A theory of incoherent scattering of radio waves by a plasma II. Scattering in a magnetic field
A theory of incoherent scattering of radio waves by a plasma II. Scattering in a magnetic field
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等离子体对无线电波的非相干散射理论 II。
DOI：
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发表时间：
1961
期刊：
影响因子：
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通讯作者：
D. W. Barron
中科院分区：
文献类型：
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作者：
D. T. Farley;J. Dougherty;D. W. Barron
导出了磁场中等离子体中电子密度的随机热涨落散射的无线电波频谱的一般表达式。这一推导是基于第一部分中使用的广义奈奎斯特噪声定理，然后通过一个近似简化，该近似相当于假设光速为无穷大。结果表明，这种近似对于理论在电离层中的应用是非常合适的。其次，在没有任何近似的情况下，证明了所持有的磁场永远不会改变总的散射信号功率；它只能在频谱上重新分配这一功率。最后，给出了散射信号频谱的详细形状。对于某些极限情况，给出了解析表达式，但对于最感兴趣的情况，必须使用数值方法。一些数值计算的结果如图1和图2所示。从这些结果可以看出，只有当入射射束与磁力线非常接近垂直时，磁场才会对频谱的形状产生显著影响。例如，对于40Mc/S的工作频率，即使当束流的正交性在5以内时，也没有观察到显著的磁效应。然而，随着这个角度的进一步减小，光谱开始在多普勒频移处迅速产生尖峰，这大约是离子陀螺频率的几倍。当光束与正交性成2°时，这些尖峰非常明显。在更高的工作频率下，波束必须按比例接近正交性才能达到同样的效果。
A general expression for the frequency spectrum of radio waves scattered by the random thermal fluctuations of electron density in a plasma in a magnetic field is derived. The derivation is based on the generalized Nyquist noise theorem used in part I. The exact result is then; simplified by means of an approximation which amounts to assuming the velocity of light to be infinite. It is shown that this approximation is quite adequate for ionospheric applications of the theory. Next it is proved, without appealing to any approximation, that the magnetic held can never alter the total scattered signal power; it can only redistribute this power over the spectrum. Finally, the detailed shape of the frequency spectrum of the scattered signal is examined. Analytic expressions are given for certain limiting cases, but for the cases of most interest, numerical methods must be used. The results of some numerical calculations are shown in figures 1 and 2. From these results, it can be seen that the magnetic field has a significant effect on the shape of the spectrum only if the incident radio beam is very nearly orthogonal to the magnetic lines of force. For example, for an operating frequency of 40 Mc/s, no significant magnetic effect is observed even when the beam is within 5 of orthogonality. As this angle is decreased further, however, the spectrum rapidly begins to develop spikes at Doppler shifts which are approximate multiples of the ion gyro-frequency. These spikes are quite pronounced when the beam is 2° from orthogonality. At higher operating frequencies, the beam must be proportionally closer to orthogonality to achieve the same effect.