4,154 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.
Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori
This paper presents an initial survey of the properties of accretion flows in
the Kerr metric from three-dimensional, general relativistic
magnetohydrodynamic simulations of accretion tori. We consider three fiducial
models of tori around rotating, both prograde and retrograde, and nonrotating
black holes; these three fiducial models are also contrasted with axisymmetric
simulations and a pseudo-Newtonian simulation with equivalent initial
conditions to delineate the limitations of these approximations.Comment: Submitted to ApJ. 30 pages, 21 figures. Animations and
high-resolution version of figures available at
http://www.astro.virginia.edu/~jd5
General Relativistic Magnetohydrodynamic Simulations of Black Hole Accretion Disks
Observations are providing increasingly detailed quantitative information
about the accretion flows that power such high energy systems as X-ray binaries
and active galactic nuclei. Analytic models of such systems must rely on
assumptions such as regular flow geometry and a simple, parameterized stress.
Global numerical simulations offer a way to investigate the basic physical
dynamics of accretion flows without these assumptions. For black hole accretion
studies one solves the equations of general relativistic magnetohydrodynamics.
Magnetic fields are of fundamental importance to the structure and evolution of
accretion disks because magnetic turbulence is the source of the anomalous
stress that drives accretion. We have developed a three-dimensional general
relativistic magnetohydrodynamic simulation code to evolve time-dependent
accretion systems self-consistently. Recent global simulations of black hole
accretion disks suggest that the generic structure of the accretion flow is
usefully divided into five regimes: the main disk, the inner disk, the corona,
the evacuated funnel, and the funnel wall jet. The properties of each of these
regions are summarized.Comment: invited review at the conference "Stellar-mass, Intermediate-mass,
and Supermassive Black Holes", held in Kyoto, Japan, Octorber 28-31, 2003, to
be published in Progress of Theoretical Physics Supplemen
Where is the Inner Edge of an Accretion Disk Around a Black Hole?
What is meant by the "inner edge" of an accretion disk around a black hole
depends on the property that defines the edge. We discuss four such definitions
using data from recent high-resolution numerical simulations. These are: the
"turbulence edge", where flux-freezing becomes more important than turbulence
in determining the magnetic field structure; the "stress edge", where plunging
matter loses dynamical contact with the outer accretion flow; the "reflection
edge", the smallest radius capable of producing significant X-ray reflection
features; and the "radiation edge", the innermost place from which significant
luminosity emerges. All these edges are dependent on the accretion rate and are
non-axisymmetric and time-variable. Although all are generally located in the
vicinity of the marginally stable orbit, significant displacements can occur,
and data interpretations placing the disk edge precisely at this point can be
misleading. If observations are to be used successfully as diagnostics of
accretion in strong gravity, the models used to interpret them must take
careful account of these distinctions.Comment: accepted by Ap.J., 26 p
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
Magnetically Driven Accretion in the Kerr Metric III: Unbound Outflows
We have carried out fully relativistic numerical simulations of accretion
disks in the Kerr metric. In this paper we focus on the unbound outflows that
emerge self-consistently from the accretion flow. These outflows are found in
the axial funnel region and consist of two components: a hot, fast, tenuous
outflow in the axial funnel proper, and a colder, slower, denser jet along the
funnel wall. Although a rotating black hole is not required to produce these
unbound outflows, their strength is enhanced by black hole spin. The
funnel-wall jet is excluded from the axial funnel due to elevated angular
momentum, and is also pressure-confined by a magnetized corona. The tenuous
funnel outflow accounts for a significant fraction of the energy transported to
large distances in the higher-spin simulations. We compare the outflows
observed in our simulations with those seen in other simulations.Comment: 33 pages, 8 figures, ApJ submitte
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