Current Distributions in Lightning Subsequent Return Strokes

云对地闪电由一个从云层到地面的“先导”闪电组成，它从云层到地面循序渐进，然后连续三到四次回击。整个过程的总时间是十分之几秒。在地面回击中测得的电流在几微秒内上升到最大值通常为20kA，而且下降得更慢。这些电流是造成建筑物雷击损坏的主要原因。雷击中的上升时间和峰值电流作为地面高度的函数从未直接测量过，也不是定量知道的，尽管这些信息对于了解飞机面临的闪电危险和闪电物理的基础研究是必要的。闪电回击的这些特性将从1987年佛罗里达州触发闪电收集的数据中推断出来。这些数据包括地面测量的电流和电场以及闪电的立体照片，从这些照片可以重建出高空间和时间分辨率的闪电通道的三维几何形状。闪电路径的几何形状对于确定在闪电附近的地面上测量的电场的时间历史很重要。在给定通道配置和测量电场的情况下，不可能唯一地求解电流分布。相反，方法是假设电流分布，使用已知的路径几何形状，并计算电场。将结果与观测场进行比较并迭代，直到达到可接受的一致。预计这将能够确定回击电流的主要特征，即峰值幅度、上升时间和传播速度随海拔的变化。这些信息对于有效设计飞行中的飞机和航天器的防雷措施是必要的。

A cloud-to-ground lightning flash consists of a "leader" that progresses in steps from the cloud to the ground followed by three or four successive return strokes. The total time for the process is several tenths of a second. Currents measured in return strokes at the ground rise to a maximum of typically 20 kA in a few microseconds and decline more slowly. These currents are responsible for much of the lightning damage to structures. The rise times and peak currents in lightning strokes as a function of the height above the ground have never been measured directly and are not quantitatively known, though the information is needed for understanding the lightning hazards faced by aircraft and for basic research on the physics of lightning. These properties of lightning return strokes will be inferred from data collected on triggered lightning in Florida in 1987. The data consist of the current and the electric field measured at the ground and stereo photographs of the flashes, from which the three-dimensional geometry of the lightning channels can be reconstructed with high spatial and temporal resolution. The geometry of the lightning path is important in determining the time history of the electric field measured at the ground near the flash. Given the channel configuration and the measured electric field, it is not possible to solve uniquely for the current distribution. Instead, the approach is to assume a current distribution, to use the known path geometry, and to compute the electric field. The results are compared with the observed field and iterated until acceptable agreement is reached. It is anticipated that this will enable the determination of the main features of the return stroke currents, namely the variation with altitude of the peak amplitude, rise time, and propagation velocity. This information is needed for the efficient design of lightning protection for flying aircraft and spacecraft.