帰還雷撃の工学モデルと雷電磁界パルス計算への応用

摘要

In this technical note, recently proposed engineering models of the lightning return stroke and their application to lightning electromagnetic pulse calculations are reviewed and evaluated. In Section 2, the transmission line model and the traveling current source model, which have been used most frequently out of engineering models of the lightning return stroke attached to flat ground, are described. Then, two models, which have been proposed recently and can reproduce all of five features observed in typical measured lightning electromagnetic field waveforms at different distances from a lightning channel, are explained. In Section 3, two models for the lightning return stroke attached to a tall grounded object are reviewed. One is the distributed shunt current source model, and the other is the lumped series voltage source model. In Section 4, expressions for calculating electric and magnetic fields radiated from a lightning return stroke channel (and a tall strike object if present), from the current distribution along the lightning channel given by an engineering return stroke model, are shown. Three different expressions for electric field are shown here. The first one is a most widely used expression derived using Lorentz condition, the second one is an expression derived recently using the continuity equation, and the last one is an expression valid only in the restricted condition that the return stroke wavefront speed is equal to the speed of light and a return stroke current wave propagates without attenuation or dispersion. In Section 5, new findings obtained from electromagnetic calculations using the engineering models for the lightning return stroke attached to a tall object are shown. For example, the vertical electric field in the vicinity of a tall strike object is reduced significantly by the presence of the tall object, while remote electric and magnetic fields are enhanced by it.%帰還雷撃の工学モデルは，帰還雷撃路上の雷電流のrn時間空間分布を，高さ，時間，帰還雷撃彼の伝搬速度等をrn変数とする簡単な数式で表現するモデルである。このようrnな数式で表現される電流分布を等価回路により実現できるrnものもあるが，通常は工学モデルが示す電流分布とそれにrn対応する等価回路やその定数との厳密な関係に注意が払わrnれることはない。むしろ，電流波の減衰，変わい，帰還雷rn撃彼の伝搬速度等の値を，回路定数や周囲媒質と直接関連rnさせずに自由に設定できることが工学モデルの利点であるといえる。