Study of Generation Mechanism of Lunimous Emissions in the Upper Atmosphere Produced by Lightning Discharge

我们发展了一种全波方法来计算闪电放电辐射的电磁脉冲的三维电离层传播，以分析雷云上方高空大气中发光的产生机制。主要结果如下：(A)利用空间和时间域的傅里叶变换，用全波方法计算了雷电辐射电磁脉冲的空间分布和时间演化。(B)假定雷电辐射的强电磁波激发的氮分子产生雷云上方的发光，我们计算了这种辐射的时间演化。计算结果表明，在雷击后0.4-0.7毫秒内，ELVE是由辐射电磁波在80-100公里高度范围内产生的，而精灵可能是在0.6毫秒后由放电产生的强静电场在较低高度(60-70公里)产生的。这些特征与对这些发光发射的实际观测相一致。(C)已发现电磁脉冲在电离层中向上传播时出现特殊的频率-时间结构，可能是由于脉冲在地球-电离层波导中的多次反射造成的。与美国沃洛普斯群岛雷云上空火箭实验观测到的电磁波形相比较，电磁波形的计算结果与实际观测结果在定量上是一致的。

We have developed a full-wave method to calculate three-dimensional ionospheric propagation of an electromagnetic pulse radiated from a lightning discharge, to analyze generation mechanisms of luminous emissions in the upper atmosphere above thunderclouds. The main results are summarized as follows :(a)Both spatial distribution and temporal evolution of an electromagnetic pulse radiated from a lightning discharge have been calculated by a full-wave method which makes use of Fourier transformations in space and time domain.(b)Assuming luminous emissions above thunderclouds are generated by nitrogen molecules excited by the intense electromagnetic wave radiated by the lightning discharge, we have computed time evolution of the emissions. Computed results suggest that the Elves are generated in the altitude range of 80-100 km by the radiated electromagnetic wave during 0.4-0.7 msec after the lightning stroke, while the Sprites are likely to be generated at lower altitudes (60-70 kin) after 0.6 msec by an intense electrostatic field created by the discharge. These features are consistent with the actual observations of these luminous emissions.(c)Peculiar frequency-time structures have been found to appear on the electromagnetic pulse when it propagates up in the ionosphere, possibly caused by the multiple reflection of the pulse in the Earth-ionospheric waveguide. Compared with the electromagnetic wave form observations by a rocket experiment conducted above thunderclouds over Wallops Islands, USA, calculated results of the electromagnetic wave forms are quantitatively consistent with the actual observations.