MODELING OBSERVED DECAY-LESS OSCILLATIONS AS RESONANTLY ENHANCED KELVIN-HELMHOLTZ VORTICES FROM TRANSVERSE MHD WAVES AND THEIR SEISMOLOGICAL APPLICATION
MODELING OBSERVED DECAY-LESS OSCILLATIONS AS RESONANTLY ENHANCED KELVIN-HELMHOLTZ VORTICES FROM TRANSVERSE MHD WAVES AND THEIR SEISMOLOGICAL APPLICATION
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DOI：
10.3847/2041-8205/830/2/l22
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发表时间：
2016-10-20
影响因子：
7.9
通讯作者：
Yokoyama, T.
中科院分区：
文献类型：
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作者：
Antolin, P.;De Moortel, I.;Yokoyama, T.
在高度结构化的日冕中，共振吸收是从整体横向MHD波向局部方位Alfven波能量传递的一种不可避免的机制。由于其局部性质，直接检测这种机制是非常困难的。然而，它是解释观测到的全球横波快速衰减的主要理论。然而，与这一理论预测不一致的是，最近的观察表明，在低振幅制度，这种横向MHD波也可以出现衰减少，仍然是一个未解决的现象。最近的数值研究表明，Kelvin-Helmholtz不稳定性（KHI）经常伴随横向MHD波。在这项工作中，我们结合联合收割机三维MHD模拟和正演模拟表明，目前达到的空间分辨率和观察到的小振幅，一个明显的衰减无振荡。这种效应是由KHI产生的周期性增亮和共振吸收放大的KHI涡旋的相干运动相结合造成的。这样的效果是特别清楚的发射线形成的温度，捕获的边界动态，而不是核心，并反映了低阻尼特性的本地方位角阿尔芬波共振耦合到扭结模式。由于相位混合，检测到的周期可以根据发射线而变化，对边界敏感的发射线的周期比对环路核心敏感的发射线的周期短。这允许我们估计边界处的密度对比。
In the highly structured solar corona, resonant absorption is an unavoidable mechanism of energy transfer from global transverse MHD waves to local azimuthal Alfven waves. Due to its localized nature, direct detection of this mechanism is extremely difficult. Yet, it is the leading theory explaining the observed fast damping of the global transverse waves. However, at odds with this theoretical prediction are recent observations that indicate that in the low-amplitude regime such transverse MHD waves can also appear decay-less, a still unsolved phenomenon. Recent numerical work has shown that Kelvin-Helmholtz instabilities (KHI) often accompany transverse MHD waves. In this work, we combine 3D MHD simulations and forward modeling to show that for currently achieved spatial resolution and observed small amplitudes, an apparent decay-less oscillation is obtained. This effect results from the combination of periodic brightenings produced by the KHI and the coherent motion of the KHI vortices amplified by resonant absorption. Such an effect is especially clear in emission lines forming at temperatures that capture the boundary dynamics rather than the core, and reflects the low damping character of the local azimuthal Alfven waves resonantly coupled to the kink mode. Due to phase mixing, the detected period can vary depending on the emission line, with those sensitive to the boundary having shorter periods than those sensitive to the loop core. This allows us to estimate the density contrast at the boundary.