阿尔芬速度

**阿爾文波**，又稱**剪切阿爾文波**，是電漿體中的一種沿磁場方向傳播的波，這種波的頻率遠低於電漿體的迴旋頻率，是一種線偏振的低頻橫波。處在磁場中的導電流體在垂直於磁場的方向上受到局部擾動時，沿著磁感線方向的磁張力提供恢復力，就會激發阿爾文波。阿爾文波是由瑞典物理學家漢尼斯・阿爾文首先預言的，因此得名。後來隆德奎斯特（Lundquist）使用 1 特斯拉左右的磁場在水銀中觀察到了阿爾文波，列納爾特（Lehnert）使用液態鈉也證實了阿爾文波的存在。

阿爾文波的色散關係為：

　
: $\omega ^{2}=k^{2}v_{A}^{2}\cos ^{2}\theta $

這樣的波稱為斜阿爾文波。_θ_=0 時是沿著磁感線的方向傳播的，稱為阿爾文波。此時阿爾文波的相速度為：

　
: $v_{A}={\frac {B}{\sqrt {\mu _{0}\rho _{0}}}}$

稱為**阿爾文速度**，其中 _μ_0 是電漿體的磁導率，_ρ_0 是離子的密度。在垂直於磁感線的方向上阿爾文波不能傳播。

## 阿爾文時間

在電漿體物理中，阿爾文時間 $\tau _{A}$ 是波動現象的一個重要時標。它和阿爾文速度的關係為：

　
: $\tau _{A}={\frac {a}{v_{A}}}$

式中 $a$ 代表了系統的特徵尺度。例如， $a$ 是托卡馬克中較小的環的半徑。

 

## 參見

* 壓縮阿爾文波

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