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 ($\lesssim 0.1$ AU) is magnetically well-coupled, as is the outermost region ($\gtrsim 10$ AU). The magnetic status of intermediate scales ($\sim 1$ 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 $\alpha$ 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