Three-dimensional magnetohydrodynamic simulations of jets from accretion disks

We report the results of three-dimensional magnetohydrodynamic (MHD) simulations of a jet formation by the interaction between an accretion disk and a large-scale magnetic field. The disk is not treated as a boundary condition but is solved self-consistently. To investigate the stability of the MHD jet, the accretion disk is perturbed with a nonaxisymmetric sinusoidal or random fluctuation of the rotational velocity. The dependences of the jet velocity (vz), mass outflow rate (M-w), and mass accretion rate (M-a) on the initial magnetic field strength in both nonaxisymmetric cases are similar to those in the axisymmetric case. That is, v(z) proportional to B-0(1/3), M-w proportional to B-0, and M-a proportional to B-0(1.4), where B-0 is the initial magnetic field strength. The former two relations are consistent with Michel's steady solution, v(z) proportional to(B-0(2)/M-w)(1/3), although the jet and accretion do not reach the steady state. In both perturbation cases, a nonaxisymmetric structure with m = 2 appears in the jet, where m is the azimuthal wavenumber. This structure cannot be explained by Kelvin-Helmholtz instability and seems to originate in the accretion disk. Nonaxisymmetric modes in the jet reach almost constant levels after about 1.5 orbital periods of the accretion disk, while all modes in the accretion disk grow with oscillation. As for the angular momentum transport by Maxwell stress, the vertical component, B phi Bz/4 pi , is comparable to the radial component, B phi Br/4 pi , in the wide range of initial magnetic field strength.