On a specialized testing installation — a high-voltage Marx generator, which allows the formation of a spark discharge up to 20 meters long, the process of the impact of lightning radio emission in the microwave range on an aircraft (AC) is simulated. A metal AC model made on a scale of 1:50 was placed in a 9 m long rod-plane spark gap. A positive voltage pulse was applied to the high-voltage electrode from the generator (front 100 μsec, duration 7500 μsec). The generated spark discharge passed through the AC model. It was found that when the spark channel passes through the body of the model, electromagnetic pulses of the microwave range with a duration of less than 500 psec and a rise time of less than 100 psec arise. The measurement of the resulting microwave pulses was carried out using special radio equipment in the subnanosecond range. High-speed photography of the discharge development, carried out synchronously with electromagnetic measurements, showed that microwave pulses (up to 10 GHz) arise at the stage of development of the spark discharge leader in the part of the “high-voltage electrode – insulated model” gap. Such pulses can be considered as a factor dangerous for various AC microwave radio systems, when the AC is struck by lightning, as well as for other objects, including power facilities, which contain equipment with microelectronics. Such microwave pulses can also occur during nearby lightning discharges (for example, in a lightning rod of a power substation), when a powerful streamer corona occurs on devices and wires of ultra-high voltage lines), and when a high-voltage spark discharge occurs when switching equipment (disconnectors, arresters) is triggered. In this case, disruptions are possible in the operation of control equipment (for example, relay protection devices) that contains microelectronics, as well as various communication means available at electric power facilities operating in the microwave range (above 1 GHz).

The microwave diagnostics of discharges occurring in an artificial cloud of charged water droplets created in an open air simulating the environment of thunderclouds is implemented. An artificial cloud with a droplet size of about 1 µm is opaque in the visible range, so intra-cloud discharges are not available for investigation by traditional methods in the spark discharge physics based on the registration of visible discharge radiation. Microwaves pass through such a cloud without noticeable attenuation, they interact only with the plasma of discharges occurring in the cloud. The probing microwave radiation had a wavelength of 8 mm. The attenuation of microwaves passed through the cloud was measured with temporary resolution of about 10 ns. The temporal characteristics of intracloud discharges were investigated.

Abstract The effects of generating pulsed radiation by a long spark discharge are important for the development of lightning models and applications related to lightning protection. In experiments with a Marx generator simulating a lightning discharge, we detected the radiation in the form of a single ultrawideband electromagnetic pulse (UWB EMP) about 200 ps in duration, and rising time about 100 ps. UWB EMP generation occurs during the breakdown of a “rod–rod” 4 m long gap. Pulses of almost unipolar shape are observed in more than half of all positive discharges. EMP emission occurs before the main stage, and corresponds to the start of the upward leader from a grounded electrode. In negative discharges, pulses are also observed, but less frequently and with a smaller amplitude. The UWB EMPs, given their large amplitude (more than 100 V/m at a distance of 90 m from the discharge), can be considered as possible new lightning damage factors.

A special experimental setup with a three-electrode discharge gap was used to study the dynamic characteristics of the ultra-high- and super-high-frequency (UHF-SHF) electromagnetic radiation of a high-voltage discharge having the streamer form with reference to the dynamics of individual streamers at the nanosecond time resolution. We performed synchronous detection of the radiation waveforms using a wideband horn antenna, on the one hand, and high-speed photography of the discharge development in the discharge gap using an ICCD camera, on the other hand. It was found that the high-voltage discharge is a source of radiation in the frequency band up to 10 GHz, which is a series of individual ultrawideband (UWB) bursts having durations of less than 1 ns and leading fronts less than 100 ps long and appears when the streamers moving from the discharge anode (thin wire) meet the discharge cathode (plane). By the order of magnitude, the number of radiation bursts corresponds to the number of streamers that reach the electrode, according to the high-speed photography data. The qualitative data confirmed by simple theoretical estimations show that the sources of UWB radiation pulses are individual streamers at the moment of their contact with the electrode, and the radiation of the streamers can be regarded as transition radiation.

On the created stand, in laboratory conditions, various phases of discharge development characteristic of lightning were simulated using an electric spark in a long air gap. With the help of special radio equipment, the radio emission of a long electric spark was investigated. It is found that at the stages of development of a spark discharge, radio emission is recorded in a long interval up to at least a frequency of 1.4 GHz. Data on radio emission in this range are of great practical importance for such areas as radio communication (during thunderstorm activity), radar, lightning direction finding. Keywords: Long electric spark, lightning discharge, radio emission, streamer discharge, lead discharge, radio communication.

