body of research addressing numerous aspects of lightning discharge, a complete understanding of physical processes concerning electrical structure of thunderstorms, breakdown initiation and flash propagation, has not been accomplished up to now. The role of lightning has recently gained even greater interest of meteorological and physical societies due to the new experimental information derived from in-situ (balloon and aircraft) measurements, radar observations, lightning mapping with ground-based and satellite systems, triggering lightning experiments, laboratory studies of charge transfer and discharge processes. In addition, luminous features in the upper atmosphere over thunderstorms, namely sprites, elves and jets, have recently captured the attention of scientists in atmospheric and ionospheric research. Theoretical analysis of all this information requires rather new approaches, based on the modern concepts of electrodynamics, nonlinear dynamics and statistical theories.
The project is intended to combine experimental investigation of thunderstorm clouds with balloon-borne and radar technique, laboratory studies of small scale processes, theoretical research to be based on the results of these experiments, modern methods of mathematical simulation, and previous extensive experience of the participants concerning atmospheric electricity problems. Original approaches of the authors to the fundamental problems of space charge generation and structure in a thundercloud, discharge initiation and electromagnetic radiation, and new mathematical models, will be developed to achieve better understanding of lightning processes under different meteorological conditions.
One of the main problems of thundercloud electricity until now is the very fast (during the time of order of 100 ms) transition to the perfectly conducting state (“metallization”) of the substantial part of the cloud, accompanied by accumulation of initially distributed over the cloud volume charge on the bottom side of the cloud, leading cloud-ground (CG) breakdown initiation. It is difficult to explain this phenomenon more quantitatively because the mean value of the intracloud great-scale electric field is by an order of magnitude less as compared to the local breakdown critical field. As follows from numerous observations, a CG lightning flash consists of several component discharges, namely, breakdown processes preceding stepped leaders, stepped leaders, return strokes, dart leaders, and interstroke processes [Lightning Electromagnetics, Ed.R.Gardner, 1990]. The first (preliminary breakdown) stage of a discharge lasts for several tens of milliseconds and characterized by the generation of broadband HF and VHF electromagnetic radiation. The latter consists of many bursts of short pulses, which imply the appearance of numerous (up to hundred thousands during the time as short as 100 ms) electrical discharges with characteristic channel lengths from 60 m to 300 m. At the preliminary stage these discharges do not change any substantial change of the gross field in a thundercloud; they are similar to the streamer channels of a laboratory spark discharge. Nevertheless, there is a great difference between natural lightning flash and laboratory long spark: in the case of a thunderstorm cloud the microdiscarges are much more intensive and occupy all the volume of a cloud. The preliminary stage of the flash is completed when one of these leaders reaches the bottom of a thundercloud, giving rise to the stepped leader stage, followed by the return stroke. During the return stroke fast neutralization of the charge accumulated at the preliminary stages occurs.
It is obvious therefore that small scale structuring of the lightning discharge and fast transition to the conducting state play a very important role in thunderclouds. The project is aimed on the understanding of these fundamental processes.
The problem of electrical structure of a thunderstorm cloud is based on the two-scale approach. The simple conceptual model of the gross charge structure of thunderclouds has been inferred from measurements and theory up the middle 1980's, which is referred usually as dipole/tripole structure. It has been described in the comprehensive review by Williams  and has prevailed to the present. Takahashi [1978, 1984] emphasized riming electrification as the primary process in the tripole structure formation. Up to now a number of models of electrical structure of clouds have been published. Common way to improve similar modelling is the increase of complexity taking into account different charging mechanisms, their different parametrizations and actual cloud dynamics patterns.
The authors of this project are developing a supplementary part of this problem [Trakhtengerts, 1989; Mareev et al., 1996; Trakhtengerts et al., 1997; Iudin et al., 1998; Iudin, Trakhtengerts and Hayakawa, 1999], based on the search of main characteristic structural elements of electrification dynamics. Particularly, we consider specific electrodynamical properties of a thunderstorm cloud, arising due to intensive multi-flow motion of charged particles (heavy charged droplets, light ice crystals etc.). At this stage a thundercloud reveals an ability to self-organize, which manifests in a small-scale electrical stratification, when intense electrical cells with the scales of order of 10 – 100 m are generated. A very important feature of this process is that the electric field in these cells may exceed the mean field value substantially, reaching locally the critical breakdown field. The breakdown inside such a cell initiates breakdown in neighbouring cells, forming a widely branched nonstationary conducting network, occupied the full volume of the cloud. This network can be defined as a “drainage” system of the macroscopic space charge gathering in the cloud [Iudin and Trakhtengerts, 1998]. The authors of the project have elaborated the fractal approach to to the quantitative description of this drainage system. Further development of these problems will be performed in the framework of the Project.
Detail studies of the space charge structure within thunderclouds are necessary particularly for understanding how the electric fields that are measured inside storms, result in and are affected by lightning flashes. Luminous features in the upper atmosphere over thunderstorms, namely sprites, elves and jets, have recently captured the attention of scientists in atmospheric and ionospheric research [Sentman et al., 1993; Lyons, 1996]. Huang et al.  reported on the distinct differences in ELF radiation from positive ground flashes associated with elves and with sprites and suggested that different types of space charge structures, leading to positive ground flashes, could result in sprite or elve generation respectively. This point is confirmed by recently obtained previous theoretical results by Smirnova, Mareev and Chugunov  who modeled the lightning generated electric field transitional processes over thunderclouds. We presented a new quasi-electrostatic model of high altitude electric field generating due to lightning-induced change of a thundercloud electric structure. The simplest possible geometry of the structure and lightning channel were studied. More complicated charge distributions (adequate to experimentally derived profiles from the Takahashi's group database) are intended to be investigated in the present project to establish the correlation between the different geometries and sprites or elves generation, as well as to compare the source-charge and bidirectional lightning models in terms of their adequacy to the field transient description over thunderclouds. Development of such a theory will promote the understanding of the runaway electron mechanism role [Gurevich, Roussel-Dupre, 1993] in the troposphere and mesosphere discharge occurence.
