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 return stroke in lightning or long spark is a wave of electric potential and current propagating from the ground over several kilometers (for lightning) or from the grounded electrode over several meters (for a long spark) along the channel created by the leader. A high current generated during the return stroke process is the source of most damaging effects of lightning. Therefore, the return-stroke process in lightning and in the laboratory long spark discharge simulating lightning (at least at early times) has attracted attention of many researchers.Many theoretical models of return stroke have been developed and tested, and the results of these studies have been published (Сooray, 2010;Rakov & Uman, 2006). In all models [(except for the one developed by Gorin (1985)], the evolution of the return-stroke current wave was considered assuming an instantaneous contact of the main downward leader channel with the ground or with the upward connecting leader (UCL) channel. As a result of this assumption, all parameters of the current pulse were determined by the parameters of the leader channels only. Actually, the conductivity of the common streamer zone formed during the breakthrough phase increases gradually, by several orders of magnitude, from a small value at the beginning of the breakthrough phase to a large value at its end, and leads to a significant increase of the current (Bazelyan & Raizer, 1998). Therefore, the front of the current pulse is largely formed during the breakthrough phase, and it can be expected that the parameters of the common streamer zone also affect the maximum discharge current. Current views of the breakthrough phase, which are still not fully understood, are summarized by Rakov and Tran (2019). For more recent finding, we refer the reader to Jiang et al. ( 2021).The collision of downward leader with the UCL usually occurs not high above the ground and is usually not seen in lightning images (e.g., Idone & Orville, 1982;Rakov & Uman, 2006), except for very tall strike objects, for which UCL length can be of the order of hundreds of meters (e.g., Rakov & Uman, 2006;Yokoyama et al., 1990). The UCL colliding with the downward leader was first observed in rocket-triggered lightning (Wang et al., 1999). In those experiments, the velocities of the main and UCLs and the average velocities of the return stroke waves propagating up and down from the junction point of the leaders were measured. The average speeds of the downward dart leader and of the UCL were found to be 4 × 10 7 and 2 × 10 7 m/s, respectively. The speed of the return stroke was found only for the upward waves, 2 × 10 8 and 3 × 10 8 m/s for two cases. First images of common streamer zone were obtained by Hill et al. (2016) andRakov (2017) for triggered and natural lightning, respectively. However, the detailed spatial and temporal variations of the channel luminosity were not obtained. In experiments with a long sparks, which are often viewed as a laboratory analog of lightning, the velocities of the main negative and conn...

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The results of a physical simulation using negatively charged artificial thunderstorm cells to test the spectrum of possible electromagnetic effects of upward streamer discharges on the model elements of transmission line monitoring systems (sensor or antennas) are presented. Rod and elongated model elements with different electric field amplification coefficients are investigated. A generalization is made about the parameters of upward streamer current impulse and its electromagnetic effect on both kinds of model elements. A wavelet analysis of the upward streamer corona current impulse and of the signal simultaneously induced in the neighboring model element is conducted. A generalization of the spectral characteristics of the upward streamer current and of the signals induced by the electromagnetic radiation of the nearby impulse streamer corona on model elements is made. The reasons for super-high and ultra-high frequency ranges in the wavelet spectrum of the induced electromagnetic effect are discussed. The characteristic spectral ranges of the possible electromagnetic effect of upward streamer flash on the elements of transmission line monitoring systems are considered.

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