1,386 research outputs found
Global MRI with Braginskii viscosity in a galactic profile
We present a global-in-radius linear analysis of the axisymmetric
magnetorotational instability (MRI) in a collisional magnetized plasma with
Braginskii viscosity. For a galactic angular velocity profile we
obtain analytic solutions for three magnetic field orientations: purely
azimuthal, purely vertical and slightly pitched (almost azimuthal). In the
first two cases the Braginskii viscosity damps otherwise neutrally stable
modes, and reduces the growth rate of the MRI respectively. In the final case
the Braginskii viscosity makes the MRI up to times faster than its
inviscid counterpart, even for \emph{asymptotically small} pitch angles. We
investigate the transition between the Lorentz-force-dominated and the
Braginskii viscosity-dominated regimes in terms of a parameter \sim \Omega
\nub/B^2 where \nub is the viscous coefficient and the Alfv\'en speed.
In the limit where the parameter is small and large respectively we recover the
inviscid MRI and the magnetoviscous instability (MVI). We obtain asymptotic
expressions for the approach to these limits, and find the Braginskii viscosity
can magnify the effects of azimuthal hoop tension (the growth rate becomes
complex) by over an order of magnitude. We discuss the relevance of our results
to the local approximation, galaxies and other magnetized astrophysical
plasmas. Our results should prove useful for benchmarking codes in global
geometries.Comment: 14 pages, 5 figure
Dynamical stability of a thermally stratified intracluster medium with anisotropic momentum and heat transport
In weakly-collisional plasmas such as the intracluster medium (ICM), heat and
momentum transport become anisotropic with respect to the local magnetic field
direction. Anisotropic heat conduction causes the slow magnetosonic wave to
become buoyantly unstable to the magnetothermal instability (MTI) when the
temperature increases in the direction of gravity and to the heat-flux--driven
buoyancy instability (HBI) when the temperature decreases in the direction of
gravity. The local changes in magnetic field strength that attend these
instabilities cause pressure anisotropies that viscously damp motions parallel
to the magnetic field. In this paper we employ a linear stability analysis to
elucidate the effects of anisotropic viscosity (i.e. Braginskii pressure
anisotropy) on the MTI and HBI. By stifling the convergence/divergence of
magnetic field lines, pressure anisotropy significantly affects how the ICM
interacts with the temperature gradient. Instabilities which depend upon the
convergence/divergence of magnetic field lines to generate unstable buoyant
motions (the HBI) are suppressed over much of the wavenumber space, whereas
those which are otherwise impeded by field-line convergence/divergence (the
MTI) are strengthened. As a result, the wavenumbers at which the HBI survives
largely unsuppressed in the ICM have parallel components too small to
rigorously be considered local. This is particularly true as the magnetic field
becomes more and more orthogonal to the temperature gradient. In contrast, the
fastest-growing MTI modes are unaffected by anisotropic viscosity. However, we
find that anisotropic viscosity couples slow and Alfven waves in such a way as
to buoyantly destabilise Alfvenic fluctuations when the temperature increases
in the direction of gravity. Consequently, many wavenumbers previously
considered MTI-stable or slow-growing are in fact maximally unstable.
(abridged)Comment: 15 pages, 7 figures, accepted by MNRAS; typos fixed and minor
corrections made; color figures available at
http://www-thphys.physics.ox.ac.uk/people/kunz/Kunz11_colorfigs.pd
Magneto-immutable turbulence in weakly collisional plasmas
We propose that pressure anisotropy causes weakly collisional turbulent
plasmas to self-organize so as to resist changes in magnetic-field strength. We
term this effect "magneto-immutability" by analogy with incompressibility
(resistance to changes in pressure). The effect is important when the pressure
anisotropy becomes comparable to the magnetic pressure, suggesting that in
collisionless, weakly magnetized (high-) plasmas its dynamical relevance
is similar to that of incompressibility. Simulations of magnetized turbulence
using the weakly collisional Braginskii model show that magneto-immutable
turbulence is surprisingly similar, in most statistical measures, to critically
balanced MHD turbulence. However, in order to minimize magnetic-field
variation, the flow direction becomes more constrained than in MHD, and the
turbulence is more strongly dominated by magnetic energy (a nonzero "residual
energy"). These effects represent key differences between pressure-anisotropic
and fluid turbulence, and should be observable in the turbulent
solar wind.Comment: Accepted for publication in J. Plasma Phy
Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability
In collisionless and weakly collisional plasmas, such as hot accretion flows
onto compact objects, the magnetorotational instability (MRI) can differ
significantly from the standard (collisional) MRI. In particular, pressure
anisotropy with respect to the local magnetic-field direction can both change
the linear MRI dispersion relation and cause nonlinear modifications to the
mode structure and growth rate, even when the field and flow perturbations are
small. This work studies these pressure-anisotropy-induced nonlinearities in
the weakly nonlinear, high-ion-beta regime, before the MRI saturates into
strong turbulence. Our goal is to better understand how the saturation of the
MRI in a low collisionality plasma might differ from that in the collisional
regime. We focus on two key effects: (i) the direct impact of self-induced
pressure-anisotropy nonlinearities on the evolution of an MRI mode, and (ii)
the influence of pressure anisotropy on the "parasitic instabilities" that are
suspected to cause the mode to break up into turbulence. Our main conclusions
are: (i) The mirror instability regulates the pressure anisotropy in such a way
that the linear MRI in a collisionless plasma is an approximate nonlinear
solution once the mode amplitude becomes larger than the background field (just
as in MHD). This implies that differences between the collisionless and
collisional MRI become unimportant at large amplitudes. (ii) The break up of
large amplitude MRI modes into turbulence via parasitic instabilities is
similar in collisionless and collisional plasmas. Together, these conclusions
suggest that the route to magnetorotational turbulence in a collisionless
plasma may well be similar to that in a collisional plasma, as suggested by
recent kinetic simulations. As a supplement to these findings, we offer
guidance for the design of future kinetic simulations of magnetorotational
turbulence.Comment: Submitted to Journal of Plasma Physic
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