SURFACE ALFVEN WAVES IN SOLAR FLUX TUBES
SURFACE ALFVEN WAVES IN SOLAR FLUX TUBES
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DOI：
10.1088/0004-637x/753/2/111
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发表时间：
2012-07-10
影响因子：
4.9
通讯作者：
Terradas, J.
中科院分区：
文献类型：
--
作者：
Goossens, M.;Andries, J.;Terradas, J.
磁流体动力学（MHD）波在太阳大气中无处不在。阿尔芬波和磁声波是MHD波的特殊类别。这些波的模式是明显不同的，只有在无限范围的均匀等离子体中才具有纯粹的性质。由于等离子体的不均匀性，MHD波具有混合性质，不能归类为纯Alfven波或磁声波。然而，涡度是一个与阿尔芬波明确相关的量，就像压缩与磁声波相关一样。本文研究了磁场恒定的一维非均匀圆柱体上叠加的MHD波。对于分段等密度剖面，我们发现非轴对称波的基本径向模态在密度真正不连续时具有与表面Alfven波相同的性质。与无限均匀等离子体中的经典阿尔芬波相反，除了圆柱体边界外，其他地方的涡度都为零。如果用密度的连续变化代替密度的不连续性，涡度就会在密度不均匀的整个区间内展开。非轴对称波的基本径向模态不像径向泛音那样需要压缩才能存在。在薄磁圆柱体中，相速度介于内外Alfven速度之间的非轴对称波的基本径向模态可以认为是表面Alfven波。相反，径向泛音可能与快速磁声模式有关。
Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere. Alfven waves and magneto-sonic waves are particular classes of MHD waves. These wave modes are clearly different and have pure properties in uniform plasmas of infinite extent only. Due to plasma non-uniformity, MHD waves have mixed properties and cannot be classified as pure Alfven or magneto-sonic waves. However, vorticity is a quantity unequivocally related to Alfven waves as compression is for magneto-sonic waves. Here, we investigate MHD waves superimposed on a one-dimensional non-uniform straight cylinder with constant magnetic field. For a piecewise constant density profile, we find that the fundamental radial modes of the non-axisymmetric waves have the same properties as surface Alfven waves at a true discontinuity in density. Contrary to the classic Alfven waves in a uniform plasma of infinite extent, vorticity is zero everywhere except at the cylinder boundary. If the discontinuity in density is replaced with a continuous variation of density, vorticity is spread out over the whole interval with non-uniform density. The fundamental radial modes of the non-axisymmetric waves do not need compression to exist unlike the radial overtones. In thin magnetic cylinders, the fundamental radial modes of the non-axisymmetric waves with phase velocities between the internal and the external Alfven velocities can be considered as surface Alfven waves. On the contrary, the radial overtones can be related to fast-like magneto-sonic modes.