Astrophysical gyrokinetics: turbulence in pressure-anisotropic plasmas at ion scales and beyond
AffiliationUniv Arizona, Lunar & Planetary Lab
MetadataShow full item record
PublisherCAMBRIDGE UNIV PRESS
CitationKunz, M., Abel, I., Klein, K., & Schekochihin, A. (2018). Astrophysical gyrokinetics: Turbulence in pressure-anisotropic plasmas at ion scales and beyond. Journal of Plasma Physics, 84(2), 715840201. doi:10.1017/S0022377818000296
JournalJOURNAL OF PLASMA PHYSICS
Rights© Cambridge University Press 2018
Collection InformationThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at firstname.lastname@example.org.
AbstractWe 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 by Kunz et al. (J. Plasma Phys., vol. 81, 2015, 325810501). 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 Alfvenic and compressive fluctuations are highlighted, with attention paid to the spectral location and width of the frequency jump that occurs as Alfven waves transition into kinetic Alfven 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 Alfven 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 Alfvenic 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.
Note6 month embargo; published online: 12 April 2018
VersionFinal accepted manuscript
SponsorsUS DOE [DE-AC02-09CH11466]; NASA [NNX16AK09G, NNX16AM23G]; Princeton Center for Theoretical Science; Vetenskapsradet [2014-5392]; UK STFC Consolidated Grant [ST/N000919/1]; ERSPC [EP/M022331/1]