A Numerical Model of the Existence of Two States in Magnetic Accretion Disks
Abstract
We present the results of threedimensional 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 (highbeta) disk and magnetic pressure dominated (lowbeta) disk. When the initial state is highbeta, magnetic field is amplified due to the BalbusHawley instability. The growth of magnetic fluctuations saturates when beta=1030. Angular momentum is transported outward due to the presence of offdiagonal components of magnetic stress. The effective value of alpha in the quasisteady state in highbeta disk is 0.01. When the initial state is lowbeta, we found that the disk stays in the lowbeta 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 lowstate disks are due to the magnetic energy release in lowbeta disks.
 Publication:

AllSky XRay Observations in the Next Decade
 Pub Date:
 1997
 Bibcode:
 1997asxo.proc...87M