阿尔文波

阿尔文波，又称剪切阿尔文波，是等离子体中的一种沿磁场方向传播的波，这种波的频率远低于等离子体的回旋频率，是一种线偏振的低频横波。处在磁场中的导电流体在垂直于磁场的方向上受到局部扰动时，沿着磁感线方向的磁张力提供恢复力，就会激发阿尔文波。阿尔文波是由瑞典物理学家汉尼斯·阿尔文首先预言的，因此得名。后来隆德奎斯特（Lundquist）使用1特斯拉左右的磁场在水银中观察到了阿尔文波，列纳尔特（Lehnert）使用液态钠也证实了阿尔文波的存在。 阿尔文波的色散关系为： ω 2 = k 2 v A 2 cos 2 ⁡ θ {\displaystyle \omega ^{2}=k^{2}v_{A}^{2}\cos ^{2}\theta } 这样的波称为斜阿尔文波。θ=0时是沿着磁感线的方向传播的，称为阿尔文波。此时阿尔文波的相速度为： v A = B μ 0 ρ 0 {\displaystyle v_{A}={\frac {B}{\sqrt {\mu _{0}\rho _{0}}}}} 称为阿尔文速度，其中μ0是等离子体的磁导率，ρ0是离子的密度。在垂直于磁感线的方向上阿尔文波不能传播。 在等离子体物理中, 阿尔文时间 τ A {\displaystyle \tau _{A}} 是波动现象的一个重要时标。它和阿尔文速度的关系为： τ A = a v A {\displaystyle \tau _{A}={\frac {a}{v_{A}}}} 式中 a {\displaystyle a} 代表了系统的特征尺度。例如， a {\displaystyle a} 是托卡马克中较小的环的半径。 压缩阿尔文波 Alfvén, H., Existence of electromagnetic-hydrodynamic waves, Nature, 1942, 150 (3805): 405–406, Bibcode:1942Natur.150..405A, doi:10.1038/150405d0  Alfvén, H., Cosmic Plasma, Holland: Reidel, 1981, ISBN 90-277-1151-8  Aschwanden, M. J.; Fletcher, L.; Schrijver, C. 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