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Dive into the research topics where Steven A. Balbus is active.

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Featured researches published by Steven A. Balbus.


The Astrophysical Journal | 1991

A powerful local shear instability in weakly magnetized disks. I - Linear analysis. II - Nonlinear evolution

Steven A. Balbus; John F. Hawley

A broad class of astronomical accretion disks is presently shown to be dynamically unstable to axisymmetric disturbances in the presence of a weak magnetic field, an insight with consequently broad applicability to gaseous, differentially-rotating systems. In the first part of this work, a linear analysis is presented of the instability, which is local and extremely powerful; the maximum growth rate, which is of the order of the angular rotation velocity, is independent of the strength of the magnetic field. Fluid motions associated with the instability directly generate both poloidal and toroidal field components. In the second part of this investigation, the scaling relation between the instabilitys wavenumber and the Alfven velocity is demonstrated, and the independence of the maximum growth rate from magnetic field strength is confirmed. 25 refs.


Annual Review of Astronomy and Astrophysics | 2003

Enhanced Angular Momentum Transport in Accretion Disks

Steven A. Balbus

▪ Abstract The status of our current understanding of angular momentum transport in accretion disks is reviewed. The last decade has seen a dramatic increase both in the recognition of key physical...


The Astrophysical Journal | 1991

A powerful local shear instability in weakly magnetized disks. II, Nonlinear evolution

John F. Hawley; Steven A. Balbus

We consider the dynamical evolution of an accretion disk undergoing Keplerian shear flow in the presence of a weak magnetic field. A linear perturbation analysis presented in a companion paper shows that such a flow is dynamically unstable; here we consider some nonlinear consequences of this instability. We solve the equations of compressible magnetohydrodynamics using a two-dimensional finite-difference code. The Keplerian disk is threaded with a weak magnetic field that has a magnetic energy density much less than the thermal pressure


The Astrophysical Journal | 1999

On the Dynamical Foundations of α Disks

Steven A. Balbus; J. C. B. Papaloizou

The dynamical foundations of α disk models are described. At the heart of the viscous formalism of accretion disk models are correlations in the fluctuating components of the disk velocity, magnetic field, and gravitational potential. We relate these correlations to the large-scale mean flow dynamics used in phenomenological viscous disk models. MHD turbulence readily lends itself to the α formalism, but transport by self-gravity does not. Nonlocal transport is an intrinsic property of turbulent self-gravitating disks, which in general cannot be captured by an α model. Local energy dissipation and α-like behavior can be reestablished if the pattern speeds associated with the amplitudes of an azimuthal Fourier decomposition of the turbulence are everywhere close to the local rotation frequency. In this situation, global wave transport must be absent. Shearing box simulations, which employ boundary conditions forcing local behavior, are probably not an adequate tool for modeling the behavior of self-gravitating disks. As a matter of principle, it is possible that disks that hover near the edge of gravitational stability may behave in accord with a local α model, but global simulations performed to date suggest matters are not this simple.


The Astrophysical Journal | 2002

The Dynamical Structure of Nonradiative Black Hole Accretion Flows

John F. Hawley; Steven A. Balbus

We analyze three-dimensional magnetohydrodynamic simulations of a nonradiative accretion flow around a black hole using a pseudo-Newtonian potential. The flow originates from a torus initially centered at 100 gravitational (Schwarzschild) radii. Accretion is driven by turbulent stresses generated self-consistently by the magnetorotational instability. The resulting flow has three well-defined dynamical components: a hot, thick, rotationally dominated Keplerian disk; a surrounding magnetized corona with vigorous circulation and outflow; and a magnetically confined jet along the centrifugal funnel wall. Inside 10 gravitational radii, the disk becomes very hot, more toroidal, and highly intermittent. These results contrast sharply with quasi-spherical, self-similar viscous models. There are no significant dynamical differences between simulations that include resistive heating and those that do not. We conclude by deducing some simple radiative properties of our solutions, and apply the results to the accretion-powered Galactic center source Sgr A*.


