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arXiv 1004.4468astro-ph.SR
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由于共振吸收引起的传播扭折波的选择性空间阻尼

Selective spatial damping of propagating kink waves due to resonant absorption

J. Terradas, M. Goossens, G. Verth

摘要:

有观测证据表明,扭结波的传播是由光球运动驱动的。这些扰动,解释为扭结磁流体动力学(MHD)波衰减,因为它们向上传播的太阳日冕。在本文中,我们表明,共振吸收提供了一个简单的解释,这些波的空间阻尼。本文用圆柱形太阳磁通量管模型研究了扭结磁流体动力学波。假设频率为实频率,纵波数为复波数,解析计算了共振吸收产生的阻尼长度和单位波长阻尼。由共振吸收引起的扭折波传播的阻尼长度是频率的单调递减函数。对于低频率的扭折波,阻尼长度正好与频率成反比,我们将其表示为TGV关系。当移动到高频率时,TGV关系仍然是阻尼长度对频率的实际依赖性的非常好的近似。这种依赖性意味着共振吸收是选择性的,因为它有利于低频波,并且可以有效地从扭结波的宽带频谱中去除高频波。它是选择性的,因为阻尼长度与频率成反比,使得阻尼随着频率的增加而变得更严重。这意味着径向不均匀性可能导致太阳能波导成为宽带干扰的自然低通滤波器。因此,扭结波列沿着,例如,日冕环,将有更大比例的高频分量消散在大气层的较低位置。这可能对太阳大气中波加热的空间分布产生重要影响。

英文摘要:

There is observational evidence of propagating kink waves driven by photospheric motions. These disturbances, interpreted as kink magnetohydrodynamic (MHD) waves are attenuated as they propagate upwards in the solar corona. In this paper we show that resonant absorption provides a simple explanation to the spatial damping of these waves. Kink MHD waves are studied using a cylindrical model of solar magnetic flux tubes which includes a non-uniform layer at the tube boundary. Assuming that the frequency is real and the longitudinal wavenumber complex, the damping length and damping per wavelength produced by resonant absorption are analytically calculated. The damping length of propagating kink waves due resonant absorption is a monotonically decreasing function of frequency. For kink waves with low frequencies the damping length is exactly inversely proportional to frequency and we denote this as the TGV relation. When moving to high frequencies the TGV relation continues to be an exceptionally good approximation of the actual dependency of the damping length on frequency. This dependency means that resonant absorption is selective as it favours low frequency waves and can efficiently remove high frequency waves from a broad band spectrum of kink waves. It is selective as the damping length is inversely proportional to frequency so that the damping becomes more severe with increasing frequency. This means that radial inhomogeneity can cause solar waveguides to be a natural low-pass filter for broadband disturbances. Hence kink wave trains travelling along, e.g., coronal loops, will have a greater proportion of the high frequency components dissipated lower down in the atmosphere. This could have important consequences with respect to the spatial distribution of wave heating in the solar atmosphere.

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