Standing sausage modes in coronal loops with plasma flow
Standing sausage modes in coronal loops with plasma flow
复制标题
等离子流冠状环中的站立香肠模式
DOI：
10.1051/0004-6361/201323352
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发表时间：
2014-08-01
影响因子：
6.5
通讯作者：
Yu, Hui
中科院分区：
文献类型：
--
作者：
Li, Bo;Chen, Shao-Xia;Yu, Hui
上下文。磁流体动力学波对诊断日冕等离子体的物理参数具有重要意义。场向流在日冕环中频繁出现。我们研究了横向密度和等离子体流结构对日冕环中固定香肠模式的影响，并研究了它们在日冕地震学背景下的观测意义。我们将日冕环建模为在静态日冕中嵌入等离子体流的直冷圆柱体。建立了控制香肠波传播的本征值问题，并利用其解构造了驻模。本文对捕获区最大周期P-max和截止长度-半径比（L/ A）（截止）与密度参数(rho（0)/rho（∞）和剖面陡度rho）和流量参数（其量级u -0和剖面陡度u）的关系进行了参数研究。对于任何一种剖面，相对于静态情况，引入流动都会降低捕获区可获得的P-max。p -max值对剖面N的p值敏感，但对剖面e的p值不敏感。到目前为止，流动引入的最重要的影响是降低了回路捕获静止香肠模式的能力：相对于静态模式，在流动的情况下，L/a（截止）可能会大大降低。此外，当流量较强时，（L/a）（截止值）较小，当流量大小固定时，（L/a）（截止值）更陡。如果密度分布可以用剖面N来描述，那么测量香肠模态周期可以帮助推断密度剖面的陡峭度。然而，如果剖面E更准确地描述了密度分布，这种做法是不可行的。此外，即使是量级远小于环境阿尔芬速度的场向流，也会使日冕环不太可能支持被困的站立香肠模式。
Context. Magnetohydrodynamic waves are important for diagnosing the physical parameters of coronal plasmas. Field-aligned flows appear frequently in coronal loops.Aims. We examine the effects of transverse density and plasma flow structuring on standing sausage modes trapped in coronal loops, and examine their observational implications in the context of coronal seismology.Methods. We model coronal loops as straight cold cylinders with plasma flow embedded in a static corona. An eigen-value problem governing propagating sausage waves is formulated and its solutions are employed to construct standing modes. Two transverse profiles are distinguished, and are called profiles E and N. A parameter study is performed on the dependence of the maximum period P-max and cutoff length-to-radius ratio (L/a)(cutoff) in the trapped regime on the density parameters (rho(0)/rho(infinity) and profile steepness rho) and the flow parameters (its magnitude U-0 and profile steepness u).Results. For either profile, introducing a flow reduces P-max obtainable in the trapped regime relative to the static case. The value of P-max is sensitive to p for profile N, but is insensitive to p for profile E. By far the most important effect a flow introduces is to reduce the capability for loops to trap standing sausage modes: (L/a)(cutoff) may be substantially reduced in the case with flow relative to the static one. In addition, (L/a)(cutoff) is smaller for a stronger flow, and for a steeper flow profile when the flow magnitude is fixed.Conclusions. If the density distribution can be described by profile N, then measuring the sausage mode period can help deduce the density profile steepness. However, this practice is not feasible if profile E more accurately describes the density distribution. Furthermore, even field-aligned flows with magnitudes substantially smaller than the ambient Alfven speed can make coronal loops considerably less likely to support trapped standing sausage modes.