A description of the developed nanosecond high-voltage generator of low-temperature plasma based on a volume streamer discharge is given. Plasma is formed in a high-voltage three-electrode gap, one of which is at a floating potential. Plasma is formed when a special switch is triggered, which connects a floating potential electrode, pre-charged with positive streamers, to a grounded electrode. The operation of the generator in a pulse-periodic mode greatly simplifies its application in experimental studies. Its design and electrical circuit are described. The main electrical characteristics and parameters of streamer plasma radiation in the optical and ultraviolet ranges are presented. An example of a specific application of a generator plasma for solving problems of water purification from metal ions (by the example of manganese) using electric discharge technology is given. The use of a low-temperature plasma of a streamer discharge for experimental research in the field of propagation of an ultrahigh-frequency (microwave) signal in an ionized region of the atmosphere (thunderstorm cell) is described.

Kostinskiy et al. (2015b), https://doi.org/10.1002/2015GL065620, using a high‐speed infrared (2.5–5.5 μm) camera, discovered the so‐called unusual plasma formations (UPFs) in artificial clouds of charged water droplets. UPFs had complex morphology including both streamer‐like regions and hot channel segments. They were observed both in the presence and in the absence of hot leader channels developing from the grounded plane toward the cloud. In this paper, which is aimed at revealing the genesis of UPFs, we present two UPFs that occurred inside the initial corona streamer burst of positive polarity emitted from the grounded plane, prior to the formation (or in the absence) of associated hot leader channel. These streamer bursts developed at speeds of 5–7 × 105 m/s over 1–1.5 m before entering the optically visible negatively charged cloud and producing UPFs at its periphery. Hot channel segments within UPFs were formed in very short times of the order of 1 μs or less. It is not clear if the UPFs were caused solely by the enhanced electric field near the charged cloud boundary or other factors also played a role. Occurrence of UPFs may be a necessary component of the lightning initiation process.

The dynamics of the brightness of a long‐spark channel during the breakthrough and the return‐stroke phases of discharge development were experimentally studied using streak‐camera images synchronized with current recordings at the high‐voltage electrode. The velocities of primary positive leader developing from the high‐voltage electrode and connecting negative leader extending from the grounded electrode during the breakthrough phase were measured as a function of the discharge current and the distance between leader tips. The velocities of the return‐stroke waves propagating upward and downward along the positive and negative leader channels, respectively, were also measured. The connection region exhibited channel splitting and reduced brightness relative to the channels above and below it.

Effects of lightning‐induced generation of high‐frequency and microwave radiation are of great interest for studying fundamental physics of lightning and its applications for monitoring of the thunderstorm activity and protection of equipment against electromagnetic interference. Ultrawideband electromagnetic pulses (UWB EMPs) of spark discharges about 1 m long were detected in a frequency band of up to 10 GHz in laboratory experiments using a cloud of water droplets charged up to the electric potential exceeding 1 MV. Electromagnetic pulses with characteristic front buildup durations from 50 to 100 ps were produced by streamer flashes, at the stage of leader propagation and the main stage of the discharge (i.e., the return stroke). Electric and magnetic fields of pulses were measured, and the radiation polarization was determined. The UWB EMP waveforms and the spectra obtained experimentally are consistent with colliding streamer models.

The streamer zone of positive leader during the breakthrough phase of long sparks was experimentally investigated with two methods. One of the methods is the analysis of streamer‐zone images obtained with a high‐speed framing camera with image enhancement. This method allowed us to estimate the spatial distribution of streamer density and low‐frequency conductivity in the streamer zone. The other method is the microwave probing, which we applied for the first time to long sparks. The attenuation of microwave beam in the streamer zone in our experiments is proportional to the total number of free electrons inside the microwave beam. Experimental data on the microwave attenuation combined with the streamer density found using the first method allowed us to estimate the total number of free electrons in one streamer. The following parameters were obtained. The streamer density in the center of the streamer zone is (0.6–1) · 105 m−3, and the total number of streamers in the streamer zone is 4 · 105–106. The average total number of free electrons in one streamer is about 3 · 1010. Low‐frequency conductivity on the axis of streamer zone was estimated to be typically 2 · 10−5 S/m, which is similar to that estimated for corona sheath in lightning. Both methods are based on the assumption of constancy of electric field and similarity of all streamers inside the streamer zone. The overall results of this study are generally consistent with this assumption.

Parameters of pulsed electric discharges arising between insulated metal parts of helicopter rotor blades, which generate electromagnetic interferences for onboard radio stations operating in the very short wave (VSW) and ultrashort wave (USW) bands have been studied. These discharges appear as a result of the in-flight engine-driven differential charging of helicopters. In-flight and laboratory experiments were used to determine the range of discharge voltages and currents and the amplitude and spectral characteristic of electromagnetic interferences produced by these discharges for onboard antennas.

