Cut-off period for slow magnetoacoustic waves in coronal plasma structures
Cut-off period for slow magnetoacoustic waves in coronal plasma structures
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DOI：
10.1051/0004-6361/201526530
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发表时间：
2015-10
影响因子：
6.5
通讯作者：
A. Afanasyev;V. Nakariakov
中科院分区：
文献类型：
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作者：
A. Afanasyev;V. Nakariakov
上下文。在日冕的等离子体结构中有大量的纵向压缩波的观测证据，这些证据可以用慢磁声波来解释。日冕慢波在等离子体诊断中的应用，以及它们对日冕加热和太阳风加速的可能贡献的分析，需要详细的理论建模。目标。我们研究了介质的倾角、磁场和非均匀性对由场向等离子体非均匀性引导的长波长慢磁声波（也称为管波）演化的影响。特别注意了由于日冕等离子体的重力分层而引起的截止效应。方法。我们研究了直线管波在垂直非扭曲直场对准等温等离子体柱体中的行为。我们采用薄通量管近似，考虑重力引起的分层效应。由于磁通管半径有限而引起的色散可以忽略不计。我们分析了填入分层等离子体的指数发散磁通管的截止周期行为。所得结果与已知的恒定阿尔芬速度和纯声波情况进行了比较。结果。推导了管波的波动方程，并将其简化为含截止频率的变系数Klein-Gordon方程。截断周期随高度的变化而变化，在低β等离子体中显著减小，在β为一个数量级的等离子体中显著减小。截止周期剖面上的凹陷会影响纵波沿日冕等离子体结构向更高日冕的传播，并形成日冕谐振器。
Context. There is abundant observational evidence of longitudinal compressive waves in plasma structures of the solar corona, which are confidently interpreted in terms of slow magnetoacoustic waves. The uses of coronal slow waves in plasma diagnostics, as well as analysis of their possible contribution to coronal heating and the solar wind acceleration, require detailed theoretical modelling. Aims. We investigate the effects of obliqueness, magnetic field, and non-uniformity of the medium on the evolution of long-wavelength slow magnetoacoustic waves guided by field-aligned plasma non-uniformities, also called tube waves. Special attention is paid to the cut-off effect due to the gravity stratification of the coronal plasma. Methods. We study the behaviour of linear tube waves in a vertical untwisted straight field-aligned isothermal plasma cylinder. We apply the thin flux tube approximation, taking into account effects of stratification caused by gravity. The dispersion due to the finite radius of the flux tube is neglected. We analyse the behaviour of the cut-off period for an exponentially divergent magnetic flux tube filled in with a stratified plasma. The results obtained are compared with the known cases of the constant Alfven speed and the pure acoustic wave. Results. We derive the wave equation for tube waves and reduce it to the form of the Klein–Gordon equation with varying coefficients, which explicitly contains the cut-off frequency. The cut-off period is found to vary with height, decreasing significantly in the low-beta plasma and in the plasma with the beta of the order of unity. The depressions in the cut-off period profiles can affect the propagation of longitudinal waves along coronal plasma structures towards the higher corona and can form coronal resonators.