Squire J, Kunz M W, Quataert E, Schekochihin A A
Theoretical Astrophysics, 350-17, California Institute of Technology, Pasadena, California 91125, USA and Walter Burke Institute for Theoretical Physics, California 91125, USA.
Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, New Jersey 08544, USA and Princeton Plasma Physics Laboratory, PO Box 451, Princeton, New Jersey 08543, USA.
Phys Rev Lett. 2017 Oct 13;119(15):155101. doi: 10.1103/PhysRevLett.119.155101. Epub 2017 Oct 12.
Using two-dimensional hybrid-kinetic simulations, we explore the nonlinear "interruption" of standing and traveling shear-Alfvén waves in collisionless plasmas. Interruption involves a self-generated pressure anisotropy removing the restoring force of a linearly polarized Alfvénic perturbation, and occurs for wave amplitudes δB_{⊥}/B_{0}≳β^{-1/2} (where β is the ratio of thermal to magnetic pressure). We use highly elongated domains to obtain maximal scale separation between the wave and the ion gyroscale. For standing waves above the amplitude limit, we find that the large-scale magnetic field of the wave decays rapidly. The dynamics are strongly affected by the excitation of oblique firehose modes, which transition into long-lived parallel fluctuations at the ion gyroscale and cause significant particle scattering. Traveling waves are damped more slowly, but are also influenced by small-scale parallel fluctuations created by the decay of firehose modes. Our results demonstrate that collisionless plasmas cannot support linearly polarized Alfvén waves above δB_{⊥}/B_{0}∼β^{-1/2}. They also provide a vivid illustration of two key aspects of low-collisionality plasma dynamics: (i) the importance of velocity-space instabilities in regulating plasma dynamics at high β, and (ii) how nonlinear collisionless processes can transfer mechanical energy directly from the largest scales into thermal energy and microscale fluctuations, without the need for a scale-by-scale turbulent cascade.
利用二维混合动力学模拟，我们探究了无碰撞等离子体中驻波和行波剪切阿尔文波的非线性“中断”。中断涉及自产生的压力各向异性消除线偏振阿尔文扰动的恢复力，并且发生在波幅δB⊥/B0≳β−1/2时（其中β是热压力与磁压力之比）。我们使用高度拉长的区域来在波和离子回旋尺度之间获得最大的尺度分离。对于高于振幅极限的驻波，我们发现波的大尺度磁场迅速衰减。动力学受到斜向软管模激发的强烈影响，这些模在离子回旋尺度处转变为长寿命的平行涨落并导致显著的粒子散射。行波的阻尼较慢，但也受到软管模衰减产生的小尺度平行涨落的影响。我们的结果表明，无碰撞等离子体不能支持δB⊥/B0∼β−1/2以上的线偏振阿尔文波。它们还生动地说明了低碰撞率等离子体动力学的两个关键方面：(i)速度空间不稳定性在高β时调节等离子体动力学中的重要性，以及(ii)非线性无碰撞过程如何能够直接将机械能从最大尺度转移到热能和微观尺度涨落，而无需逐个尺度的湍流级联。
