Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- $\beta$ Plasma

Journal Article · Thu Oct 12 00:00:00 EDT 2017 · Physical Review Letters

DOI:https://doi.org/10.1103/PhysRevLett.119.155101· OSTI ID:1404881

Squire, J. ^[1]; Kunz, M. W. ^[2]; Quataert, E. ^[3]; Schekochihin, A. A. ^[4]

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1. California Inst. of Technology (CalTech), Pasadena, CA (United States); Walter Burke Institute for Theoretical Physics, Pasadena, CA (United States)
2. Princeton Univ., Princeton, NJ (United States); Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
3. Univ. of California, Berkeley, CA (United States)
4. Univ. of Oxford, Oxford (United Kingdom); Merton College, Oxford (United Kingdom)

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_⊥/B₀≳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_⊥/B₀~β^–1/2. Here, 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.

• Research Organization: Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: GBMF5076; AST 13-33612; ACI-1548562; NNX16AK09G; AC02-09CH11466
• OSTI ID: 1404881
• Alternate ID(s): OSTI ID: 1399168
• Journal Information: Physical Review Letters, Vol. 119, Issue 15; ISSN 0031-9007
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY