Inertial-range kinetic turbulence in pressure-anisotropic astrophysical plasmas
File(s)
Accepted version
Author(s)
Kunz, MW
Schekochihin, AA
Chen, CHK
Abel, IG
Cowley, SC
Type
Journal Article
Abstract
A theoretical framework for low-frequency electromagnetic (drift-)kinetic
turbulence in a collisionless, multi-species plasma is presented. The result
generalises reduced magnetohydrodynamics (RMHD) and kinetic RMHD (Schekochihin
et al. 2009) for pressure-anisotropic plasmas, allowing for species drifts---a
situation routinely encountered in the solar wind and presumably ubiquitous in
hot dilute astrophysical plasmas (e.g. intracluster medium). Two main
objectives are achieved. First, in a non-Maxwellian plasma, the relationships
between fluctuating fields (e.g., the Alfven ratio) are order-unity modified
compared to the more commonly considered Maxwellian case, and so a quantitative
theory is developed to support quantitative measurements now possible in the
solar wind. The main physical feature of low-frequency plasma turbulence
survives the generalisation to non-Maxwellian distributions: Alfvenic and
compressive fluctuations are energetically decoupled, with the latter passively
advected by the former; the Alfvenic cascade is fluid, satisfying RMHD
equations (with the Alfven speed modified by pressure anisotropy and species
drifts), whereas the compressive cascade is kinetic and subject to
collisionless damping. Secondly, the organising principle of this turbulence is
elucidated in the form of a generalised kinetic free-energy invariant. It is
shown that non-Maxwellian features in the distribution function reduce the rate
of phase mixing and the efficacy of magnetic stresses; these changes influence
the partitioning of free energy amongst the various cascade channels. As the
firehose or mirror instability thresholds are approached, the dynamics of the
plasma are modified so as to reduce the energetic cost of bending
magnetic-field lines or of compressing/rarefying them. Finally, it is shown
that this theory can be derived as a long-wavelength limit of non-Maxwellian
slab gyrokinetics.
turbulence in a collisionless, multi-species plasma is presented. The result
generalises reduced magnetohydrodynamics (RMHD) and kinetic RMHD (Schekochihin
et al. 2009) for pressure-anisotropic plasmas, allowing for species drifts---a
situation routinely encountered in the solar wind and presumably ubiquitous in
hot dilute astrophysical plasmas (e.g. intracluster medium). Two main
objectives are achieved. First, in a non-Maxwellian plasma, the relationships
between fluctuating fields (e.g., the Alfven ratio) are order-unity modified
compared to the more commonly considered Maxwellian case, and so a quantitative
theory is developed to support quantitative measurements now possible in the
solar wind. The main physical feature of low-frequency plasma turbulence
survives the generalisation to non-Maxwellian distributions: Alfvenic and
compressive fluctuations are energetically decoupled, with the latter passively
advected by the former; the Alfvenic cascade is fluid, satisfying RMHD
equations (with the Alfven speed modified by pressure anisotropy and species
drifts), whereas the compressive cascade is kinetic and subject to
collisionless damping. Secondly, the organising principle of this turbulence is
elucidated in the form of a generalised kinetic free-energy invariant. It is
shown that non-Maxwellian features in the distribution function reduce the rate
of phase mixing and the efficacy of magnetic stresses; these changes influence
the partitioning of free energy amongst the various cascade channels. As the
firehose or mirror instability thresholds are approached, the dynamics of the
plasma are modified so as to reduce the energetic cost of bending
magnetic-field lines or of compressing/rarefying them. Finally, it is shown
that this theory can be derived as a long-wavelength limit of non-Maxwellian
slab gyrokinetics.
Date Issued
2015-07-16
Date Acceptance
2015-06-05
Citation
Journal of Plasma Physics, 2015, 81 (5)
ISSN
1469-7807
Publisher
Cambridge University Press (CUP)
Journal / Book Title
Journal of Plasma Physics
Volume
81
Issue
5
Copyright Statement
© Cambridge University Press 2015 . The final publication is available via Cambridge Journals Online at https://dx.doi.org/10.1017/S0022377815000811
Subjects
astro-ph.HE
astro-ph.HE
physics.plasm-ph
physics.space-ph
Publication Status
Published
Article Number
325810501