Return-Stroke Modeling Based on Observation of Close Electromagnetic Fields

提出了一种云对地负闪电的电磁回程模型。该模型旨在研究直接击中地面的雷击与击中高层建筑物的雷击的雷击电流参数的差异。分析结果表明，在距离雷击点几百米的范围内，高层建筑物被雷击后对其伴生电场和伴生磁场的影响有很大的不同。若能同时测量雷击电流、近电场和近磁场，则可对模型进行评估。经过验证的模型将对仪器塔测得的雷电电流统计值与远程电磁场估计的雷电电流统计值之间的差异进行论证。对于随后的回击，开发了一个新的工程模型，可以很好地再现已发表的与火箭触发闪电相关的电流场和近场观测结果。在随后的回冲程通道中，电流从距离地面几米的连接导线的连接点开始向上和向下传播。据估计，电流波的速度不到光速的一半，向下传播的波在地面上被反射。新提出的首回击击回击模型还预测了首回击击与后续击击的雷电电流峰值与远端电磁场之间的关系是不相同的。这种差异来自于一个众所周知的事实，即第一次击球的上升时间通常比随后的击球要长得多。通过回顾积累的观测资料来验证这一点，将消除直接测量得到的雷电电流统计数据与电磁估计的雷电电流统计数据之间的差异这一尚未解决的问题。

An electromagnetic return stroke model of negative cloud-to-ground lightning is newly proposed. This model is aiming at studying the difference of lightning current parameters between those of lightning strokes hitting directly ground and of those hitting a tall structure. The analyzed results predict that the influence of a tall structure hit by lightning on associated electric field is quite different from that on associated magnetic field within a few hundred meters from the hitting point. If simultaneous measurement of lightning current, close electric field and close magnetic field is achieved, the proposed model will be evaluated. A verified model will conclude the argument on the discrepancy in the statistics of lightning currents measured on instrumented towers and those estimated from remote electromagnetic field.Regarding subsequent return strokes, a new engineering model which can well reproduce published observation results on current and close fields associated with rocket-triggered lightning has been developed. The current waves in a subsequent return-stroke channel start to travel upward and downward from the junction point of connecting leaders, a few meters above the ground. The velocity of the current waves is estimated to be less than a half of the speed of light, and the downward traveling wave is reflected at the ground.The newly proposed return-stroke model for a first return stroke also predicts that the relationships between the peaks of lightning current and remote electromagnetic field are not the same for first and subsequent strokes. The difference comes from the well-known fact that the rise time of a first return stroke is generally much longer than that of a subsequent stroke. Verification of this point by reviewing accumulated observation data will clear the unresolved problem on the discrepancy between the statistics of lightning current obtained by direct measurement and of those estimated electromagnetically.