地闪回击通道几何结构对VLF/VHF辐射场特征的影响

The Influence of Channel Geometry on VLF/VHF Radiations from Lightning Return Strokes

云地闪电回击通道分叉或者曲折引起的电流传播方向改变可以在回击辐射波形上面引入一系列次峰结构乃至影响其VHF辐射特征。本文用回击传输线模式探讨了回击通道几何结构对回击VLF/VHF辐射场的影响,结果显示:(1)VLF信号波形次峰幅度与通道片断的尺度以及取向有关,尺度为100m量级且具有较大的垂直分量的电流变化过程可以在回击波形下降沿上面引入与回击幅度相当的次峰结构,而小尺度(1m量级或者更小)的通道曲折在回击波形上面引入的次峰幅度只有回击主峰幅度的10%以下。水平方向的电流传播过程几乎不在回击波形上面产生可以辨别的次峰结构。曲折通道的所有曲折片段的平均长度是描述曲折通道的一个重要参数,次峰时间间隔与幅度均与平均曲折尺度成正相关。(2)曲折通道能够明显增加回击高频辐射分量的能量,出现最大辐射能量增加的频率相当于波长等于曲折尺度的电磁波频率的1/8。小尺度的曲折或者分叉结构能够使得回击高频分量的强度提升一个数量级,但是这一强度的增加仍比我们实际测量的VHF辐射强度小一个数量级,说明通道曲折和分叉很可能不是回击过程VHF辐射产生的主要原因。

The change in propagation direction of the lightning current causedby the channel geometry (branch or tortuosity) may introduce subsidiary peaks into lightning return stroke waveformsand also alter its VHF radiation. By using the transmission\|line\|mode the influence of channel geometry on VLF/VHF radiation from lightning return strokes is presented. It is indicated that the strength of the subsidiary peak depends on the length and orientation of the channel geometry filaments. Among those heavily vertically\|oriented filaments the large one (with the length of 100 m in scale) tends to produce subsidiary peak comparative to return strokes in strength and the small one (with the length of 1 m in scale or less) can only produce peaks weaker than 10% of return strokes, while the horizontally\|oriented filament seems to produce no distinguishable subsidiary peaks. The average lengths of all channel branch or turtuosity filaments(ALF) are among the important channel geometry parameters and the intensity of the subsidiary peak and the interval time between successive peaks strongly relate to this parameter. In frequency domain the channel geometry can add to by a factor in maximum the high frequency energy of lightning return stroke radiation at the wavelength approximate 8 times ALF. However, this high frequency component is a factor weaker than the measured VHF radiation strength during lightning return strokes, strongly indicating that the channel branch or tortuosity is not the main source of VHF radiation from lightning return strokes.