186 research outputs found
Magnetic self-organisation in Hall-dominated magnetorotational turbulence
The magnetorotational instability (MRI) is the most promising mechanism by
which angular momentum is efficiently transported outwards in astrophysical
discs. However, its application to protoplanetary discs remains problematic.
These discs are so poorly ionised that they may not support magnetorotational
turbulence in regions referred to as `dead zones'. It has recently been
suggested that the Hall effect, a non-ideal magnetohydrodynamic (MHD) effect,
could revive these dead zones by enhancing the magnetically active column
density by an order of magnitude or more. We investigate this idea by
performing local, three-dimensional, resistive Hall-MHD simulations of the MRI
in situations where the Hall effect dominates over Ohmic dissipation. As
expected from linear stability analysis, we find an exponentially growing
instability in regimes otherwise linearly stable in resistive MHD. However,
instead of vigorous and sustained magnetorotational turbulence, we find that
the MRI saturates by producing large-scale, long-lived, axisymmetric structures
in the magnetic and velocity fields. We refer to these structures as zonal
fields and zonal flows, respectively. Their emergence causes a steep reduction
in turbulent transport by at least two orders of magnitude from extrapolations
based upon resistive MHD, a result that calls into question contemporary models
of layered accretion. We construct a rigorous mean-field theory to explain this
new behaviour and to predict when it should occur. Implications for
protoplanetary disc structure and evolution, as well as for theories of planet
formation, are briefly discussed.Comment: 18 pages, 16 figures, accepted for publication in MNRA
Linear Vlasov theory of a magnetised, thermally stratified atmosphere
The stability of a collisionless, magnetised plasma to local convective
disturbances is examined, with a focus on kinetic and finite-Larmor-radius
effects. Specific application is made to the outskirts of galaxy clusters,
which contain hot and tenuous plasma whose temperature increases in the
direction of gravity. At long wavelengths (the "drift-kinetic" limit), we
obtain the kinetic version of the magnetothermal instability (MTI) and its
Alfv\'enic counterpart (Alfv\'enic MTI), which were previously discovered and
analysed using a magnetofluid (i.e. Braginskii) description. At sub-ion-Larmor
scales, we discover an overstability driven by the electron temperature
gradient of kinetic-Alfv\'en drift waves -- the electron MTI (eMTI) -- whose
growth rate is even larger than the standard MTI. At intermediate scales, we
find that ion finite-Larmor-radius effects tend to stabilise the plasma. We
discuss the physical interpretation of these instabilities in detail, and
compare them both with previous work on magnetised convection in a collisional
plasma and with temperature-gradient-driven drift-wave instabilities well-known
to the magnetic-confinement-fusion community. The implications of having both
fluid and kinetic scales simultaneously driven unstable by the same temperature
gradient are briefly discussed.Comment: 51 pages, 9 figures; to appear in Journal of Plasma Physic
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
Magnetorotational Turbulence and Dynamo in a Collisionless Plasma
We present results from the first 3D kinetic numerical simulation of
magnetorotational turbulence and dynamo, using the local shearing-box model of
a collisionless accretion disc. The kinetic magnetorotational instability grows
from a subthermal magnetic field having zero net flux over the computational
domain to generate self-sustained turbulence and outward angular-momentum
transport. Significant Maxwell and Reynolds stresses are accompanied by
comparable viscous stresses produced by field-aligned ion pressure anisotropy,
which is regulated primarily by the mirror and ion-cyclotron instabilities
through particle trapping and pitch-angle scattering. The latter endow the
plasma with an effective viscosity that is biased with respect to the
magnetic-field direction and spatio-temporally variable. Energy spectra suggest
an Alfv\'en-wave cascade at large scales and a kinetic-Alfv\'en-wave cascade at
small scales, with strong small-scale density fluctuations and weak
non-axisymmetric density waves. Ions undergo non-thermal particle acceleration,
their distribution accurately described by a kappa distribution. These results
have implications for the properties of low-collisionality accretion flows,
such as that near the black hole at the Galactic center.Comment: 6 pages, 6 figures, accepted for publication in Physical Review
Letter
Firehose and Mirror Instabilities in a Collisionless Shearing Plasma
Hybrid-kinetic numerical simulations of firehose and mirror instabilities in
a collisionless plasma are performed in which pressure anisotropy is driven as
the magnetic field is changed by a persistent linear shear . For a
decreasing field, it is found that mostly oblique firehose fluctuations grow at
ion Larmor scales and saturate with energies ; the pressure
anisotropy is pinned at the stability threshold by particle scattering off
microscale fluctuations. In contrast, nonlinear mirror fluctuations are large
compared to the ion Larmor scale and grow secularly in time; marginality is
maintained by an increasing population of resonant particles trapped in
magnetic mirrors. After one shear time, saturated order-unity magnetic mirrors
are formed and particles scatter off their sharp edges. Both instabilities
drive sub-ion-Larmor--scale fluctuations, which appear to be
kinetic-Alfv\'{e}n-wave turbulence. Our results impact theories of momentum and
heat transport in astrophysical and space plasmas, in which the stretching of a
magnetic field by shear is a generic process.Comment: 5 pages, 8 figures, accepted for publication in Physical Review
Letter
Pegasus: A New Hybrid-Kinetic Particle-in-Cell Code for Astrophysical Plasma Dynamics
We describe Pegasus, a new hybrid-kinetic particle-in-cell code tailored for
the study of astrophysical plasma dynamics. The code incorporates an
energy-conserving particle integrator into a stable, second-order--accurate,
three-stage predictor-predictor-corrector integration algorithm. The
constrained transport method is used to enforce the divergence-free constraint
on the magnetic field. A delta-f scheme is included to facilitate a
reduced-noise study of systems in which only small departures from an initial
distribution function are anticipated. The effects of rotation and shear are
implemented through the shearing-sheet formalism with orbital advection. These
algorithms are embedded within an architecture similar to that used in the
popular astrophysical magnetohydrodynamics code Athena, one that is modular,
well-documented, easy to use, and efficiently parallelized for use on thousands
of processors. We present a series of tests in one, two, and three spatial
dimensions that demonstrate the fidelity and versatility of the code.Comment: 27 pages, 12 figures, accepted for publication in Journal of
Computational Physic
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
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