Microwave diagnostics was first applied to the study of long spark and some new parameters of this discharge not available by the traditional experimental methods in this field were measured. The conditions of applicability of the method of measuring the high-frequency conductivity of the streamer zone by absorption of probing microwave radiation were theoretically analyzed.

Properties of positive and negative leaders developing in air gaps ranging from 4 to 10 m that were subjected to 100/7,500‐μs voltage impulses were examined using a two‐frame, high‐speed video camera with image enhancement. Abrupt extension (stepping) that culminated in a bright and structured corona streamer burst was observed for both negative (expected for the “classical” stepping process) and positive (expected for the so‐called restrike process) leaders. Selected high‐quality images of five negative and four positive leaders with pronounced corona streamer bursts are presented here. The morphology of corona streamer bursts was essentially independent of polarity. Streamer bursts exhibiting nearly spherical symmetry were observed. For the four positive leaders, the newly added channel sections (steps) were almost straight and had lengths ranging from about 50 to over 120 cm. For the five negative leaders, most of the steps were curved and their 2‐D lengths were some tens of centimeters. It is generally thought that positive leaders in both long sparks and lightning extend continuously or exhibit optically unresolvable steps whose length is comparable to the leader tip size (1 cm or less) and that for sparks only when the absolute humidity is relatively high (>10 g/m3 or so) or voltage rise time is relatively long (around 1 ms or more) can larger steps occur. In this study, both modes of propagation for different branches of the same positive leader were observed.

This review covers selected results of recent observations of natural lightning and laboratory sparks performed using high-speed video cameras at the Lightning Observatory in Gainesville (LOG), Florida, and at the high-voltage research facility in Istra, Russia, respectively. The most important results include (a) the first high-speed video images of bidirectional leader that made contact with the ground and produced a return stroke, (b) the first speed profile of positive leader that developed in the channel of preceding negative stroke, (c) discovery of unusual plasma formations that coexist with "normal" discharges inside the artificially-charged cloud, and (d) the first two-frame record of the connection between negative and positive leaders after the common streamer zone has been formed.

The possibility of initiation of electric discharges by a crossbow bolt (projectile) moving in the electric field of a cloud of negatively charged water droplets has been demonstrated for the first time [1]. Over one hundred of discharges have been produced. For each event, a high-speed video camera recorded the images of upward positive leaders developing from both the nearby grounded sphere and the projectile, followed by the return-stroke-like process. Corresponding currents were measured and integrated photos of the events were obtained. The results can help to improve our understanding of lightning initiation by airborne vehicles and by a vertical conductor rapidly extended below the thundercloud in order to trigger lightning with the rocket-and-wire technique.

Detailed observations of the connection between positive and negative leaders in meter-scale electric discharges generated by clouds of negatively charged water droplets are presented, and their possible implications for the attachment process in lightning are discussed. Optical images obtained with three different high-speed cameras (visible range with image enhancement, visible-range regular, and infrared) and corresponding current recordings were used. Two snapshots of the breakthrough phase of the leader connection, showing significant leader branching inside the common streamer zone, are presented for the first time. Positive and negative leader speeds inside the common streamer zone for two events were found to be similar. Higher leader speeds were generally associated with higher leader currents. In the case of head-to-head leader connection, the infrared brightness of the junction region (probably representing the gas temperature and, hence, the energy input) was typically a factor of 5 or so higher than for channel sections either below or above that region. In 16% of cases, the downward negative leader connected to the upward positive leader below its tip (attached to the lateral surface of the positive leader), with the connection being accomplished via a channel segment that appeared to be perpendicular to one or both of the leader channels.

We have observed unusual plasma formations (UPFs) in artificial clouds of charged water droplets using a high-speed infrared camera operating in conjunction with a high-speed visible-range camera. Inferred plasma parameters were close to those of long-spark leaders observed in the same experiments, while the channel morphology was distinctly different from that of leaders, so that UPFs can be viewed as a new type of in-cloud discharge. These formations can occur in the absence of spark leaders and appear to be manifestations of collective processes building, essentially from scratch, a complex hierarchical network of interacting channels at different stages of development (some of which are hot and live for milliseconds). We believe that the phenomenon should commonly occur in thunderclouds and might give insights on the missing link in the still poorly understood lightning initiation process.

A unique type of corona discharge-modulated corona nanosecond discharge-has been obtained, the parameters of which have been determined in a geometric system of electrodes with a sharply heterogeneous electric field in air under ambient pressure and natural humidity.