低电离层elves现象的模拟研究

Simulation Study on elves in the Lower Ionosphere

建立了雷暴放电产生的电磁场在对流层-低电离层之间耦合的时变模式,并用模式研究elves现象的时空特征．模拟结果表明,雷暴放电产生的电场包括准静电场和辐射场,其中由放电电流脉冲产生的辐射场是产生elves现象的直接原因．辐射场的电场强度分布与偶极辐射产生的电场强度的分布相似,在雷暴放电的正上方电场较弱,在地面附近辐射电场较强．模拟结果还表明,elves现象在向外扩张时具有双峰或单峰结构,在摄像仪的底片上相应出现双环或单环结构． elves现象在83km高度水平向外扩张的过程中,最大光辐射强度开始增大,然后逐渐减小．此外,利用球形电磁波假设,探讨了elves现象超光速水平扩张的原因．

A time-dependent electromagnetic field code is developed in this paper and is applied to simulate time-space characteristics of elves （emission of light and very low frequency perturbations from electro-magnetic pulse sources）. The simulation results indicate the electric fields produced by thunderstorm discharge include the Quasi-Electrostatic Fields （QEF） and the radiation fields. The radiation fields, which are produced by the discharge current pulse, directly produce the elves. The distributions of radiation fields are similar to the electric fields produced by the electric dipole radiation. The radiation fields above the thunderstorm discharge are relatively weaker, but the radiation fields are relatively greater near the ground. Moreover, the simulation results show that the double or single ring structure of optical emission, which corresponds to the double or single peak structure of elves, will appear on the film of a camera. During the outward expanding process of elves at the altitude of 83km, the intensity of optical emission originally increases with the radial distance till it gets the maximum, then decreases with the radial distance. In addition to, the causes that the expanding velocity of elves is bigger than the speed of light are explored using the assumption of spherical electromagnetic wave.