High-speed Video Observations on Abrupt Elongations of the Positive End of Bidirectional Leader

Abstract

One of the most important advances in lightning physics research in recent decades is the introduction of the the bidirectional developmengt of leaders and its observation and verification, but this theory is not paid much attention until more and more observation results of natural lightning, long gap spark discharge in laboratory and artificially triggered lightning have proved the correctness of the concept.The theory is used to establish the model of leader to interpretate the physical mechanism of lightning initiation and development. High-speed video camera observation not only provides direct evidence for the development of bidirectional leader, but also gives details of positive and negative ends. Previous observations show that the negative leader developed in a step-wise manner by relying on the space stem/leader and the corona streamer at the front of the new leader's tip. Positive leaders propagate in a continuous or step-wise manner, but to date, characteristics and mechanisms of the development of positive leaders are still unclear. In recent years, the abrupt elongations of the positive leader (or the positive end of bidirectional leader) are found in the high-speed video observation of the leader. This phenomenon might be closely related to the propagation mechanism of the positive leader and deserves further observation and analysis.Based on synchronization data of high-speed video camera and electric field change of an upward flash at the Canton Tower, the abrupt elongations phenomenon of the positive end before and after the return stroke is analyzed in detail.Results show that the positive end of the second dart leader intermittently extends into the air. There are three abrupt elongations of the second dart leader of the positive end, and the second abrupt elongation is caused by the connection between the positive end and the floating channel in which the tip of the positive end appears. After the second subsequent return stroke, there are two abrupt elongations of the channel tip. The two-dimensional average speed of the three abrupt elongations of the positive end is approximately 2.3×106 m·s-1, and the average length of the three abrupt elongations is approximately 115 m. After the return stroke, the two-dimensional average speed of the two abrupt elongations of the channel tip is approximately 4.3×106 m·s-1, and the average length of the two abrupt elongations is approximately 212 m.

Keywords

• upward lightning flash
• bidirectional leader
• abrupt elongation
• high-speed video observations