A Numerical Model of the Existence of Two States in Magnetic Accretion Disks
We present the results of three-dimensional magnetohydrodynamic simulations of Keplerian accretion disks under local shearing box approximation. The effect of vertical gravity is included. Initial magnetic field is assumed to be in the azimuthal direction. We confirmed the model proposed by Shibata et al. (1990) that magnetic accretion disks fall into two types: gas pressure dominated (high-beta) disk and magnetic pressure dominated (low-beta) disk. When the initial state is high-beta, magnetic field is amplified due to the Balbus-Hawley instability. The growth of magnetic fluctuations saturates when beta=10-30. Angular momentum is transported outward due to the presence of off-diagonal components of magnetic stress. The effective value of alpha in the quasi-steady state in high-beta disk is 0.01. When the initial state is low-beta, we found that the disk stays in the low-beta state for time scale much longer than the rotation period. Unless beta is extremely low, magnetorotational instability coupled with the Parker instability generates fluctuating magnetic fields which transport angular momentum. We discuss the possibility that large amplitude sporadic time variations in low-state disks are due to the magnetic energy release in low-beta disks.
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