No abstract

Abstract. An axisymmetrical electrostatic model of charges in a thunderstorm cloud at the mature stage is used to initiate a bidirectional vertical leader that develops into either a cloud-to-ground or an intracloud flash from the regions of maximum electric field. The principal result of this study is the determination of the physical and quantitative relationships among cloud charges, potentials, and electric fields, and the induced charges, currents, and electric field changes of the lightning channel from the numerical solution of the Poisson equation for a cloud charge model with lightning. An important consequence of the model is that the upper part of the cloud-to-ground leader and the lower part of the intracloud leader terminate inside the cloud.

Common physical processes are identified in various types of natural and artificially triggered lightning flashes, both in summer and winter storms. By applying an electrostatic model of bidirectional, uncharged and monopolar, charged leaders, the main physical principles are deftned for interpretation of common lightning processes. These principles focus on lightning initiation, charges on the leader, the leader's electrical potential, the electrical breakdown at the leader tip, leader branching and current cutoff, the occurrence of recoil streamers, and the conclusion of lightning propagation. The bidirectional, uncharged leader model is compared, on both physical grounds and by analyzing the electric field changes, with the conventional model of a unidirectional, uniformly charged leader which originates from a space charge source. These two models are also tested against experimental data obtained with a VHF mapping system and in situ measurements of electric fields in storms on the heights of lightning origins in cloud-to-ground flashes. INTRODUCTIONLightning research is still predominantly in the observational or analysis stage, and many studies have been devoted to describing, labeling and comparing various lightning processes. Even by combining different measuring techniques and using a variety of remote sensors on the ground, most records obtained are difficult to interpret definitely because of missing links, particularly in the understanding of lightning development in space. Laboratory experiments with long gap discharges (long sparks) between electrodes and a plane have been directed to problems of insulation and lightning protection of power transmission linesbut have also benefited lightning studies because they produced valuable data on negative and positive leader formation and on the effects of several environmental factors on leader propagation. There are, however, some constrains concerning the applicability of laboratory results to cases of natural lightning, because of (1) the complex conditions in nature, including the space charge environment, that are not reproduced in the laboratory; (2) the much larger spatial scale of processes in thunderclouds; and (3) the fact that natural lightning is initially an electrodeless discharge with an ambient electric field as the source of energy. Better linkages between laboratory studies of long sparks and natural lightning have been made possible through the introduction of rocket-triggered lightning techniques, subsequent studies of initial lightning processes using direct current measurements [e.g., Laroche et al., 1985, 1991], and observations with optical and video systems [e.g., Idone et al., 1984]. In the 1950s, Kasemir [1950] applied the fundamental principles of electrostatics to lightning, viewing it as a conductor in an ambient electric field (E field). The concept of the bidirectional, Paper number 93JD00626. 0148-0227/93/93JD-00626505.00 uncharged leader proposed and developed by Kasemir [1950, 1960, 1983] was overlooked or ignored by most...

The objective of this paper is to review our present understanding of the physical processes in lightning flashes during their development within or outside a cloud, following lightning initiation. This represents the 'big picture' of lightning development, in the scale of the cloud dimensions themselves. Since the acceptance of the bi-directional, zero-net-charge leader concept, significant changes have occurred in our understanding of the key physical processes of which a lightning flash is comprised, and in the analytical relationship between the electrical structure of a cloud and lightning parameters. These changes are discussed with an emphasis on the unifying nature of the bi-directional leader concept. To cite this article: V. Mazur, C. R. Physique 3 (2002) 1393-1409.  2002 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS Mécanismes généraux de développement de l'éclair RésuméCet article présente les connaissances actuelles des processus physiques régissant le développement de l'éclair à l'intérieur ou à l'extérieur du nuage, après la phase d'initiation. Il s'agit de proposer une vision d'ensemble du processus, à l'échelle du nuage lui-même. Depuis que le concept de leader bi-directionnel non chargé est complètement admis, notre compréhension des principaux mécanismes de l'éclair a évolué de façon significative et il est possible d'appréhender de façon analytique les relations entre la structure électrique du nuage d'orage et les paramètres caractéristiques de l'éclair. Ces nouvelles approches sont discutées en insistant sur le caractère unificateur du concept de leader bi-directionnel. Pour citer cet article : V.

Abstract. The nature of visible, horizontally stratified lightning channels propagating over large distances near the cloud base during the decaying stage of a storm (also called "spider" lightning) was investigated. The study was effectuated through the use of the coordinated observations of a VHF interferometer, a high-speed image-intensified video system, measurements of electric and magnetic fields, and optical transients. Spider-lightning events were found to be negative leaders similar to stepped leaders in negative cloud-to-ground flashes, with a similar average speed of propagation horizontally of 2-4 x 10 5 m s -1. Being slow negative leaders, spider-lightning events are part of intracloud flashes and positive cloud-to-ground flashes occurring prior to and during the inverted (fair weather polarity) phase of the End of the Storm Oscillation in the ground electric field. Spider lightning is characterized by both the pulsing luminosity at the tips of its branched channels and the continuous luminosity (for tens to hundreds of milliseconds) which is maintained by the continuing current flow. The interferometer produced mapping of radiation sources closely resembling the spider-lightning channels (negative leaders) but only a weak trace of radiation sources associated with positive leaders to ground. Both the video images and a few radiation sources of positive leaders were obtained within 1 ms of the leader's ground attachment. The interferometer, however, failed to map fast negative leaders that occurred intermittently during the spider-lightning events.