Numerical simulation of wave-plasma interactions in the ionosphere
Numerical simulation of wave-plasma interactions in the ionosphere
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发表时间：
2016
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通讯作者：
P. Cannon
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作者：
P. Cannon
利用高功率电磁波对电离层进行改造，可以激发出各种等离子体波和不稳定性。本论文旨在发展及应用一个GPU加速的时域有限差分法（FDTD）程序，以模拟电离层电浆对入射电磁波的时间显式响应。验证测试中的代码实现了良好的协议等离子体理论的预测和基准软件的计算。该代码被用来调查的机制背后的几个最近的实验观察，尚未完全理解，包括2D密度不均匀性的效果的O-模式到Z-模式的转换过程中，从而转换窗口的形状，和EM波的偏振和频率的影响密度不规则性的增长。的O-到Z-模式转换过程被证明是负责的强烈依赖的人工诱导的等离子体扰动的电磁波的倾角和背景等离子体的二维特性。允许激发的Z模式波反射回相互作用区域被发现会导致电场的增强和电子温度的大幅增加。O-模式和X-模式极化波的模拟表明，两者都能够激发地磁场对齐的密度不规则性，特别是在高度的背景等离子体频率对应于电子回旋谐波。包括估计的静电场与不规则性的模拟算法导致在增强的电子温度。这些密度特征的激发可以解决在EISCAT报告的异常吸收和最近无法解释的X模式加热结果中观察到的不对称性。比较模拟与离子运动允许或抑制表明，一个参数的不稳定性是负责不规则生产。电磁波场的模拟证实，X模式波是能够超过阈值的参数不稳定性激发在一定条件下。
Ionospheric modification by means of high-power electromagnetic (EM) waves can result in the excitation of a diverse range of plasma waves and instabilities. This thesis presents the development and application of a GPU-accelerated finite-difference time-domain (FDTD) code designed to simulate the time-explicit response of an ionospheric plasma to incident EM waves. Validation tests are presented in which the code achieved good agreement with the predictions of plasma theory and the computations of benchmark software. The code was used to investigate the mechanisms behind several recent experimental observations which have not been fully understood, including the effect of 2D density inhomogeneity on the O-mode to Z-mode conversion process and thus the shape of the conversion window, and the influence of EM wave polarisation and frequency on the growth of density irregularities. The O-to-Z-mode conversion process was shown to be responsible for a strong dependence of artificially-induced plasma perturbation on both the EM wave inclination angle and the 2D characteristics of the background plasma. Allowing excited Z-mode waves to reflect back towards the interaction region was found to cause enhancement of the electric field and a substantial increase in electron temperature. Simulations of O-mode and X-mode polarised waves demonstrated that both are capable of exciting geomagnetic field-aligned density irregularities, particularly at altitudes where the background plasma frequency corresponds to an electron gyroharmonic. Inclusion of estimated electrostatic fields associated with irregularities in the simulation algorithm resulted in an enhanced electron temperature. Excitation of these density features could address an observed asymmetry in anomalous absorption and recent unexplained X-mode heating results reported at EISCAT. Comparing simulations with ion motion allowed or suppressed indicated that a parametric instability was responsible for irregularity production. Simulation of EM wave fields confirmed that X-mode waves are capable of exceeding the threshold for parametric instability excitation under certain conditions.