VLF波从大气层到低电离层的传输特性分析

基于传播矩阵法计算了均匀半空间电离层的反透射系数, 同时解Booker复系数四次方程得到电离层的复折射指数, 分别研究了电离层反射透射系数及折射指数随VLF频段入射电波频率、入射角和地磁倾角、电离层电子浓度及碰撞频率的变化规律. 计算结果表明, 在VLF频段, 垂直电偶极子辐射的横磁(Traverse Magnetic)波更易透射进入电离层, 而水平电偶极子辐射的横电(Traverse Electric) 波易被限制在地-电离层波导内来回反射. 电离层电子密度较低时 (如夜间), 在高纬度地区, 观测到地震电离层VLF异常的概率更大. 当考虑地磁场 的影响时, 电离层将允许地震辐射的超低频(Ultra Low Frequency ,ULF)/甚低频 (Very Low Frequency, VLF)部分的电磁波透射进入电离层, 这一点已有很多卫星观测事实为证, 但其进一步的物理机制尚需深入研究.

Penetration Characteristics of VLF Wave From Atmosphere Into the Lower Ionosphere

Many papers have indicated that earthquake happens associated with a width frequency band of VLF (Very Low Frequency), ULF (Ultra Low Frequency), ELF (Extremely Low Frequency) electromagnetic radiation, which have been recorded by satellites in the ionosphere and low magnetosphere. In this paper, the reflection and transmission coefficient of the homogeneous half-space ionosphere has been calculated using propagation matrix method. Simultaneously the Booker quartic equation is solved to get the refractive index in the ionosphere. The analysis of the factors which influence the reflection and transmission coefficient of the ionosphere and the propagation characteristics in the ionosphere of VLF wave have been performed, such as wave frequency, incident angle, geomagnetic inclination, electron density and collision frequency in the ionosphere to provide fundamental basis for further studies on numerical calculation of propagation of ULF/VLF waves in the stratified ionosphere and support the study of seismo-ionosphere coupling mechanism. The results show that it is easier for the TE (Traverse Electric) wave radiated by vertical electric dipole to penetrate into the ionosphere, but the TM (Traverse Magnetic) wave radiated by horizontal electric dipole is reflected again and again in the ground-ionosphere waveguide. It is easier to observe the VLF wave anomaly when the electron density is lower or at the high-latitude region. The O wave experiences severer attenuation than X wave, so X wave is a penetration mode whereas O wave is a non penetration mode in the ionosphere. The attenuation of the two characteristic wave decrease as the decrease of the angle between the geomagnetic field and wave vector, that is to say the attenuation is the smallest when longitudinal propagation. When the geomagnetic field is considered, the ULF/VLF wave related with earthquake can penetrate into the ionosphere which is confirmed by the satellite observation. But the further physical mechanism will be delved in the future.