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**Session 27 - Variable Stars, Novae, & Supernovae.**

*Display session, Tuesday, June 10*

*South Main Hall, *

## [27.11] Quasi-steady Disks, Magnetically Cataclysmic Disks and Jet Forming Disks

*R. Matsumoto, T. Matsuzaki (Chiba U.), T. Tajima (U.Texas), K. Shibata (NAOJ)*
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=P_gas/P_mag \sim 30 and
\alpha_B=-\left \sim 0.01. Once the
magnetic pressure is comparable to the gas pressure, however,
the Balbus amp; Hawley instability coupled with the Parker instability
further amplifies magnetic fields. We found that the disk stays in a
low-beta (\beta \le 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_\varphi \propto 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.

**Program
listing for Tuesday**