Numerical modelling of MHD waves in the solar chromosphere
Numerical modelling of MHD waves in the solar chromosphere
复制标题
太阳色球层 MHD 波的数值模拟
DOI：
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
T. Bogdan
中科院分区：
文献类型：
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作者：
M. Carlsson;T. Bogdan
声波是由太阳对流区的对流运动产生的。当向上传播进入色球层时，它们到达声速等于阿尔芬速度的高度，并经历模式转换，折射和反射。我们使用数值模拟来研究这些过程中的实际配置中的波的波长是类似的磁场的长度尺度。即使这种制度是以前的分析研究或研究的有效性，使用射线跟踪理论，我们表明，他们的一些基本结果仍然有效：模式转换的临界量是磁场和k矢量之间的角度：攻角。在小于30°的角度下，来自光球层的大部分声快模作为声慢模沿场线沿着传播。在较大的角度下，大部分能量被折射/反射并作为快速模式返回，从而在向上和向下传播的波之间产生干涉图案。在三维空间中，波之间的这种小角度干涉产生了具有大水平相速度的图案，特别是接近磁场集中的图案。在考虑激波耗散和辐射阻尼的情况下，中低色球层中的波主要具有向上传播的声波特征，只有在反射层附近，向上传播的声波和折射反射波的振幅才接近。振荡功率在磁场集中时被抑制，而在磁场集中时被增强。即使在简单的入射波和简单的磁场几何形状下，由模式转换、折射和反射引起的复杂干涉图案也使得直接反演观测值变得非常困难。在动态色球层中，确定平均量是否有意义是值得怀疑的。
Acoustic waves are generated by the convective motions in the solar convection zone. When propagating upwards into the chromosphere they reach the height where the sound speed equals the Alfvén speed and they undergo mode conversion, refraction and reflection. We use numerical simulations to study these processes in realistic configurations where the wavelength of the waves is similar to the length scales of the magnetic field. Even though this regime is outside the validity of previous analytic studies or studies using ray-tracing theory, we show that some of their basic results remain valid: the critical quantity for mode conversion is the angle between the magnetic field and the k-vector: the attack angle. At angles smaller than 30° much of the acoustic, fast mode from the photosphere is transmitted as an acoustic, slow mode propagating along the field lines. At larger angles, most of the energy is refracted/reflected and returns as a fast mode creating an interference pattern between the upward and downward propagating waves. In three-dimensions, this interference between waves at small angles creates patterns with large horizontal phase speeds, especially close to magnetic field concentrations. When damping from shock dissipation and radiation is taken into account, the waves in the low–mid chromosphere have mostly the character of upward propagating acoustic waves and it is only close to the reflecting layer we get similar amplitudes for the upward propagating and refracted/reflected waves. The oscillatory power is suppressed in magnetic field concentrations and enhanced in ring-formed patterns around them. The complex interference patterns caused by mode-conversion, refraction and reflection, even with simple incident waves and in simple magnetic field geometries, make direct inversion of observables exceedingly difficult. In a dynamic chromosphere it is doubtful if the determination of mean quantities is even meaningful.