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We present analytical and numerical studies of magnetorotationalinstabilities occuring in magnetized accretion disks. In these studies we makeuse of the linearised compressible MHD equations. These calculations areperformed for general radially stratified disks in the cylindrical limit. Inparticular, we investigate the influence of nonvanishing toroidal magneticfield component on the growth rate and oscillation frequency ofmagnetorotational instabilities in Keplerian disks. We find the persistence ofthese instabilities in accretion disks close to equipartition. Our calculationsshow that these eigenmodes become overstable (complex eigenvalue), due to thepresence of a toroidal magnetic field component, while their growth ratereduces slightly. Furthermore, we demonstrate the presence ofmagneto-rotational overstabilities in weakly magnetized sub-Keplerian rotatingdisks. We show that the growth rate scales with the rotation frequency of thedisk. These eigenmodes also have a nonzero oscillation frequency, due to thepresence of the dominant toroidal magnetic field component. The overstablecharacter of the MRI increases as the rotation frequency of the disk decreases.Comment: 11 pager, 18 Postscript figures, accepted for publication in  Astronomy & Astrophysic

Magneto-rotational overstability in accretion disks