Squire J, Schekochihin A A, Quataert E, Kunz M W
Physics Department, University of Otago, 730 Cumberland St., Dunedin 9016, New Zealand.
TAPIR, Mailcode 350-17, California Institute of Technology, Pasadena, CA 91125, USA.
J Plasma Phys. 2019 Feb;85(1). doi: 10.1017/s0022377819000114. Epub 2019 Feb 18.
We propose that pressure anisotropy causes weakly collisional turbulent plasmas to self-organize so as to resist changes in magnetic-field strength. We term this effect "magneto-immutability" by analogy with incompressibility (resistance to changes in pressure). The effect is important when the pressure anisotropy becomes comparable to the magnetic pressure, suggesting that in collisionless, weakly magnetized (high-) plasmas its dynamical relevance is similar to that of incompressibility. Simulations of magnetized turbulence using the weakly collisional Braginskii model show that magneto-immutable turbulence is surprisingly similar, in most statistical measures, to critically balanced MHD turbulence. However, in order to minimize magnetic-field variation, the flow direction becomes more constrained than in MHD, and the turbulence is more strongly dominated by magnetic energy (a nonzero "residual energy"). These effects represent key differences between pressure-anisotropic and fluid turbulence, and should be observable in the ≳ 1 turbulent solar wind.
我们提出，压力各向异性会使弱碰撞湍流等离子体进行自组织，从而抵抗磁场强度的变化。通过与不可压缩性（对压力变化的抵抗）作类比，我们将这种效应称为“磁不变性”。当压力各向异性变得与磁压力相当时，这种效应就很重要，这表明在无碰撞、弱磁化（高β）等离子体中，其动力学相关性与不可压缩性相似。使用弱碰撞Braginskii模型对磁化湍流进行的模拟表明，在大多数统计量上，磁不变湍流与临界平衡磁流体动力学（MHD）湍流惊人地相似。然而，为了使磁场变化最小化，流动方向比在MHD中受到更多限制，并且湍流受磁能（非零“剩余能量”）的主导更强。这些效应代表了压力各向异性湍流和流体湍流之间的关键差异，并且应该在β≳1的湍流太阳风中可以观测到。
