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Title: Astrophysical gyrokinetics: turbulence in pressure-anisotropic plasmas at ion scales and beyond

Here, we present a theoretical framework for describing electromagnetic kinetic turbulence in a multi-species, magnetized, pressure-anisotropic plasma. The turbulent fluctuations are assumed to be small compared to the mean field, to be spatially anisotropic with respect to it and to have frequencies small compared to the ion cyclotron frequency. At scales above the ion-Larmor radius, the theory reduces to the pressure-anisotropic generalization of kinetic reduced magnetohydrodynamics (KRMHD) formulated. At scales at and below the ion-Larmor radius, three main objectives are achieved. First, we analyse the linear response of the pressure-anisotropic gyrokinetic system, and show it to be a generalization of previously explored limits. The effects of pressure anisotropy on the stability and collisionless damping of Alfvénic and compressive fluctuations are highlighted, with attention paid to the spectral location and width of the frequency jump that occurs as Alfvén waves transition into kinetic Alfvén waves. Secondly, we derive and discuss a very general gyrokinetic free-energy conservation law, which captures both the KRMHD free-energy conservation at long wavelengths and dual cascades of kinetic Alfvén waves and ion entropy at sub-ion-Larmor scales. We show that non-Maxwellian features in the distribution function change the amount of phase mixing and the efficiency of magnetic stresses,more » and thus influence the partitioning of free energy amongst the cascade channels. Thirdly, a simple model is used to show that pressure anisotropy, even within the bounds imposed on it by firehose and mirror instabilities, can cause order-of-magnitude variations in the ion-to-electron heating ratio due to the dissipation of Alfvénic turbulence. Our theory provides a foundation for determining how pressure anisotropy affects turbulent fluctuation spectra, the differential heating of particle species and the ratio of parallel and perpendicular phase mixing in space and astrophysical plasmas.« less
Authors:
ORCiD logo [1] ; ORCiD logo [2] ; ORCiD logo [3] ;  [4]
  1. Princeton Univ., Princeton, NJ (United States); Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  2. Princeton Univ., Princeton, NJ (United States); Chalmers Univ. of Technology, Gothenburg (Sweden)
  3. Univ. of Michigan, Ann Arbor, MI (United States); Univ. of Arizona, Tucson, AZ (United States)
  4. Univ. of Oxford, Oxford (United Kingdom); Merton College, Oxford (United Kingdom)
Publication Date:
Grant/Contract Number:
AC02-09CH11466 and NASA grant NNX16AK09G
Type:
Accepted Manuscript
Journal Name:
Journal of Plasma Physics
Additional Journal Information:
Journal Volume: 84; Journal Issue: 02; Journal ID: ISSN 0022-3778
Publisher:
Cambridge University Press
Research Org:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
77 NANOSCIENCE AND NANOTECHNOLOGY; 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; astrophysical plasmas
OSTI Identifier:
1465675

Kunz, M.  W., Abel, I.  G., Klein, K.  G., and Schekochihin, A.  A.. Astrophysical gyrokinetics: turbulence in pressure-anisotropic plasmas at ion scales and beyond. United States: N. p., Web. doi:10.1017/S0022377818000296.
Kunz, M.  W., Abel, I.  G., Klein, K.  G., & Schekochihin, A.  A.. Astrophysical gyrokinetics: turbulence in pressure-anisotropic plasmas at ion scales and beyond. United States. doi:10.1017/S0022377818000296.
Kunz, M.  W., Abel, I.  G., Klein, K.  G., and Schekochihin, A.  A.. 2018. "Astrophysical gyrokinetics: turbulence in pressure-anisotropic plasmas at ion scales and beyond". United States. doi:10.1017/S0022377818000296. https://www.osti.gov/servlets/purl/1465675.
@article{osti_1465675,
title = {Astrophysical gyrokinetics: turbulence in pressure-anisotropic plasmas at ion scales and beyond},
author = {Kunz, M.  W. and Abel, I.  G. and Klein, K.  G. and Schekochihin, A.  A.},
abstractNote = {Here, we present a theoretical framework for describing electromagnetic kinetic turbulence in a multi-species, magnetized, pressure-anisotropic plasma. The turbulent fluctuations are assumed to be small compared to the mean field, to be spatially anisotropic with respect to it and to have frequencies small compared to the ion cyclotron frequency. At scales above the ion-Larmor radius, the theory reduces to the pressure-anisotropic generalization of kinetic reduced magnetohydrodynamics (KRMHD) formulated. At scales at and below the ion-Larmor radius, three main objectives are achieved. First, we analyse the linear response of the pressure-anisotropic gyrokinetic system, and show it to be a generalization of previously explored limits. The effects of pressure anisotropy on the stability and collisionless damping of Alfvénic and compressive fluctuations are highlighted, with attention paid to the spectral location and width of the frequency jump that occurs as Alfvén waves transition into kinetic Alfvén waves. Secondly, we derive and discuss a very general gyrokinetic free-energy conservation law, which captures both the KRMHD free-energy conservation at long wavelengths and dual cascades of kinetic Alfvén waves and ion entropy at sub-ion-Larmor scales. We show that non-Maxwellian features in the distribution function change the amount of phase mixing and the efficiency of magnetic stresses, and thus influence the partitioning of free energy amongst the cascade channels. Thirdly, a simple model is used to show that pressure anisotropy, even within the bounds imposed on it by firehose and mirror instabilities, can cause order-of-magnitude variations in the ion-to-electron heating ratio due to the dissipation of Alfvénic turbulence. Our theory provides a foundation for determining how pressure anisotropy affects turbulent fluctuation spectra, the differential heating of particle species and the ratio of parallel and perpendicular phase mixing in space and astrophysical plasmas.},
doi = {10.1017/S0022377818000296},
journal = {Journal of Plasma Physics},
number = 02,
volume = 84,
place = {United States},
year = {2018},
month = {4}
}