Under the slow varying ambient electric field, positive leader propagation exhibits steps characterized by intense reilluminations and abrupt elongations. These steps are currently not well understood. In this work, we investigate these steps in laboratory atmospheric discharges, using a high‐speed video camera and a synchronized electrical parameter measurement system. The discharge, emitting weak light and preceding the intense reillumination, is discovered. This finding suggests that the leader channel actually restarts and extends forward before the intense reillumination, which deepens our understanding of the dynamic process of the positive leader step. The discharge before the intense reillumination contributes to the corona inception from the electrode, leading to the intense reillumination of the leader channel and the emergence of an intense corona streamer burst from the leader tip.
The geometric form of the leader trajectory is an interesting aspect of long air gap discharge research. In this paper, the 3-D discharge channel tortuosity in 3m rodplane gaps is obtained by following steps: Firstly, using two static cameras fixed orthogonally to record the face photo and the side photo of the discharge channel. Secondly, the 3-D discharge channel is reconstructed by employing image recognition technology. Thirdly, the discharge channel tortuosity in two-dimensional space and three-dimensional space are derived respectively, which satisfied the Gaussian distribution law. The statistical results indicate that the channel tortuosity will be conservatively estimated by using a two-dimensional method. Statistical results of the channel tortuosity with three-dimensional method indicate the following conclusions. The angle between the discharge channel segments and the gap axis meets the distribution of rational function and its average value and standard deviation is 23.8°and ±19.0°respectively; the angle between the adjacent channel segments also meets the distribution of rational function, and its average value and standard deviation is 13.3°and ±9.9°.
The most important physics underlying lightning is the leader discharge. The presence or absence of space stems/leaders in leader steps is the key to the polarity asymmetry of leaders, which describes the difference in macroscopic behavior between positive and negative leaders and is a long-term consensus among lightning physicists. It is generally believed that negative leader steps are led by space stem, and there is no space stem/leader in positive leader discharges. Here we report the emergence of the space stem and the bidirectional development of the space leader in positive leader steps in moist air, using a high-speed camera with unprecedented spatial-temporal resolution. The lifetime of space stem/leader in positive leader steps is shorter than that in negative leader steps, causing the uncover of space stem/leader in previous studies. The bidirectional development of space leaders in positive leader steps may be an important source for VHF radiations, illuminating insight into the outstanding problem that how positive lightning leaders produce VHF radiation.
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