用VLF/VHF信号大容量采集系统观测云地闪电放电过程

Observations of Cloud-to-Ground Lightning Discharge by Using the Bulky VLF/ VHF Acquisition System

简述了一种高分辨、大容量闪电VLF/VHF信号记录系统,报道了一次包含12次对地回击、平均回击间隔70ms、持续时间超过800 ms的地闪放电过程的VLF/VHF辐射波形全景以及分析结果.这一个典型事例揭示了一些有趣的现象:(1)这次过程的头5 ms出现了强烈的VLF双极性大脉冲序列,标志着云内初始击穿过程启动;对应于一系列VLF辐射事件出现了强烈的VHF辐射爆发,总体上看,前380 ms期间VHF辐射异常强烈,呈现为间歇式准连续辐射,之后强烈VHF辐射则更多地表现为分立脉冲式爆发特征.(2)与回击主电流峰期间VHF辐射较弱不同,地闪最强的VHF辐射来自初始击穿过程和回击后云内放电通道扩展或者新通道形成过程;在初始击穿阶段和回击间歇期,出现了不止一次强烈的VHF辐射爆发并不伴随明显的VLF辐射.(3)回击间歇期间一类云中放电过程产生一系列半宽为3～4μs左右、出现频率约105Hz的VLF快脉冲串,整个脉冲串持续时间约1ms,频谱峰值区域在60～90 kHz,并伴随较强的VHF辐射,这些特征都与直窜先导特征一致.很可能这是一种云内K事件.(4)还给出了江淮地区地闪过程回击VLF/VHF辐射波形的统计特征,统计还显示当相继两次回击间隔小于40 ms时后面回击幅度倾向于比前面回击弱,当回击间隔时间大于100 ms时,后面回击比前面回击强的可能性大.

A multi-channel bulky acquiring system, which can synchronously record VLF/VHF signals radiated during lightning discharges with a time resolution of 0.8 μs for about more than 800 ms is described. The general view of VHF radiation as well as associated VLF radiation produced by a typical cloud-to-ground (CG) lightning flash is presented. This flash lasts about more than 800 ms and has in total 12 return strokes with a mean interstroke interval of about 70 ms. The case study of this flash shows that: (1) In view of the total discharge, VLF radiation events are accompanied by VHF radiation events. The flash tends to produce intermittently a series of quasi-continuous strong VHF radiations for the first 380 ms and then a series of isolated VHF bursts occur for the rest of time. (2) The strongest VHF radiation can be produced by both preliminary breakdown before the first return stroke and extension of old channel or formation of new channel after the return stroke while VHF radiation associated with the return stroke peak is quite weak. It is noted that during the preliminary breakdown and the Intermission between return strokes, strong VHF radiations are observed for more than once to associate with weak or quiet VLF radiations. (3) Dart leaders as well as a type of K events produce a train of VLF pulses (about 3~4μs in half width) at a frequency of about 105 pulses per second for a duration of about 1 ms. The amplitude spectrum of these events tends to peak in the range of 60~ 90 kHz. (4) Statistics on VLF/VHF waveforms of return strokes in the Changjiang-Huaihe region of Anhui Province is also presented. It is interesting to note that the former of two successive return strokes at an interval of smaller than 40 ms tends to be more powerful than the latter, while the case with an interval of greater than 100 ms shows a reverse tendency.