117 research outputs found
Solar Nebula Magnetohydrodynamics
The dynamical state of the solar nebula depends critically upon whether or
not the gas is magnetically coupled. The presence of a subthermal field will
cause laminar flow to break down into turbulence. Magnetic coupling, in turn,
depends upon the ionization fraction of the gas. The inner most region of the
nebula ( AU) is magnetically well-coupled, as is the outermost
region ( AU). The magnetic status of intermediate scales (
AU) is less certain. It is plausible that there is a zone adjacent to the inner
disk in which turbulent heating self-consistently maintains the requisite
ionization levels. But the region adjacent to the active outer disk is likely
to be magnetically ``dead.'' Hall currents play a significant role in nebular
magnetohydrodynamics.
Though still occasionally argued in the literature, there is simply no
evidence to support the once standard claim that differential rotation in a
Keplerian disk is prone to break down into shear turbulence by nonlinear
instabilities. There is abundant evidence---numerical, experimental, and
analytic---in support of the stabilizing role of Coriolis forces.
Hydrodynamical turbulence is almost certainly not a source of enhanced
turbulence in the solar nebula, or in any other astrophysical accretion disk.Comment: 19 pages, LaTEX, ISSI Space Sciences Series No.
A Magnetohydrodynamic Nonradiative Accretion Flow in Three Dimensions
We present a global magnetohydrodynamic (MHD) three dimensional simulation of
a nonradiative accretion flow originating in a pressure supported torus. The
evolution is controlled by the magnetorotational instability which produces
turbulence. The flow forms a nearly Keplerian disk. The total pressure scale
height in this disk is comparable to the vertical size of the initial torus.
Gas pressure dominates only near the equator; magnetic pressure is more
important in the surrounding atmosphere. A magnetically dominated bound outflow
is driven from the disk. The accretion rate through the disk exceeds the final
rate into the hole, and a hot torus forms inside 10 r_g. Hot gas, pushed up
against the centrifugal barrier and confined by magnetic pressure, is ejected
in a narrow, unbound, conical outflow. The dynamics are controlled by magnetic
turbulence, not thermal convection, and a hydrodynamic alpha model is
inadequate to describe the flow. The limitations of two dimensional MHD
simulations are also discussed.Comment: 5 pages, 2 figures, submitted to ApJ Letters. For web version and
mpeg animations see http://www.astro.virginia.edu/~jh8h/nraf
An Exact, Three-Dimensional, Time-Dependent Wave Solution in Local Keplerian Flow
We present an exact three-dimensional wave solution to the shearing sheet
equations of motion. The existence of this solution argues against transient
amplification as a route to turbulence in unmagnetized disks. Moreover, because
the solution covers an extensive dynamical range in wavenumber space, it is an
excellent test of the dissipative properties of numerical codes.Comment: 22 pages, 4 figures. To appear Apj Dec 1 200
Chaos in Turbulence Driven by the Magnetorotational Instability
Chaotic flow is studied in a series of numerical magnetohydrodynamical
simulations that use the shearing box formalism. This mimics important features
of local accretion disk dynamics. The magnetorotational instability gives rise
to flow turbulence, and quantitative chaos parameters, such as the largest
Lyapunov exponent, can be measured. Linear growth rates appear in these
exponents even when the flow is fully turbulent. The extreme sensitivity to
initial conditions associated with chaotic flows has practical implications,
the most important of which is that hundreds of orbital times are needed to
extract a meaningful average for the stress. If the evolution time in a disk is
less than this, the classical formalism will break down.Comment: 6 pages, 8 figures. To be appear in MNRA
The interaction of a giant planet with a disc with MHD turbulence I: The initial turbulent disc models
This is the first of a series of papers aimed at developing and interpreting
simulations of protoplanets interacting with turbulent accretion discs. Here we
study the disc models prior to the introduction of a protoplanet.We study
models in which a Keplerian domain is unstable to the magnetorotational
instability (MRI). Various models with B-fields having zero net flux are
considered.We relate the properties of the models to classical viscous disc
theory.All models attain a turbulent state with volume averaged stress
parameter alpha ~ 0.005. At any particular time the vertically and azimuthally
averaged value exhibited large fluctuations in radius. Time averaging over
periods exceeding 3 orbital periods at the outer boundary of the disc resulted
in a smoother quantity with radial variations within a factor of two or so. The
vertically and azimuthally averaged radial velocity showed much larger spatial
and temporal fluctuations, requiring additional time averaging for 7-8 orbital
periods at the outer boundary to limit them. Comparison with the value derived
from the averaged stress using viscous disc theory yielded schematic agreement
for feasible averaging times but with some indication that the effects of
residual fluctuations remained. The behaviour described above must be borne in
mind when considering laminar disc simulations with anomalous Navier--Stokes
viscosity. This is because the operation of a viscosity as in classical viscous
disc theory with anomalous viscosity coefficient cannot apply to a turbulent
disc undergoing rapid changes due to external perturbation. The classical
theory can only be used to describe the time averaged behaviour of the parts of
the disc that are in a statistically steady condition for long enough for
appropriate averaging to be carried out.Comment: 10 pages, 23 figures, accepted for publication in MNRAS. A gzipped
postscript version including high resolution figures is available at
http://www.maths.qmul.ac.uk/~rp
Viscous and Resistive Effects on the MRI with a Net Toroidal Field
Resistivity and viscosity have a significant role in establishing the energy
levels in turbulence driven by the magnetorotational instability (MRI) in local
astrophysical disk models. This study uses the Athena code to characterize the
effects of a constant shear viscosity \nu and Ohmic resistivity \eta in
unstratified shearing box simulations with a net toroidal magnetic flux. A
previous study of shearing boxes with zero net magnetic field performed with
the ZEUS code found that turbulence dies out for values of the magnetic Prandtl
number, P_m = \nu/\eta, below P_m \sim 1; for P_m \gtrsim 1, time- and
volume-averaged stress levels increase with P_m. We repeat these experiments
with Athena and obtain consistent results. Next, the influence of viscosity and
resistivity on the toroidal field MRI is investigated both for linear growth
and for fully-developed turbulence. In the linear regime, a sufficiently large
\nu or \eta can prevent MRI growth; P_m itself has little direct influence on
growth from linear perturbations. By applying a range of values for \nu and
\eta to an initial state consisting of fully developed turbulence in the
presence of a background toroidal field, we investigate their effects in the
fully nonlinear system. Here, increased viscosity enhances the turbulence, and
the turbulence decays only if the resistivity is above a critical value;
turbulence can be sustained even when P_m < 1, in contrast to the zero net
field model. While we find preliminary evidence that the stress converges to a
small range of values when \nu and \eta become small enough, the influence of
dissipation terms on MRI-driven turbulence for relatively large \eta and \nu is
significant, independent of field geometry.Comment: Accepted to ApJ; version 2 - minor changes following review; 35 pages
(preprint format), 10 figure
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