Cut-off of Alfvén Waves in Weakly Ionized Near-Earth Atmosphere
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摘要: 利用全电流双流体模型分析了地面磁扰动信号在近地层大气中的传播.结果表明磁扰动信号的传播模式受中性粒子elax-elax离子碰撞频率、磁扰动频率、磁粘滞系数以及大气离化系数等参数的影响.中性粒子碰撞效应导致近地层大气中不存在阿尔芬波传播通道.当磁扰动频率远小于0.001Hz时，磁扰动传播存在趋肤效应，且在近地大气中从高往下逐渐增强；当磁扰动频率远大于0.001Hz时，磁扰动以类似于真空电磁波的模式在近地大气中传播.Abstract: Propagation of magnetic perturbation in the near-Earth atmosphere is associated with many applications, such as analysis of earthquake signals received by satellites and magnetic anomaly detection. These propagation properties are thought to be similar with those of electromagnetic waves in free space. However, some literatures show that magnetic perturbation signals also can penetrate into near-earth atmosphere by means of Alfvén waves, considering the effect of geomagnetic field. In this paper, the propagation properties of magnetic perturbation signal in the near-earth atmosphere, by using a total-current and two-fluid model, are studied. The results show that the propagation models of magnetic perturbation signal depend on the neutral-ion collision frequency, the perturbation frequency, the coefficient of magnetic diffusion and the ionization fraction. Alfvén Waves cannot exist in the near-earth atmosphere because of neutral-ion collisions. When the frequency of magnetic perturbation is smaller than 0.001Hz, the magnetic perturbation signals decay according to the skin effect, which increases as the height to the ground decreases. When the frequency of magnetic perturbation is larger than 0.001Hz, the propagation of magnetic perturbation signal in the near-earth atmosphere is similar to that of electromagnetic wave in vacuum.
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Key words:
- Magnetic perturbation /
- Alfvé /
- n wave /
- Atmosphere
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