Coupling of fast and Alfvén waves in a straight bounded magnetic field with density stratification
Coupling of fast and Alfvén waves in a straight bounded magnetic field with density stratification
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具有密度分层的直有界磁场中快波和阿尔文波的耦合
DOI：
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
J. Ballester
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文献类型：
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作者：
I. Arregui;R. Oliver;J. Ballester
在各种太阳日冕结构中的线性驻波或传播的磁流体动力学波的理论理解是远远不够的，因为线性化的MHD方程的解析解只能找到非常简单的磁配置。在本文中，我们使用数值代码来解决一个非常简单的，有界的磁配置，结合了两个特点，通常不考虑类似的作品，即纵向磁场分量和波的传播在纵向方向（$k_y）的线性快速和阿尔芬波方程
eq 0$）。我们使用数字代码（Arregui et al. 2001年），已被修改，包括交错网格，使我们能够正确地捕捉波动方程的解决方案的空间行为。详细研究了快模和阿尔芬模之间的耦合，发现当纵向场分量为零且快模的频率在沿沿着场线具有相同空间结构的阿尔芬连续区之外时，不发生耦合。在这些情况下，快模保持其整体空间行为，并且也具有ω ^2 $随k_y^2 $线性变化的特征，例如在均匀介质中（尽管这里阿尔芬速度在垂直于磁力线的方向上呈指数变化）。关于模式耦合，它的主要特征是在某些模式中具有接近频率的快解和阿尔芬解的混合，以及它们的性质的混合，即围绕某些磁表面的不连续性或跳跃（例如在纯阿尔芬波中），法向速度分量的全局空间分布和非零密度扰动（例如在快波中）。
The theoretical understanding of the linear standing or propagating magnetohydrodynamic waves in a variety of solar coronal structures is far from complete since analytical solutions to the linearised MHD equations can only be found for very simple magnetic configurations. In this paper, we use a numerical code to solve the linear fast and Alfven wave equations in a very simple, bounded magnetic configuration that incorporates two features that are not usually considered in similar works, namely the longitudinal magnetic field component and wave propagation in the longitudinal direction ($k_y
eq 0$). We use a numerical code (Arregui et al. 2001) that has been modified by including a staggered mesh that allows us to properly capture the spatial behaviour of solutions to the wave equations. Coupling between fast and Alfven modes has been studied in detail and it has been found that it does not take place when the longitudinal field component is zero and the frequency of the fast mode is outside the Alfven continuum with the same spatial structure along field lines. Under these circumstances, fast modes retain their global spatial behaviour and are also characterised by $omega^2$ varying linearly with $k_y^2$, such as in a uniform medium (although here the Alfven speed changes exponentially in the direction normal to field lines). Regarding mode coupling, its main feature is the blend of fast and Alfven solutions with close frequencies in some modes with a mixture of their properties, namely discontinuities or jumps around certain magnetic surfaces (such as in pure Alfven waves), global spatial distribution of the normal velocity component and non-zero density perturbations (such as in fast waves).