Propagation of medium frequency (1–4 MHz) auroral radio waves to the ground via the Z‐mode radio window
Propagation of medium frequency (1–4 MHz) auroral radio waves to the ground via the Z‐mode radio window
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通过 Z 模式无线电窗口将中频 (1–4 MHz) 极光无线电波传播到地面
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
S. Shepherd
中科院分区：
文献类型：
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作者：
P. Yoon;A. Weatherwax;T. Rosenberg;J. Labelle;S. Shepherd
最近对极光无线电波的地面观测已经确认了窄带辐射，其频率接近低电离层电子回旋频率（fce）的2到3倍，被称为极光咆哮。本文用射线追踪技术研究了这些波在极光电离层中的传播。我们模拟了一个特定的场景，其中一个大规模（数十公里）的水平密度结构，根据密度结构观察到的Sondrestrom雷达在极光轰鸣发射的时候，起着至关重要的作用，在引导波到地面，使模式转换。初始波的位置和模式特性的局部稳定性的基础上确定的，这表明Z模式波激发是有利的2fce附近。然而，由于Z模式不能传播到地面，它们必须首先经历模式转换到自由空间模式（X和O）之一。结果表明，在较窄的频率和初始波相位角范围内，Z模可以通过Ellis窗口转换为O模。这一发现与极光咆哮发射几乎100% O模式偏振的事实是一致的。然而，重要的是要注意，Z-模式波的阻尼沿着射线路径的评价没有考虑在本初步研究的背景下，将是至关重要的，最终确定确切的物理场景的极光咆哮产生机制。
Recent ground-based observations of auroral radio waves have identified narrowband emissions near 2 and 3 times the lower ionospheric electron cyclotron frequency (fce) known as auroral roars. In this paper the propagation of these waves in the auroral ionosphere is investigated by means of a ray-tracing technique. We model one particular scenario in which a large-scale (tens of kilometers) horizontal density structure, based on density structures observed with the Sondrestrom radar at times of auroral roar emissions, plays a crucial role in both guiding the waves to the ground and enabling mode conversion. The location and the mode characteristics of the initial waves are determined on the basis of local stability properties, which suggests that Z-mode wave excitation is favored near 2fce. However, since Z-mode cannot propagate to the ground they must first undergo a mode conversion to one of the free-space modes (X and O). It is found that for a narrow range of frequencies and initial wave phase angles the trapped Z mode can be converted to O mode via the Ellis radio window. This finding is consistent with the fact that auroral roar emissions are nearly 100% O-mode polarized. However, it is important to note that the evaluation of the damping of the Z-mode waves along the ray path is not considered within the context of this preliminary study and will be critical for eventually determining the exact physical scenario of the auroral roar generation mechanism.