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/B0β-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. 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.
Original language | English (US) |
---|---|
Article number | 155101 |
Journal | Physical review letters |
Volume | 119 |
Issue number | 15 |
DOIs | |
State | Published - Oct 12 2017 |
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)
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In: Physical review letters, Vol. 119, No. 15, 155101, 12.10.2017.
Research output: Contribution to journal › Article › peer-review
TY - JOUR
T1 - Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- β Plasma
AU - Squire, J.
AU - Kunz, Matthew Walter
AU - Quataert, E.
AU - Schekochihin, A. A.
N1 - Funding Information: We have presented hybrid-kinetic simulations of large-amplitude SA waves in a collisionless plasma. Our results demonstrate clearly the exceptional influence of microinstabilities on the large-scale ( λ A ≫ ρ i ) dynamics of high- β collisionless plasmas, illustrating how the evolution of self-excited oblique firehose modes controls the plasma’s fluid properties. The simulations also verify, using a realistic model with kinetic ions, that linearly polarized shear-Alfvénic perturbations do not exist in their linear wave form above the amplitude limit δ B ⊥ / B 0 ∼ β - 1 / 2 [9] . The SA wave dynamics depend strongly on how oblique firehose modes evolve as the plasma becomes stable ( Δ p ≳ - B 2 / 4 π ). We find that firehose fluctuations become parallel ( k ⊥ = 0 ) and move to smaller scales ( k ∥ ρ i ∼ 1 ), surviving nonlinearly throughout the large-scale δ B ⊥ decay and scattering particles at a high rate. These long-lived k ∥ ρ i ∼ 1 modes cause SA standing-wave dynamics in a collisionless plasma to resemble those in a collisional (Braginskii) one [10] . The initial evolution of the traveling wave is effectively collisionless and matches analytic predictions [10] ; however, after generating a global negative anisotropy and exciting firehose modes, its final decay resembles the standing wave. For both standing and traveling waves, the simulations provide an interesting example of direct transfer of energy from the largest scales to thermal energy and microscale fluctuations, without a turbulent cascade. Our simulations cannot fully address what occurs at yet higher λ A / ρ i . This will depend on how oblique firehose modes decay and scatter particles, physics that is currently poorly understood. That said, it is clear that SA wave interruption provides a robust mechanism for dissipating energy directly from large-scale perturbations into heat and microinstabilities. Our results suggest that numerical models of weakly collisional high- β plasmas would be better off damping large-amplitude SA waves, rather than letting them freely propagate. One concrete way to achieve this aim might be a LF model with pressure-anisotropy limiters [47] that enhance the collisionality to a rate that is determined by the large-scale Alfvén frequency. More work on developing and validating subgrid models of this kind is underway. Given the strong deviations from MHD predictions, SA wave interruption could significantly impact the turbulent dynamics of weakly collisional plasmas in a variety of astrophysical environments [43] . Some effects have already been observed in the β ∼ 1 solar wind [17] . Other astrophysical plasmas—for instance the ICM, with β ∼ 100 [12,15] —are likely to be more strongly affected by interruption, and work is underway to assess its impact on turbulence under such conditions. We thank S. Balbus, S. D. Bale, C. H. K Chen, S. Cowley, B. 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PY - 2017/10/12
Y1 - 2017/10/12
N2 - 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/B0β-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. 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.
AB - 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/B0β-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. 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.
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U2 - 10.1103/PhysRevLett.119.155101
DO - 10.1103/PhysRevLett.119.155101
M3 - Article
C2 - 29077437
AN - SCOPUS:85031323255
SN - 0031-9007
VL - 119
JO - Physical review letters
JF - Physical review letters
IS - 15
M1 - 155101
ER -