Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- β Plasma

Abstract

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.

Self-sustaining sound in collisionless, high-β plasma

“…As a result, IAWs of sufficiently large amplitude can be self-sustaining, and propagate in a manner akin to sound waves in a weakly collisional fluid. This result complements recent work showing that low-collisionality plasmas cannot support linearly polarized shear-Alfvén fluctuations above a critical β-dependent amplitude (Squire, Quataert & Schekochihin 2016;Squire et al 2017a;Squire, Schekochihin & Quataert 2017b).…”

Braginskii viscosity on an unstructured, moving mesh accelerated with super-time-stepping

“…Extensive modeling of this phenomenon, using both a collisionless Landau fluid closure and weakly-collisional Braginskii MHD, has been presented in Squire et al (2017a). The consequences for the magneto-rotational instability (MRI) have also been investigated (Squire et al 2018) and the interruption was studied using hybrid kinetic simulations in Squire et al (2017b).…”

Shearing-box simulations of MRI-driven turbulence in weakly collisional accretion discs

“…It is therefore unclear to what extent we should expect to recover the usual Prandtl-number dependence in Braginskii MHD. Another important factor to consider is how the very building blocks of MHD turbulence are modified in lowcollisionality plasmas and whether this may change the large-scale turbulent state in low-collisionality accretion discs. Squire et al (2016) and Squire et al (2017b) showed that collisionless and weakly collisional plasmas cannot support linearly polarized shear-Alfvén waves above a critical amplitude due to a cancellation between the Lorentz force and the anisotropic-pressure force. It is unclear if and how this might affect the large-scale turbulent properties in accretion discs.…”