Along with theoretical treatment and computer modeling the authors of the project have extensive experience in laboratory plasma experiments. They have elaborated the principles of laboratory modeling of multi-flow instability of highly charged aerosol particles and started with the design of a set-up for this experiment. It is planned to complete the construction of this set-up and conduct the experiments in the framework of the present project. The results of this experiment will be treated in close connection to the results of well-known experiments by Takahashi [1978, 1996] on the laboratory modeling of charge transfer processes in thunderclouds. They will help to understand the nature of space charge structure formation and lightning discharge development in thunderstorms.
Particular emphasis in the project will be paid to the following advanced aspects, recently received much attention:
1. Analysis of different mechanisms of the charging of thunderstorm particles with regard to the electrical filamentation of a thundercloud. Participation in data processing and analysis for balloon measurements in winter thunderstorms in Japan (carried out by Takahashi group) is foreseen.
2. Laboratory modeling of electrical filamentation of multi-flow charged aerosol particles under thunderstorm cloud conditions.
3. Development of physical basics of electric breakdown phenomenon in the presence of electrical cells of different intensities. Analysis of corona discharge near the large hydrometeors in view of its contribution to the large-scale cloud electrodynamics before the leader inception.
4. Development of fractal approach in terms of its applicability for the quantitative description of an electric system of the gathering of macroscopic charge within a thundercloud. Analysis of experimental data, supplied by a Collaborator, will allow us to reveal the fractal properties and small-scale electric structure in thunderclouds.
5. Computer modeling of intracloud discharge dynamics with allowance for the electrical filamentation of a thundercloud.
6. Modeling of electromagnetic radiation from lightning in the radio frequency band with allowance for nonstationary dynamics and fractal structure of great-scale and small-scale currents in the cloud. Participation in the performance of experiments on the RF radiation from thundercloud conducted by the Collaborators is planned, as well as in data processing and analysis of their results to derive new data concerning the presence of coherent current structures in a cloud.
7. Field experiments on the receiving of acoustic radiation from a thunderstorm cloud.
8. High altitude discharge (sprites and elves) characterization and modeling to reveal the connection between electrical structure of thunderclouds and recently reported optical phenomena in the middle atmosphere, correlated with severe positive cloud-to-ground flashes. The data of the Collaborator on the ELF radiation of high-altitude discharge will be used.
The methods developed in the project will be used for the solution of a number of practical problems, in particular for the modelling of creeping discharge on insulator surfaces as applied to aircraft dynamics, and computer simulation of the downward leader channel beneath a thundercloud.
The following results are intended to be obtained at the end of the project:
1. Creation of a unified database of in-situ balloon sounding profiles of electric and meteorological parameters in winter thunderclouds in Japan, theoretical explanation of these profiles.
2. Creation of a unique laboratory set-up for the study of small-scale electric structure of thunderstorm clouds.
3. Development of the advanced theory of electric breakdown phenomenon in the presence of electrical cells of different intensities in thunderclouds with allowance of corona discharge near the hydrometeors.
4. Revealing of the fractal properties and small-scale electric structure in thunderclouds as connected to the results of concrete radar and probe field experiments..
5. Quantitative explanation of temporal dynamics, intensity and frequency spectra of electromagnetic and acoustic radiation from a thunderstorm cloud.
6. New data concerning the presence of coherent current structures in a cloud. Estimation of the fractal dimension of the electric drainage system of thunderclouds.
7. The better understanding of sprite (elves) events, their correlation with positive lightning and their role in the atmospheric dynamics.
8. The creation of an extensive numerical model of space charge distribution and intracloud discharge development. Better understanding of electrification processes which are in charge of different kinds of space charge profiles formation.
In the course of the project realization a comprehensive investigation of lightning discharge will be performed. We hope to create general model of lightning discharge initiation and progression. The result of project realization will be of practical significance.
The Project proposed will be performed in the framework of a close co-operation of leading teams in the field of electric measurement performance and their interpretation. The project consortium unifies well-known teams from the Obirin University (headed by Prof. Takahashi), the University of Electro-Communications (headed by Prof. Hayakawa) as well as the team from the Institute of Applied Physics, RAS, headed by Prof. Trakhtengerts. The team from RRI headed by Dr. Iudin, has an extensive theoretical background and numerical modeling of fractal processes as well as the experimental investigations of electromagnetic and acoustic radiation from different sources including thunderstorms. Project participants from IAP RAS and RRI have gained also the great experience in observation and calculation of electromagnetic processes related to generation and propagation of powerful electromagnetic pulses in the atmospheres. During the Project performance the participants will exchange the information with collaborators, joint discussion of the results obtained. Activities on the Project will make possible for scientists and specialists from the Russia, engaged earlier in development of nuclear weapons and test, to retarget their capabilities to peaceful activities, encourage integration of the Russian scientists into the International scientific community, promote fundamental and applied research for peaceful purposes and will facilitate resolving of national and international technical problems in the field of monitoring of the environment, that meets the goals and directions of activities of the ISTC.
All the teams of this consortium have extensive experience in the subject involved, and their joint participation will allow us to enhance the co-operation between Japanese and Russian teams for the analysis of space charge, electric field and lightning discharge in the atmosphere. It will lead to new knowledge on space charge structures in the atmosphere and their effects.
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