Quasi-steady Disks, Magnetically Cataclysmic Disks and Jet Forming Disks

Details Quasi-steady Disks, Magnetically Cataclysmic Disks and Jet Forming Disks Quasi-steady Disks, Magnetically Cataclysmic Disks and Jet Forming Disks

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...190.2711m&link_type=abstract

American Astronomical Society, 190th AAS Meeting, #27.11; Bulletin of the American Astronomical Society, Vol. 29, p.813

Astronomy and Astrophysics

Astronomy

Scientific paper

We carried out three-dimensional magnetohydrodynamic simulations of differentially rotating disks. First, we present the results of local simulations of a gravitationally stratified, Keplerian disk initially threaded by azimuthal magnetic field. Numerical results indicate that magnetic accretion disks have two states; a gas pressure dominated quasi-steady state and a magnetic pressure dominated cataclysmic state. In weakly magnetized disks, we confirmed the results by Stone et al. (1996) that the disk approaches to a quasi-steady state with beta =Pgas/Pmag ~ 30 and alpha_B =- ~ 0.01. Once the magnetic pressure is comparable to the gas pressure, however, the Balbus & Hawley instability coupled with the Parker instability further amplifies magnetic fields. We found that the disk stays in a low-beta (beta <= 1) state for time scale longer than the rotation period. Large amplitude sporadic time variations in low-state disks may be due to the magnetic energy release in low-beta disks. By including the effects of radial boundaries and curvature of azimuthal magnetic field lines, we also carried out global three-dimensional simulations of the whole disk. We assumed a differentially rotating polytropic plasma threaded by azimuthal magnetic field (B_ϕ ~ 1/r). In a cylindrical model neglecting vertical stratification, we found that when the rotation law is nearly Keplerian, non-axisymmetric high-m (azimuthal wave number) modes grow. The essential results of local simulations of high-beta disks are confirmed by global simulations. When the initial magnetic field is parallel to the rotation axis, since the rotating cylinder tends to become low-beta, we included vertical gravity and simulated the evolution of a torus from which plasma can expand along the rotation axis. We show that a bipolar jet is ejected from the torus and that non-axisymmetric, helical structures are created in the jet.

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