Computational magnetohydrodynamics of turbulence, dynamos, and jet formation in differentially rotating astrophysical disks

We present the results of three-dimensional global magnetohydrodynamic (MHD) simulations of differentially rotating astrophysical disks. We simulate the time evolution of the disk by using a parallelized three-dimensional MHD code. Typical number of grid points is (N_r, Nφ, N_z) = (200, 64, 240) in a cylindrical coordinate system. We found that when the initial magnetic field is toroidal and weak (β = P_gas/P_mag ≫ 1), magnetic energy is amplified exponentially due to the dynamo action driven by the magnetorotational instability. In the nonlinear stage, magnetic turbulence excited in the disk tangles magnetic field lines. We found that the amplification of magnetic energy saturates when β ∼ 10 and that the system approaches a quasi-steady state. Inside the disk, filamentary shaped, magnetic pressure dominated (β < 1) regions appear. Magnetic energy release in low-β regions leads to violent time variations of X-ray emission from the disk. When the initial magnetic field is poloidal, magnetically driven collimated jet emanates from the surface of the disk.