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High-speed Video Observations on Abrupt Elongations of the Positive End of Bidirectional Leader
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摘要: 基于广州高建筑物雷电观测站的观测设备，于2016年6月4日在广州塔上发生的一次上行闪电过程中观测到双向发展的直窜先导正端在回击前、后突然延展的现象。利用高时间分辨率的光学和电场变化同步数据，分析双向先导正端突然延展现象的细节特征。结果表明：回击前直窜先导双向发展时正端可能会出现多次突然延展的现象；突然延展现象有时由双向先导的正端与已有的悬空先导序列相连而引发，并促使双向先导正端传输至未击穿空气中；在一次继后回击后，通道正端头部也观测到两次突然延展现象，但未沿回击前正端伸展通道传输，而是通过开辟新通道进入了未击穿空气；回击前直窜先导正端三次突然伸展的二维平均速率约为2.3×106 m·s-1，伸展长度平均值约为115 m；回击后通道头部两次突然延展的二维平均速率约为4.3×106 m·s-1，伸展长度平均值约为212 m。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.
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图 1 广州塔上行闪电快电场(a)、慢电场(b)变化波形
(记录时间窗口为-462.4~215 ms，R1~R7代表上行闪电的7次回击)
Fig. 1 Synchronized fast(a) and slow(b) electric field change records of the Canton Tower upward flash
(within-462.4 to 215 ms time window, R1-R7 refer to 7 return strokes of the upward flash)
图 2 广州塔上行闪电的通道亮度
(HC-3拍摄的图像所有像素点灰度值之和，记录时间窗口为-50~200 ms) (a)、快电场变化(b)、慢电场变化(c) (R1~R7代表 7次回击)
Fig. 2 Synchronized image brightness
(sum of the gray values of all pixels in each image, based on HC-3 images, within-50 to 200 ms time window) (a), fast(b) and slow(c) electric field change records of the Canton Tower upward flash (R1-R7 refer to 7 return strokes)
图 3 1000帧/s高速摄像(HC-3)拍摄的上行正先导30帧选定图像合成图
(图像进行对比度增强和反相处理，图中虚线方框区域为HC-1视野范围)
Fig. 3 Composite image of 30 selected frames (from -150 to -40 ms) obtained by HC-3 (1000 fps) showing the upward positive leader
(image is inverted and contrast enhanced, the dashed rectangular box denotes the field of view of HC-1)
图 4 20000帧/s高速摄像(HC-1)拍摄的第2次直窜先导的16帧连续图像
(图像进行裁剪、对比度增强和反相处理，图像上时间为曝光结束时间)
Fig. 4 Sixteen consecutive images of the second dart leader obtained by the high-speed video camera 1 (HC-1, 20000 fps)
(images are cropped, inverted and contrast-enhanced, time on each image is the end of the exposure time)
图 5 20000帧/s高速摄像(HC-1)拍摄的第2次回击后12帧连续图像
(图像进行裁剪、对比度增强和反相处理，图像上时间为曝光结束时间)
Fig. 5 Twelve consecutive images after the second return stroke obtained by the high-speed video camera 1
(HC-1, 20000 fps)(images are cropped, inverted and contrast-enhanced, time on each image is the end of the exposure time)
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