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With the support of hybrid-kinetic simulations and analytic theory, we describe the nonlinear behaviour of long-wavelength non-propagating (NP) modes and fast magnetosonic waves in high- $$\beta$$ collisionless plasmas, with particular attention to their excitation of and reaction to kinetic micro-instabilities. The perpendicularly pressure balanced polarization of NP modes produces an excess of perpendicular pressure over parallel pressure in regions where the plasma $$\beta$$ is increased. For mode amplitudes $$|\delta B/B_0| \gtrsim 0.3$$ , this excess excites the mirror instability. Particle scattering off these micro-scale mirrors frustrates the nonlinear saturation of transit-time damping, ensuring that large-amplitude NP modes continue their decay to small amplitudes. At asymptotically large wavelengths, we predict that the mirror-induced scattering will be large enough to interrupt transit-time damping entirely, isotropizing the pressure perturbations and morphing the collisionless NP mode into the magnetohydrodynamic (MHD) entropy mode. In fast waves, a fluctuating pressure anisotropy drives both mirror and firehose instabilities when the wave amplitude satisfies $$|\delta B/B_0| \gtrsim 2\beta ^{-1}$$ . The induced particle scattering leads to delayed shock formation and MHD-like wave dynamics. Taken alongside prior work on self-interrupting Alfvén waves and self-sustaining ion-acoustic waves, our results establish a foundation for new theories of electromagnetic turbulence in low-collisionality, high- $$\beta$$ plasmas such as the intracluster medium, radiatively inefficient accretion flows and the near-Earth solar wind.
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Immunity to Backscattering of Bulk Waves in Topological Acoustic Superlattices
We herein investigate the scattering of orthogonal counterpropagating waves and one-way propagating bulk waves in discrete acoustic superlattices subjected to a scattering potential applied to one of the superlattice unit cells. We demonstrate theoretically that the orthogonality of counterpropagating modes does not provide robust protection against backscattering. By contrast, the one-way propagating modes do satisfy a no-reflection condition, i.e., they exhibit immunity to backscattering, for a wide range of applied scattering potentials, which represent defects and disorder.
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- Award ID(s):
- 2242925
- PAR ID:
- 10520135
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Crystals
- Volume:
- 14
- Issue:
- 4
- ISSN:
- 2073-4352
- Page Range / eLocation ID:
- 1-12
- Subject(s) / Keyword(s):
- immunity to scattering phononic superlattice pseudospin topological acoustics
- Format(s):
- Medium: X Other: pdf
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/cryst14040344
