Milanese Lucio M, Loureiro Nuno F, Daschner Maximilian, Boldyrev Stanislav
Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA and ETH Zurich, CH-8093 Zürich, Switzerland.
Phys Rev Lett. 2020 Dec 31;125(26):265101. doi: 10.1103/PhysRevLett.125.265101.
In weakly collisional plasma environments with sufficiently low electron beta, Alfvénic turbulence transforms into inertial Alfvénic turbulence at scales below the electron skin depth, k_{⊥}d_{e}≳1. We argue that, in inertial Alfvénic turbulence, both energy and generalized kinetic helicity exhibit direct cascades. We demonstrate that the two cascades are compatible due to the existence of a strong scale dependence of the phase alignment angle between velocity and magnetic field fluctuations, with the phase alignment angle scaling as cosα_{k}∝k_{⊥}^{-1}. The kinetic and magnetic energy spectra scale as ∝k_{⊥}^{-5/3} and ∝k_{⊥}^{-11/3}, respectively. As a result of the dual direct cascade, the generalized helicity spectrum scales as ∝k_{⊥}^{-5/3}, implying progressive balancing of the turbulence as the cascade proceeds to smaller scales in the k_{⊥}d_{e}≫1 range. Turbulent eddies exhibit a phase-space anisotropy k_{∥}∝k_{⊥}^{5/3}, consistent with critically balanced inertial Alfvén fluctuations. Our results may be applicable to a variety of geophysical, space, and astrophysical environments, including the Earth's magnetosheath and ionosphere, solar corona, and nonrelativistic pair plasmas, as well as to strongly rotating nonionized fluids.
在电子比压足够低的弱碰撞等离子体环境中，当尺度低于电子趋肤深度，即(k_{⊥}d_{e}≳1)时，阿尔文湍流会转变为惯性阿尔文湍流。我们认为，在惯性阿尔文湍流中，能量和广义动能螺旋度都呈现直接串级。我们证明，由于速度和磁场涨落之间的相位对齐角存在强烈的尺度依赖性，这两种串级是兼容的，相位对齐角的标度为(\cosα_{k}∝k_{⊥}^{-1})。动能谱和磁能谱的标度分别为(∝k_{⊥}^{-5/3})和(∝k_{⊥}^{-11/3})。由于双重直接串级，广义螺旋度谱的标度为(∝k_{⊥}^{-5/3})，这意味着随着串级在(k_{⊥}d_{e}≫1)范围内向更小尺度发展，湍流逐渐达到平衡。湍流涡旋呈现出相空间各向异性(k_{∥}∝k_{⊥}^{5/3})，这与临界平衡的惯性阿尔文涨落一致。我们的结果可能适用于各种地球物理、空间和天体物理环境，包括地球的磁鞘和电离层、日冕、非相对论性对等离子体，以及强旋转的非电离流体。
