In collisionless and weakly collisional plasmas, such as hot accretion flows onto compact objects, the magnetorotational instability (MRI) can differ significantly from the standard (collisional) MRI. In particular, pressure anisotropy with respect to the local magnetic-field direction can both change the linear MRI dispersion relation and cause nonlinear modifications to the mode structure and growth rate, even when the field and flow perturbations are very small. This work studies these pressure-anisotropy-induced nonlinearities in the weakly nonlinear, high-ion-beta regime, before the MRI saturates into strong turbulence. Our goal is to better understand how the saturation of the MRI in a low-collisionality plasma might differ from that in the collisional regime. We focus on two key effects: (i) the direct impact of self-induced pressure-anisotropy nonlinearities on the evolution of an MRI mode, and (ii) the influence of pressure anisotropy on the ‘parasitic instabilities’ that are suspected to cause the mode to break up into turbulence. Our main conclusions are: (i) The mirror instability regulates the pressure anisotropy in such a way that the linear MRI in a collisionless plasma is an approximate nonlinear solution once the mode amplitude becomes larger than the background field (just as in magnetohyrodynamics). This implies that differences between the collisionless and collisional MRI become unimportant at large amplitudes. (ii) The break up of large-amplitude MRI modes into turbulence via parasitic instabilities is similar in collisionless and collisional plasmas. Together, these conclusions suggest that the route to magnetorotational turbulence in a collisionless plasma may well be similar to that in a collisional plasma, as suggested by recent kinetic simulations. As a supplement to these findings, we offer guidance for the design of future kinetic simulations of magnetorotational turbulence.

中文翻译：

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动磁旋不稳定性中压力各向异性引起的非线性

在无碰撞和弱碰撞等离子体中，例如热吸积流到致密物体上，磁旋不稳定性 (MRI) 可能与标准（碰撞）MRI 显着不同。特别是，相对于局部磁场方向的压力各向异性既可以改变线性 MRI 色散关系，也可以对模式结构和增长率造成非线性修改，即使在场和流动扰动非常小的情况下也是如此。这项工作在 MRI 饱和到强湍流之前，研究了弱非线性、高离子 beta 状态下的这些压力各向异性引起的非线性。我们的目标是更好地了解低碰撞等离子体中 MRI 的饱和度可能与碰撞状态中的饱和度有何不同。我们关注两个关键效应：(i) 自致压力各向异性非线性对 MRI 模式演变的直接影响，以及 (ii) 压力各向异性对可能导致模式分解为湍流的“寄生不稳定性”的影响。我们的主要结论是：（i）镜像不稳定性以这样一种方式调节压力各向异性，即一旦模式幅度变得大于背景场（就像在磁流体动力学中一样），无碰撞等离子体中的线性 MRI 是近似非线性解。这意味着无碰撞和碰撞 MRI 之间的差异在大振幅下变得不重要。(ii) 大振幅 MRI 模式通过寄生不稳定性分解成湍流在无碰撞和碰撞等离子体中是相似的。一起，这些结论表明，正如最近的动力学模拟所表明的，无碰撞等离子体中磁旋涡流的路径很可能与碰撞等离子体中的路径相似。作为对这些发现的补充，我们为未来磁旋湍流动力学模拟的设计提供了指导。