Modelling MHD accretion-ejection - episodic ejections of jets triggered by a mean-field disk dynamo

摘要

We present MHD simulations exploring the launching, acceleration andcollimation of jets and disk winds. The evolution of the disk structure isconsistently taken into account. Extending our earlier studies, we now considerthe self-generation of the magnetic field by an $\alpha^2\Omega$ mean-fielddynamo. The disk magnetization remains on a rather low level, that helps toevolve the simulations for $T > 10,000$ dynamical time steps on a domainextending 1500 inner disk radii. We find a magnetic field of the inner disksimilar to the commonly found open field structure, favoringmagneto-centrifugal launching. The outer disk field is highly inclined andpredominantly radial. Here, differential rotation induces a strong toroidalcomponent that plays a key role in outflow launching. These outflows from theouter disk are slower, denser, and less collimated. If the dynamo action is notquenched, magnetic flux is continuously generated, diffuses outward the disk,and fills the entire disk. We have invented a toy model triggering atime-dependent mean-field dynamo. The duty cycles of this dynamo lead toepisodic ejections on similar timescales. When the dynamo is suppressed as themagnetization falls below a critical value, the generation of the outflows andalso accretion is inhibited. The general result is that we can steer episodicejection and {\em large-scale jet knots} by a {\em disk-intrinsic dynamo} thatis time-dependent and regenerates the jet-launching magnetic field.