Amplitude limits and nonlinear damping of shear-Alfvén waves in high-beta low-collisionality plasmas

Abstract

This work, which extends Squire et al. [ApJL, 830 L25 (2016)], explores the effect of self-generated pressure anisotropy on linearly polarized shear-Alfvén fluctuations in low-collisionality plasmas. Such anisotropies lead to stringent limits on the amplitude of magnetic perturbations in high-β plasmas, above which a fluctuation can destabilize itself through the parallel firehose instability. This causes the wave frequency to approach zero, "interrupting" the wave and stopping its oscillation. These effects are explored in detail in the collisionless and weakly collisional "Braginskii" regime, for both standing and traveling waves. The focus is on simplified models in one dimension, on scales much larger than the ion gyroradius. The effect has interesting implications for the physics of magnetized turbulence in the high-β conditions that are prevalent in many astrophysical plasmas.

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

“…We illustrate the similarity in Fig. 2(b), which also shows δB z and 4πΔp=B 2 for an SA wave governed by the Braginskii model (including heat fluxes; see [10], Appendix B). The "humped" shape occurs because the perturbation splits into regions where 4πΔp ≈ −B 2 and dδB z =dt < 0 (around the antinodes), and regions where 4πΔp > −B 2 and δB z ¼ 0 (these spread from the nodes).…”

“…Because ω A ≪ ν c ≪ Ω i , the plasma dynamics now resemble the Braginskii collisional limit [42] and the SA wave behaves as discussed in [10]. We illustrate the similarity in Fig.…”

“…Of these stages, (iii) is notably different from the predictions of one-dimensional Landau-fluid (LF) models [9,10]. Figure 2 shows one-dimensional (y-averaged) wave profiles.…”

“…Near the wave nodes, where S ¼ j∇uj ≈ 0 and δB z ≈ 0, Δp is not driven by a large-scale dB=dt and can freely decay [29,32,34,35]. Near the wave antinodes, where S ∼ β −1=2 ω A ≈ 6 × 10 −5 Ω i [10] and δB z ≠ 0, Δp is continuously driven by the decreasing field [29,31,[36][37][38].…”

“…Figure 2 shows one-dimensional (y-averaged) wave profiles. Because of heat fluxes [10], as Δp decreases initially it is nearly homogenous in space [ Fig. 2(a)].…”