Vladislav Mazur

Common physical processes in natural and artificially triggered lightning

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 defined for interpretation of common lightning processes. These principles focus on lightning initiation, charges on the leader, the leaders electrical potential, the electrical break-down 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.

Model of electric charges in thunderstorms and associated lightning

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.

“Spider” lightning in intracloud and positive cloud-to-ground flashes

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 × 105 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 leaders ground attachment. The interferometer, however, failed to map fast negative leaders that occurred intermittently during the spider-lightning events.

Correlated high-speed video and radio interferometric observations of a cloud-to-ground lightning flash

A six-stroke cloud-to-ground lightning flash has been studied using observations from a high-speed video camera (1000 frames s −1 ) and a VHF radio interferometer (1-μs time resolution), as well as additional electric, magnetic, and optical measurements. The flash produced strokes along two channels to ground and a long (550 ms) continuing current. The video observations provided time-resolved pictures of stepped and dart leaders, short and long continuing currents, and M components during the continuing currents, and complemented and confirmed the interferometer observations of flash structure and development. The M components were initiated by fast negative streamers inside the cloud which propagated into the conducting channel of the continuing current and subsequently brightened the channel to ground. We call this sequence an M event. Dart leaders and the fast streamers of M and K events were found to be significantly brighter than stepped leaders and continuing currents to ground. A number of streamers did not initiate M events that were identical to in-cloud K streamers. Analysis of the M and K event occurrences indicated that the conducting channels of the continuing current both expanded and contracted with time. An M-type event was also observed during a dart leader. It is proposed that the channel multiplicity within the flash resulted from cutoff of the channel to ground while charge continued to flow down the channel from the stroke source region, stranding negative charge along the channel. Dart-stepped leaders (such as occurred during the third stroke) are similarly explained. Because the stranded charge is observed to be greatest for initial strokes, new channels to ground of stepped and dart-stepped leaders are expected to follow initial strokes, as is usually observed. The video and electric field observations indicate that all return strokes have at least a short continuing current, of the order of one or a few milliseconds. The results also reinforce the well-known observation that long continuing currents tend to follow relatively weak return strokes.

Common physical processes in natural and triggered lightning in winter storms in Japan

An analysis of measurements of electric, magnetic, and radiation field variations produced by rocket-triggered discharges in winter storms in Japan shows that a discharge initiated by a rocket with trailing and grounded wire is a negative leader that consists of continuous current and a pulse series associated with streamer development at the tip. The negative leader is not followed by a process analogous to a return stroke in cloud-to-ground flashes. Application of the electrostatic model of lightning as a bidirectional and uncharged leader in an ambient electric field to analysis of rocket-initiated discharges and positive cloud-to-ground flashes uncovers the commonality of processes occurring in both types of discharges.

Aircraft lightning initiation and interception from in situ electric measurements and fast video observations

Multipoint electric field measurements on the French C-160 research airplane presented additional verification for a physical model of aircraft-triggered lightning as a bidirectional leader initiated on the aircraft. For the few lightning strikes to aircraft that were not aircraft-triggered, the multipoint electric field measurements proved that these strikes resulted from interception of the aircraft by natural lightning flashes. Video images of lightning channels at 200 frames per second provided the first and only physical evidence of the presence of a positive leader exiting from the positive pole of the airplanes electrical dipole. The existence of a small (<1 A) positive leader current had been predicted in the physical model but had not yet been measured. Weak pulse processes found in magnetic field variations during positive leader development confirm the presence of small amplitude streamers in addition to a continuous leader process.

The physical processes of current cutoff in lightning leaders

Current cutoff in lightning channels, which takes place in the development of both intracloud and cloud-to-ground flashes, is still poorly understood. A new evaluation of the conditions leading to current cutoff, and also of the two existing hypotheses of the cutoff mechanism, is the main objective of the paper. We reviewed the literature with results of measurements and modeling of free-burning arcs in a laboratory (the closest analogs of lightning leaders) focusing on the relationship between the internal electric field and current. This relationship governs the leaders behavior in the current cutoff. In our analysis of the mechanisms leading to current cutoff, we identify the two types of current cutoff in lightning channels: the current cutoff in a single, unbranched leader channel, which occurs as the result of reaching the threshold conditions for leader propagation; and the current cutoff in branched leaders, when screening by the leader branches alters the ambient electrical environment, thus diminishing the leader current and causing cutoff at a branching point or at the base of the straight channel that preceded branching. We advance the electrostatic model of the screening effect of branching on current cutoff, introduced by Mazur and Ruhnke (1993), and we provide the evidence of this mechanism from lightning observations. We also critically evaluate the concept of lightning-channel instability, proposed by Heckman (1992), as a suggested mechanism leading to current cutoff. We show that the fundamentals of this concept and therefore the concept in its entirety are invalid.

Simulating electrodeless discharge from a hydrometeor array

The objective of this research was to test, by means of an experiment in a high-voltage laboratory, the effect of an array of hydrometeors on the processes involved in the streamer-leader formation of lightning. Because the common types of hydrometeors present whenever lightning initiation in thunderstorms occurs are ice particles (graupel, hail, or ice crystals), we used, in this experiment, conductive particles similar to hail in size, with various spacing between them, but all under normal atmospheric pressure and room temperature. The laboratory array was suspended on dielectric threads in a uniform electric field of 1 MV m−1 in the middle of the gap between the high-voltage and ground electrodes. During the first phase of the experiment, we studied the formation of a bidirectional arc discharge from the array and the effects of the arrays size on the electrical characteristics and on the speed of development of the discharge. We continued with the same objectives in the second phase of the experiment, adding high-speed video observations with a recording speed of 10 Mfps, to observe all stages of the streamer-leader formation.

The relationship of leader and return stroke processes in cloud‐to‐ground lightning

The unique relationship between the leader and return stroke in cloud-to-ground flashes and the ambient cloud potential distribution is formulated. This relationship in the form of a ratio of an electric field produced by a leader, ΔEl, to that of a return stroke, ΔEr, is analyzed for two electrostatic leader models, the bi-directional leader and the Schonland “source charge” models, and is then compared with field measurements. It is found that the ΔEl/ΔEr ratios of real flashes fall between the curves of the ratios calculated for the two leader models, with each model representing an extreme condition of ambient electric field distribution.

Lightning Investigation with Radar

In 1943, within two years after the first thunderstorm observations with a microwave radar (Ligda, 1951), the first published account of a lightning observation occurred, with a radar of 1.5-m wavelength (Pawsey, 1957). About the same time, Shono (1947) reported detection of lightning with a 4-m wavelength radar in Japan. The general method of lightning observation, which has been followed by many subsequent investigators, was to direct a fixed radar beam above the most vertically developed region of thunderstorms in order to avoid strong backscattering signal from precipitation that masks the lightning echoes. The first photographs of lightning echoes were made in 1949, using a 10-cm wavelength radar at MIT (Ligda, 1950).