Nonlinear full-wave analysis of atmospheric luminous emissions above lightning discharges

我们研究了“精灵”的产生机制，“精灵”是在闪电上方85-95公里处产生的不断膨胀的光学环。精灵被认为是通过激发氮分子的第一个正带而产生的，该正带是由于闪电产生的强烈电磁脉冲(EMPS)照亮低电离层而产生的电子加热所刺激的。利用全波分析方法，严格计算了雷电冲击下电磁脉冲在磁化电离层中的传播。当入射到低电离层时，强烈的电磁脉冲加热电子，然后提高那里的碰撞频率，导致电磁脉冲能量的吸收。虽然全波分析不能处理电离层参数的这种非线性时间变化，但我们通过修正电离层中的上行电磁脉冲，近似地将这种影响考虑到计算中。利用新开发的10台PC机并行计算系统，计算了精灵的时间和空间演化。计算结果表明，在闪电上方85-100公里的高度产生了一个光学环，在1ms内迅速水平扩展到200公里以上，这与精灵的实际观测结果一致。研究还发现，由于电磁脉冲沿90公里以上的地球磁力线以哨声模式传播，精灵环结构出现明显的南北不对称和东西不对称。

We have investigated the generation mechanism of "elves," the expanding optical rings created 85-95 km above lightning flashes. Elves are believed to be generated through the excitation of the first positive band of nitrogen molecule, which is stimulated by the electron heating due to the lightning-generated intense electromagnetic pulses (EMPs) illuminating the lower ionosphere. By using a full-wave analysis, we have computed rigorously the propagation in a magnetized ionosphere of the EMPs radiated by lightning current strokes. When incident into the lower ionosphere, an intense EMP heats the electrons and then enhances the collision frequency there, causing the absorption of the EMP energy. Though the full wave analysis cannot treat such a nonlinear temporal change of ionospheric parameters, we have approximately incorporated this effect into the calculation by modifying the up-going EMP in the ionosphere. Making use of a newly developed parallel-computation system with 10 PCs, we have computed the temporal and spatial evolution of elves. Computed results have shown that an optical ring is created at the altitudes of 85-100 km above the lightning, quickly expanding horizontally over 200 km within < 1 ms, which is consistent with the actual observations of elves. It has also been found that because of the EMP propagation in the whistler mode along the Earth's magnetic field line above 90 km, there appears appreciable north-south and east-west asymmetry in the elves ring structures.