Effects of Field-Aligned Flows on Standing Kink and Sausage Modes Supported by Coronal Loops
Effects of Field-Aligned Flows on Standing Kink and Sausage Modes Supported by Coronal Loops
复制标题
场对准流对冠状环支持的直立扭结和香肠模式的影响
DOI：
10.1007/s11207-013-0420-7
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发表时间：
2013-09
期刊：
影响因子：
2.8
通讯作者：
Yu, H.
中科院分区：
文献类型：
--
作者：
Li, B.;Xia, L. -D.;Chen, Y. -J.;Yu, H.
基本模式及其泛音在日冕地震学中发挥着重要作用。我们研究了显着的场对准流对日冕环支持的站立模式的影响，此处将其建模为冷磁板。特别令人感兴趣的是扭结和香肠模式的基波与其第 (n−1) 泛音的周期比 [P1/nPn]，以及香肠模式的阈值半宽与长度比。对于固定扭结模式，一般而言，流动会显着降低 P1/nP，对于环路与其周围环境之间的较高和较弱的密度对比度 [$\rho_{0}/\rho_{\rm e}$]，效果尤其强烈。也就是说，即使当 $\rho_{0}/\rho_{\rm e}$ 接近无穷大时，这种影响仍然很大，当阿尔夫文马赫数 [MA] 达到 0.8 时，相对于静态情况，最小 P1/nPn 减少高达 13.7 % (24.5 %) forn=2 (n=4)，其中 MA 测量以内部单位为单位的环流速度阿尔文速度。虽然对于直立香肠模式来说不可忽略，但降低 P1/nP 的流动效果并不那么强。然而，流动情况下的阈值半宽与长度比比静态情况下要高得多。对于 [9,1024] 范围内的 $\rho_{0}/\rho_{\rm e}$ 和 [0,0.5] 范围内的 MA，详尽的参数研究表明该阈值可以通过 $(d/L)_{\rm cutoff, fit} = \frac{1}{2}\sqrt{\frac{1}{\rho_{0}/\rho_{\rm e}-1}} \exp 很好地拟合(3.7 M_{\mathrm{A}}^{2} )$，以简单的方式涉及两个参数。当识别出直立香肠振荡时，这使得人们可以分析地约束具有已知宽长比的环的组合 $(\rho_{0}/\rho_{\rm e}, M_{\mathrm {A}})$。它还允许人们更详细地检查部分香肠模式的想法，并且发现流动显着减少了允许部分模式的空间范围。
Fundamental standing modes and their overtones play an important role in coronal seismology. We examine the effects of a significant field-aligned flow on standing modes that are supported by coronal loops, which are modeled here as cold magnetic slabs. Of particular interest are the period ratios of the fundamental to its (n−1)th overtone [P1/nPn] for kink and sausage modes, and the threshold half-width-to-length ratio for sausage modes. For standing kink modes, the flow significantly reducesP1/nPnin general, the effect being particularly strong for highernand weaker density contrast [$\rho_{0}/\rho_{\rm e}$] between loops and their surroundings. That said, even when $\rho_{0}/\rho_{\rm e}$ approaches infinity, this effect is still substantial, reducing the minimalP1/nPnby up to 13.7 % (24.5 %) forn=2 (n=4) relative to the static case, when the Alfvén Mach number [MA] reaches 0.8, whereMAmeasures the loop flow speed in units of the internal Alfvén speed. Although it is not negligible for standing sausage modes, the flow effect in reducingP1/nPnis not as strong. However, the threshold half-width-to-length ratio is considerably higher in the flowing case than in its static counterpart. For $\rho_{0}/\rho_{\rm e}$ in the range [9,1024] andMAin the range [0,0.5], an exhaustive parameter study yields that this threshold is well fitted by $(d/L)_{\rm cutoff, fit} = \frac{1}{2}\sqrt{\frac{1}{\rho_{0}/\rho_{\rm e}-1}} \exp (3.7 M_{\mathrm{A}}^{2} )$, which involves the two parameters in a simple way. This allows one to analytically constrain the combination $(\rho_{0}/\rho_{\rm e}, M_{\mathrm {A}})$ for a loop with a known width-to-length ratio when a standing sausage oscillation is identified. It also allows one to examine the idea of partial sausage modes in more detail, and the flow is found to significantly reduce the spatial extent where partial modes are allowed.
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