Field line resonances in a nonaxisymmetric magnetic field
Field line resonances in a nonaxisymmetric magnetic field
复制标题
非轴对称磁场中的场线谐振
DOI：
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
Yan Song
中科院分区：
文献类型：
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作者：
Dong‐Hun Lee;R. Lysak;Yan Song
在近地磁尾区，磁几何形状变得非轴对称。研究了该系统中的场线共振（FLR）。非轴对称的影响意味着，与轴对称几何不同，每个磁壳上的方位角波数不再是不变的。在这个数值模拟研究中，我们研究了几何不对称性如何影响FLR理论的正确性，即任何径向振荡都是渐近阻尼的，并将波能转移到剪切驻模。数值结果表明，在这种非轴对称情况下，FLR仍然存在。与这些剪切模态相对应的振荡与周围的阿尔芬速度面相切。结果表明，Alfven速度面对于建立局部坐标系来定义波的偏振是非常有用的。在轴对称情况下，局部Alfven速度梯度法向与径向相似，切向与方位角方向相似。此外，我们还研究了有限电离层耗散对FLR和场向电流形成的影响。
In the near-Earth magnetotail region the magnetic geometry becomes nonaxisymmetric. Field line resonances (FLR) are studied in such a system. The effects of nonaxisymmetry imply that the azimuthal wavenumber is no longer invariant on each magnetic shell, unlike in an axisymmetric geometry. In this numerical simulation study we investigate how geometric asymmetry affects the ansatz of FLR theory that any radial oscillations are asymptotically damped and transfer wave energy to shear standing modes. It is found in our numerical results that the FLR persist in such nonaxisymmetric cases. The oscillations corresponding to these shear modes are tangential to the ambient Alfven speed surface. It is suggested that the Alfven speed surface can be very useful in establishing a local coordinate system to define the wave polarization. The direction normal to the local Alfven speed gradient is found to be similar to the radial direction in an axisymmetric case, while the tangential direction is similar to the azimuthal direction. In addition, we examine the effects of the finite ionospheric dissipation on the formation of FLR and field-aligned currents.