Long-term evolution of a di pole type magnetosphere interacting with an accretion disk II. Transition into a quasi-stationary spherically radial outflow

The evolution of an initially stellar dipole type magnetosphere interacting with an accretion disk is investigated numerically using the ideal MHD ZEU S-3D code in the 2D-axisymmetry option. Depending mainly on the magnetic fi eld strength, our simulations may last several thousands of Keplerian perio ds of the inner disk. A Keplerian disk is assumed as a boundary condition p rescribing a mass inflow into the corona. Additionally, a stellar wind from a rotating central star is prescribed. We compute the innermost region aro und the stellar object applying a non-smoothed gravitational potential. Our major result is that the initially dipole type field develops into a sp herically radial outflow pattern with two main components, a disk wind and a stellar wind component. These components evolve into a quasi-stationary f inal state. The poloidal field lines follow a conical distribution. As a co nsequence of the initial dipole, the field direction in the stellar wind is opposite to that in the disk wind. The half opening angle of the stellar w ind cone varies from 30 degrees to 55 degrees depending on the ratio of the mass flow rates of disk wind and stellar wind. The maximum speed of the ou tflow is about the Keplerian speed at the inner disk radius. An expanding bubble of hot, low density gas together with the winding-up pr ocess due to differential rotation between star and disk disrupts the initi al dipole type field structure. An axial jet forms during the first tens of disk/star rotations, however, this feature does not survive on the very lo ng time scale. A neutral field line divides the stellar wind from the disk wind. Depending on the numerical resolution, small plasmoids are ejected in irregular time intervals along this field line. Within a cone of 15 degree s along the axis the formation of small knots can be observed if only a wea k stellar wind is present. With the chosen mass flow rates and field strength we see almost no indicat ion for a flow self-collimation. This is due to the small net poloidal elec tric current in the (reversed field) magnetosphere which is in difference t o typicaljet models.