Characteristic of Lightning Channel in the Thundercloud from Thunder Measurement

用三个传声器测得的雷击信号可以重建闪电通道的形状。这种声学技术的优点是可以用简单的设备重建云内不可见的闪电通道。我们改进了声学方法，利用个人计算机系统重建长时雷声的声道。从1987年冬天开始，在龟泽附近海拔928米的奥久久山进行了火箭引发人工闪电的实验。1993年，我们在实验地点以北3.6 km的石库高地测量了雷电数据。根据7个冬季雷击记录重建了闪电通道。声源平均高度即通道高度在1.5 ~ 2.4 km之间，水平距离平均为4.4 km。通过对间隙长度为1mm的火花放电产生的压力波进行叠加仿真，研究了线声源的特性。计算结果与实验值进行了比较。震源赤道面上的波幅与脉冲长度成正比。波长随着与垂直平分线的辐射角的增大而变长。云放电的雷击信号与地面放电的雷击信号不同，地面放电在云和地面之间有一条较长的线路通道。因此，地面放电的SN比大于云放电。

The shape of lightning channels can be reconstructed from the thunder signals measured with three microphones. This acoustic technique has the advantage of reconstructing the invisible lightning channels inside the colud by simple equipments. We have improved the accoustic method to reconstruct the channels from long duration thunder by the personal computer system. Experiments of artificial lightning triggered by a rocket have been carried out at Mt. Okushishiku with an altitude 928 m near Kamazawa, since the winter of 1987. We measured thunder data at Shishiku Heights located at 3.6 km north of the experiment site in 1993. Lightning channels were reconstructed from 7 winter thunder records. Then, it becomes clear that the mean hight of sound sources, e.i.the channel altitude is ranged between 1.5 km and 2.4 km and the horizontal distance is 4.4 km on the average.Characters of line sound source are examined by superposing simulation of a presure wave generated from a spark discharge of 1mm gap-length. The calculated characteristics are compared with the experimental vales. The wave amplitude in the eqratorial plane of source is propotipnal to the spurce length. The wave length becomes longer according as the radiated angle from the perpendicular bisector increases. The thunder signals of the cloud discharge are different from those of the ground discharge that has a long line channel between the cloud and the ground. Therefore, the SN ratio of the ground discharge is larger than the cloud discharge.