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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 highspeed 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.

Plain Language Summary Lightning is a positive or negative super long spark discharge between the cloud and ground or cloud and cloud. The propagation of positively and negatively charged plasma channels, termed, respectively, positive and negative leaders, is the physical nature of lightning development. Under the slow varying ambient electric field, positive leader propagation is not continuous and exhibits steps with waiting time of tens of microseconds punctuated by sudden elongations and intense reilluminations. These steps are currently not fully understood. Leader discharges can also be simulated in a laboratory. Important insights about natural lightning phenomena have come emerging from explorations of laboratory-scale atmospheric discharges. Here, we investigate the positive leader steps under the slow varying ambient electric field in long laboratory spark discharges, using a high-speed video camera and a synchronized electrical parameter measurement system. A discharge that precedes the intense leader channel reillumination is discovered, which provides a new understanding of the dynamic process of positive leader steps and provides new insights into positive stepped lightning leaders under the slow varying ambient electric field.