近距离地闪电场变化及对通道电荷密度分布的响应

一般认为,如果回击过程将先导通道的电荷完全中和,则几十米到几百米范围内的近距离先导电场与回击电场的大小相同,否则,二者之间则存在一定的差异。为了对上述观点进行分析,将源电荷先导模式和MTLL回击模式组合建立了先导-回击模式(这两个模式的组合意味着先导通道的电荷被回击完全中和),对近距离先导-回击电场变化的"V"形结构进行了模拟分析。结果表明,即使假定先导电场被回击完全中和,在近距离30—550m范围回击电场与先导电场强弱关系并不确定。另外,根据进一步的讨论,上述模拟结果几乎不受其他因素,如土壤湿度(该因素直接决定土壤电参数)和地形起伏的影响。因此,在回击开始几十微秒内,基于地面测量的先导-回击电场之差来判断先导通道电荷是否被回击完全中和可能存在一定的不确定。

Characteristics of the close leader/return stroke electric field change and its response to the corresponding charge density along the lightning channel.

Generally speaking, at very close distances, any return stroke electric field change value that is smaller than that of the preceding leader is indicative of the presence of the leader charge unneutralized by the return stroke. In order to reproduce the V shape structure characteristics of close dart leader/return stroke field change, we employ two existing models, one for the 'source charge' leader model and the other for MTLL return stroke model, both based on the assumption of uniform leader charge distribution along the channel and the complete neutralization of the leader charges by the following return stroke process. The simulated results show that the return stroke electric field is inversely related with the return stroke speed at early times (within few tens of microseconds from the beginning of the return stroke), while at later times the field is dominated by the deposited charge density component, the close electric field is independent of speed and the return stroke electric field change equals to that of the preceding leader. Therefore, at early times it might cause some uncertainty to judge whether the charges deposited by the dart leader are completely neutralized by the following return stroke process based on the difference between the return stroke and the leader field on the ground.