Fine Structure of the Breakthrough Phase of the Attachment Process in a Natural Lightning Flash

A lightning flash striking a 325-m tower was captured using a high-speed camera operated at 380 kfps and the characteristics of breakthrough phase (BTP) were recorded in two consecutive frames. The fine time-resolved images indicate that the common streamer zone (CSZ) formed when the bright tips of negative and positive leaders were >= 23 m apart from each other. During BTP, both the negative and positive leaders extended their channels with enhanced luminosity and the gap between them was shortened to 16 m in the frame prior to return stroke. A space leader-like luminous segment was found within the route of leader connection. This route is inferred to have already been determined at the initial formation of CSZ, by the interconnected streamers of approaching leaders. The dominant growth of this route is interpreted as a new scenario for the conversion of a high-impedance CSZ to a hot plasma channel during BTP.