The Astrophysical Journal | 1994

Local shear instabilities in weakly ionized, weakly magnetized disks

Omer Blaes; Steven A. Balbus

We extend the analysis of axisymmetric magnetic shear instabilities from ideal magnetohydrodynamic (MHD) flows to weakly ionized plasmas with coupling between ions and neutrals caused by collisions, ionization, and recombination. As part of the analysis, we derive the single-fluid MHD dispersion relation without invoking the Boussinesq approximation. This work expands the range of applications of these instabilities from fully ionized accretion disks to molecular disks in galaxies and, with somewhat more uncertainty, to protostellar disks. Instability generally requires the angular velocity to decrease outward, the magnetic field strengths to be subthermal, and the ions and neutrals to be sufficiently well coupled. If ionization and recombination processes can be neglected on an orbital timescale, adequate coupling is achieved when the collision frequency of a given neutral with the ions exceeds the local epicyclic freqency. When ionization equilibrium is maintained on an orbital timescale, a new feature is present in the disk dynamics: in contrast to a single-fluid system, subthermal azimuthal fields can affect the axisymmetric stability of weakly ionized two-fluid systems. We discuss the underlying causes for this behavior. Azimuthal fields tend to be stabilizing under these circumstances, and good coupling between the neutrals and ions requires the collision frequency to exceed the epicyclic frequency by a potentially large secant factor related to the magnetic field geometry. When the instability is present, subthermal azimuthal fields may also reduce the growth rate unless the collision frequency is high, but this is important only if the field strengths are very subthermal and/or the azimuthal field is the dominant field component. We briefly discuss our results in the context of the Galactic center circumnuclear disk, and suggest that the shear instability might be present there, and be responsible for the observed turbulent motions.


The Astrophysical Journal | 1992

A powerful local shear instability in weakly magnetized disks. IV: Nonaxisymmetric perturbations

Steven A. Balbus; John F. Hawley

In this paper, we continue our study of a powerful axisymmetric MHD instability recently put forward by the authors as the underlying cause of anomalous transport in accretion disks. The theory of local non-axisymmetric perturbations in weakly magnetized disks is presented. Such disturbances are of interest for several reasons, most notably in the context of a dynamo magnetic field amplification scheme. The most volatile disturbances are those associated with the presence of a poloidal field, which grow by tens of orders of magnitude with e-folding times measured in fractions of an orbital period


The Astrophysical Journal | 1999

Differential Rotation and Turbulence in Extended H I Disks

J. A. Sellwood; Steven A. Balbus

When present, extended disks of neutral hydrogen around spiral galaxies show a remarkably uniform velocity dispersion of ~6 km s-1. Since stellar winds and supernovae are largely absent in such regions, neither the magnitude nor the constancy of this number can be accounted for in the classical picture in which interstellar turbulence is driven by stellar energy sources. Here we suggest that magnetic fields with strengths of a few microgauss in these extended disks allow energy to be extracted from galactic differential rotation through MHD-driven turbulence. The magnitude and constancy of the observed velocity dispersion may be understood if its value is Alfvenic. Moreover, by providing a simple explanation for a lower bound to the gaseous velocity fluctuations, MHD processes may account for the sharp outer edge to star formation in galaxy disks.


The Astrophysical Journal | 1992

A powerful local shear instability in weakly magnetized disks. III - Long-term evolution in a shearing sheet. IV - Nonaxisymmetric perturbations

John F. Hawley; Steven A. Balbus

The nonlinear evolution of the recently identified accretion disk magnetic shear instability is investigated through a series of numerical simulations. Finite-difference computations of the equations of compressible MHD are carried out on an axisymmetric shearing sheet system with periodic boundary conditions designed to approximate a local region within an accretion disk. Initial field configurations that include some net vertical component evolve into a nonlinear, exponentially growing solution with large poloidal velocities and magnetic fields with energies comparable to the thermal energy density. The stability of a purely azimuthal field configuration is examined, and it is found that nonaxisymmetric instability is present, but with a growth time measured in tens of orbital periods. In general, the most rapid growth occurs for very small radial and azimuthal wavenumbers, leading to coherent magnetic field structure in planes parallel to the disk. It is suggested that this instability is a key ingredient for the generation of magnetic fields in disks.


The Astrophysical Journal | 2001

A Magnetohydrodynamic Nonradiative Accretion Flow in Three Dimensions

John F. Hawley; Steven A. Balbus; James M. Stone

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 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 10rg. 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 α-model is inadequate to describe the flow. The limitations of two-dimensional MHD simulations are also discussed.

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N. O. Weiss

University of Cambridge

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Emmanuel Dormy

École Normale Supérieure

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