Numerical study of accretion discs

Results of three-dimensional global magnetohydrodynamic/ radiation magnetohydrodynamic simulations of accretion discs will be presented. Magnetohydrdodynamic simulations using a high-order MHD code indicate that the dissipation rate of the magnetic energy increases with numerical resolution because the small scale magnetic turbulence driven by the magneto-rotational instability (MRI) can be resolved. We discuss the condition of the generation of large-scale azimuthal magnetic fields, and quasi-periodic reversals of mean magnetic fields. We also show the results of radiation magnetohydrodynamic simulations when the accretion rate of radiativeluy inefficient black hole accretion flow increases up to 10% of the maximum accretion rate for spherical accretion. It has been shown that hot accretion flow near the black hole co-exists with the outer cool, optically thick disk. Magnetic fields amplified in the intermediate region drives intermittent ejection of jets. This state explains the luminous hard X-ray emission and jet ejections observed during the outbursts of black hole candidates.