可压缩MHD中Elsässer变量的性质
The nature of Elsässer variables in compressible MHD
摘要：
Elsässer变量常用于等离子体湍流的研究，以帮助区分与主磁场平行或反平行传播的MHD波。虽然纯Alfvén波在均匀等离子体的方法是严格有效的，我们表明，可压缩的，磁声波一般描述的两个Elsässer变量。此外，在可压缩和非均匀等离子体中，纯MHD波（Alfvén，快和慢）不再是正常模式，但波变得线性耦合或显示Alfvén和磁声性质的混合性质。这些波必须由两个Elsässer变量描述，因此Elsässer形式主义不能用于严格分离平行和反平行传播波。然而，即使在非均匀等离子体中，对于高度阿尔夫文波，与传播方向对应的埃尔塞塞变量似乎仍然占主导地位。我们认为，对于Alfvénic波，Elsässer变量的相对振幅取决于局部的不均匀性程度和其他等离子体和波的性质。这一发现对非均匀和可压缩等离子体中的湍流研究有意义，例如日冕和太阳风。
英文摘要：
The Elsässer variables are often used in studies of plasma turbulence, in helping differentiate between MHD waves propagating parallel or anti-parallel to the main magnetic field. While for pure Alfvén waves in a homogeneous plasma the method is strictly valid, we show that compressible, magnetoacoustic waves are in general described by both Elsässer variables. Furthermore, in a compressible and inhomogeneous plasma, the pure MHD waves (Alfvén, fast and slow) are no longer normal modes, but waves become linearly coupled or display mixed properties of Alfvén and magnetoacoustic nature. These waves are necessarily described by both Elsässer variables and therefore the Elsässer formalism cannot be used to strictly separate parallel and anti-parallel propagating waves. Nevertheless, even in an inhomogeneous plasma, for a highly Alfvénic wave the Elsässer variable corresponding to the propagation direction appears still dominating. We suggest that for Alfvénic waves, the relative amplitude of Elsässer variables depends on the local degree of inhomogeneity and other plasma and wave properties. This finding has implications for turbulence studies in inhomogeneous and compressible plasmas, such as the solar corona and solar wind.
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