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arXiv 1507.02653astro-ph.SRphysics.space-ph
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日冕环中磁扭曲对扭折磁流体动力学波的共振吸收

Resonant absorption of kink magnetohydrodynamic waves by a magnetic twist in coronal loops

Zanyar Ebrahimi, Kayoomars Karami

摘要:

有充分的证据表明日冕中存在扭曲的磁结构。这促使我们考虑磁扭曲的原因阿尔芬频率连续的日冕环,这可以支持共振吸收作为一个快速阻尼机制所观察到的日冕扭结磁流体动力学(MHD)振荡。我们用一个圆柱形的磁通量管来模拟日冕环,这个圆柱形的磁通量管在内部和外部区域具有恒定但不同的密度。假设磁场是恒定的,并且除了通量管的边界附近的具有附加的小磁场扭曲的薄层之外,到处与圆柱轴对齐。然后,我们研究了一些可能出现在我们的模型中的不稳定性。在细管薄边界近似下,我们导出色散关系,并解析求解,得到基波(l=1)和一次/二次泛音(l= 2,3)扭折(m=1)MHD模的频率和阻尼率。我们的结论是磁扭曲的共振吸收可以证明在日冕环中观察到的扭结MHD波的快速阻尼。此外,在非均匀层中的磁扭曲可以导致偏离P1/P2=2和P1/P3=3,这与观测相当。

英文摘要:

There is ample evidence of twisted magnetic structures in the solar corona. This motivates us to consider the magnetic twist as the cause of Alfven frequency continuum in the coronal loops, which can support the resonant absorption as a rapid damping mechanism for the observed coronal kink magnetohydrodynamic (MHD) oscillations. We model a coronal loop with a straight cylindrical magnetic flux tube which has constant but different densities in the interior and exterior regions. The magnetic field is assumed to be constant and aligned with the cylinder axis everywhere except a thin layer near the boundary of the flux tube which has an additional small magnetic field twist. Then, we investigate a number of possible instabilities that may arise in our model. In the thin tube thin boundary approximation, we derive the dispersion relation and solve it analytically to obtain the frequencies and damping rates of the fundamental (l=1) and first/second overtone (l=2,3) kink (m=1) MHD modes. We conclude that the resonant absorption by the magnetic twist can justify the rapid damping of kink MHD waves observed in coronal loops. Furthermore, the magnetic twist in the inhomogeneous layer can cause deviations from P1/P2=2 and P1/P3=3 which are comparable with the observations.

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