666 research outputs found
Buoyancy Instabilities in Weakly Magnetized Low Collisionality Plasmas
I calculate the linear stability of a stratified low collisionality plasma in
the presence of a weak magnetic field. Heat is assumed to flow only along
magnetic field lines. In the absence of a heat flux in the background plasma,
Balbus (2000) demonstrated that plasmas in which the temperature increases in
the direction of gravity are buoyantly unstable to convective-like motions (the
``magnetothermal instability''). I show that in the presence of a background
heat flux, an analogous instability is present when the temperature decreases
in the direction of gravity. The instability is driven by the background heat
flux and the fastest growing mode has a growth time of order the local
dynamical time. Thus, independent of the sign of the temperature gradient,
weakly magnetized low collisionality plasmas are unstable on a dynamical time
to magnetically-mediated buoyancy instabilities. The instability described in
this paper is predicted to be present in clusters of galaxies at radii from ~
0.1-100 kpc, where the observed temperature increases outwards. Possible
consequences for the origin of cluster magnetic fields, ``cooling flows,'' and
the thermodynamics of the intercluster medium are briefly discussed.Comment: 5 pages; 1 cartoon; ApJ in pres
The Character of Transport Caused by ExB Drift Turbulence
The basic character of diffusive transport in a magnetised plasma depends on
what kind of transport is modelled. ExB turbulence under drift ordering has
special characteristics: it is nearly incompressible, and it cannot lead to
magnetic flux diffusion if it is electrostatic. The ExB velocity is also
related to the Poynting energy flux. Under quasineutral dynamics, electric
fields are not caused by transport of electric charge but by the requirement
that the total current is divergence free. Consequences for well constructed
computational transport models are discussed in the context of a general mean
field analysis, which also yields several anomalous transfer mechanisms not
normally considered by current models.Comment: 31 pages including 2 figures, submitted to Physics of Plasma
Compressible MHD Turbulence in Interstellar Plasmas
Radio-wave scintillation observations reveal a nearly Kolmogorov spectrum of
density fluctuations in the ionized interstellar medium. Although this density
spectrum is suggestive of turbulence, no theory relevant to its interpretation
exists. We calculate the density spectrum in turbulent magnetized plasmas by
extending the theory of incompressible MHD turbulence given by Goldreich &
Sridhar to include the effects of compressibility and particle transport. Our
most important results are as follows. (1) Density fluctuations are due to the
slow mode and the entropy mode. Both modes are passively mixed by the cascade
of shear Alfven waves. Since the shear Alfven waves have a Kolmogorov spectrum,
so do the density fluctuations. (2) Observed density fluctuation amplitudes
imply either that the magnetic and gas pressures are comparable, or that the
outer scale of the turbulence is very small. (3) A high degree of ionization is
required for the cascade to survive damping by neutrals and thereby to extend
to small lengthscales. Regions that are insufficiently ionized produce density
fluctuations only on lengthscales larger than the neutral damping scale. These
regions may account for the excess of power that is found on large scales. (4)
Both the entropy mode and the slow mode are damped on lengthscales below that
at which protons can diffuse across an eddy during the eddy's turnover time.
