AAS 195th Meeting, January 2000
Session 91. Drizzling Down the Potential Well: Accreting Compact Objects I
Oral, Friday, January 14, 2000, 10:00-11:30am, Centennial III

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[91.02] 3D MHD Simulations of Accretion Disks and Coronae

K.A. Miller, J.M. Stone (Department of Astronomy, University of Maryland)

We use time-dependent numerical MHD simulations to investigate a number of problems related to the evolution of the interaction region between the inner edge of an accretion disk and the magnetosphere of the central object. In our first study, we perform 2D simulations of a disk threaded by a dipolar stellar field. When stellar rotation is slow, polar accretion occurs periodically via major reconnection events in the magnetosphere. However, we find polar accretion can occur regardless of the stellar rotation rate when strong global disk fields combine with the stellar field to create a favorable net field topology.

In our next study, we use 3D, local MHD simulations to study the formation of a corona above an initially weakly magnetized, isothermal accretion disk. We find that MHD turbulence driven by the magnetorotational instability (MRI) produces strong amplification of weak fields within two scale heights of the disk midplane in a few orbital times. About 25% of the magnetic energy generated by the MRI within two scale heights (Hz) escapes due to buoyancy, producing a strongly magnetized corona above the disk. Most of the buoyantly rising magnetic energy is dissipated between 3 and 5 Hz, suggesting the corona will be hot: ~104 K for protostellar disks, and 108 K for disks around neutron stars. On long timescales the average vertical disk structure consists of a weakly magnetized (\beta ~50) turbulent core, and a strongly magnetized (\beta \lesssim 10-1) corona which is stable to the MRI. The largescale field structure in both the disk and the coronal regions is predominately toroidal. The functional form of the stress with vertical height is flat within ± 2Hz, but proportional to the density above ±2Hz.

Finally, we perform 3D, global simulations of the evolution of optically thin, geometrically thick disks. We investigate the viability of convection as an angular momentum transport mechanism, and examine the evolution when weak magnetic fields are present.

The author(s) of this abstract have provided an email address for comments about the abstract: kam@astro.umd.edu

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