Consequently, eddies whose extents along the magnetic field are smaller than
the proton collisional mean free path do not contribute to the density
spectrum. However, in MHD turbulence eddies are highly elongated along the
magnetic field. From an observational perspective, the relevant lengthscale is
that transverse to the magnetic field. Thus the cut-off lengthscale for density
fluctuations is significantly smaller than the proton mean free path.Comment: 19 pages, 2 figures, submitted to Ap
Nonlinear polarisation and dissipative correspondence between low frequency fluid and gyrofluid equations
The correspondence between gyrofluid and low frequency fluid equations is
examined. The lowest order conservative effects in ExB advection, parallel
dynamics, and curvature match trivially. The principal concerns are
polarisation fluxes, and dissipative parallel viscosity and parallel heat
fluxes. The emergence of the polarisation heat flux in the fluid model and its
contribution to the energy theorem is reviewed. It is shown that gyroviscosity
and the polarisation fluxes are matched by the finite gyroradius corrections to
advection in the long wavelength limit, provided that the differences between
gyrocenter and particle representations is taken into account. The dissipative
parallel viscosity is matched by the residual thermal anisotropy in the
gyrofluid model in the collision dominated limit. The dissipative parallel heat
flux is matched by the gyrofluid parallel heat flux variables in the collision
dominated limit. Hence, the gyrofluid equations are a complete superset of the
low frequency fluid equations.Comment: RevTeX 4, 28 pages, no figures, final revised version for Physics of
Plasmas prior to proof stag
The Weak Field Limit of the Magnetorotational Instability
We investigate the behavior of the magneto-rotational instability in the
limit of extremely weak magnetic field, i.e., as the ratio of ion cyclotron
frequency to orbital frequency (X) becomes small. Considered only in terms of
cold two-fluid theory, instability persists to arbitrarily small values of X,
and the maximum growth rate is of order the orbital frequency except for the
range m_e/m_i < |X| < 1, where it can be rather smaller. In this range, field
aligned with rotation (X > 0) produces slower growth than anti-aligned field (X
< 0). The maximum growth rate is generally achieved at smaller and smaller
wavelengths as |X| diminishes. When |X| < m_e/m_i, new unstable
"electromagnetic-rotational" modes appear that do not depend on the equilibrium
magnetic field. Because the most rapidly-growing modes have extremely short
wavelengths when |X| is small, they are often subject to viscous or resistive
damping, which can result in suppressing all but the longest wavelengths, for
which growth is much slower. We find that this sort of damping is likely to
curtail severely the frequently-invoked mechanism for cosmological magnetic
field growth in which a magnetic field seeded by the Biermann battery is then
amplified by the magneto-rotational instability. On the other hand, the small
|X| case may introduce interesting effects in weakly-ionized disks in which
dust grains carry most of the electric charge.Comment: 30 pages, including 4 figures; revised version resubmitted to Ap
Inertial range turbulence in kinetic plasmas
The transfer of turbulent energy through an inertial range from the driving
scale to dissipative scales in a kinetic plasma followed by the conversion of
this energy into heat is a fundamental plasma physics process. A theoretical
foundation for the study of this process is constructed, but the details of the
kinetic cascade are not well understood. Several important properties are
identified: (a) the conservation of a generalized energy by the cascade; (b)
the need for collisions to increase entropy and realize irreversible plasma
heating; and (c) the key role played by the entropy cascade--a dual cascade of
energy to small scales in both physical and velocity space--to convert
ultimately the turbulent energy into heat. A strategy for nonlinear numerical
simulations of kinetic turbulence is outlined. Initial numerical results are
consistent with the operation of the entropy cascade. Inertial range turbulence
arises in a broad range of space and astrophysical plasmas and may play an
important role in the thermalization of fusion energy in burning plasmas.Comment: 11 pages, 2 figures, submitted to Physics of Plasmas, DPP Meeting
Special Issu
Impurity and Trace Tritium Transport in Tokamak Edge Turbulence
The turbulent transport of impurity or minority species, as for example
Tritium, is investigated in drift-Alfv\'en edge turbulence. The full effects of
perpendicular and parallel convection are kept for the impurity species. The
impurity density develops a granular structure with steep gradients and locally
exceeds its initial values due to the compressibility of the flow. An
approximate decomposition of the impurity flux into a diffusive part and an
effective convective part (characterized by a pinch velocity) is performed and
a net inward pinch effect is recovered. The pinch velocity is explained in
terms of Turbulent Equipartition and is found to vary poloidally. The results
show that impurity transport modeling needs to be two-dimensional, considering
besides the radial direction also the strong poloidal variation in the
transport coefficients.Comment: 12 Pages, 5 Figure
Late-Time Convection in the Collapse of a 23 Solar Mass Star
The results of a 3-dimensional SNSPH simulation of the core collapse of a 23
solar mass star are presented. This simulation did not launch an explosion
until over 600ms after collapse, allowing an ideal opportunity to study the
evolution and structure of the convection below the accretion shock to late
times. This late-time convection allows us to study several of the recent
claims in the literature about the role of convection: is it dominated by an
l=1 mode driven by vortical-acoustic (or other) instability, does it produce
strong neutron star kicks, and, finally, is it the key to a new explosion
mechanism? The convective region buffets the neutron star, imparting a 150-200
km/s kick. Because the l=1 mode does not dominate the convection, the neutron
star does not achieve large (>450 km/s) velocities. Finally, the neutron star
in this simulation moves, but does not develop strong oscillations, the energy
source for a recently proposed supernova engine. We discuss the implications
these results have on supernovae, hypernovae (and gamma-ray bursts), and
stellar-massed black holes.Comment: 31 pages (including 13 figures), submitted to Ap
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