Structure and magnetic configurations of accretion disk - dynamo models

The influence of large-scale magnetic fields on the structure of accretion disks is studied. The magnetic field is obtained by a self-consistent nonlinear dynamo model with magnetic pressure strongly influencing the density stratifica-tion which itself feeds back to the field generation. The resulting magnetic field geometry is discussed in relation to the accretion disk wind theory. Regarding new results of MHD turbulence simulations, both possible signs of the α-effect are allowed (Brandenburg & Don-ner 1997). In the canonical case of positive α the resulting field is of quadrupolar symmetry. The field strength is about 50% of the value for dynamo models nonlinearly limited by α-quenching. The temperature profiles as well as the disk geometry remain nearly unchanged. The viscous stress remains the key transporter of angular momentum driving the accretion inflow. For negative α, however, a stationary dipolar structure of the magnetic field results. The additional magnetic torque at the disk surface changes the profile of the effective temperature significantly to a profile which is more flat. The magnetic torque becomes of the same order as the radial viscous torque. The inclination angle of the poloidal field exceeds 30 • even for a magnetic Prandtl number of order unity, and also the criterion for poloidal collimation after Spruit et al. (1997) is fulfilled. The dynamo-generated magnetic field configuration thus supports the magnetic wind launching concept for accretion disks for realistic turbulent magnetic Prandtl numbers. 1. Motivation The mean-field dynamo theory describes the maintenance of large-scale magnetic fields by flow patterns in conducting fluids. As is well known, in particular the feedback of the induced magnetic fields onto the fluid limits the magnetic field amplitude. While for stars the magnetic feedback basically drives internal large-scale flows ('Malkus-Proctor effect') and/or suppresses and deforms the turbulent convection ('α-quenching', Send offprint requests to: G. Rüdiger 'η T-quenching'), in galaxies and accretion disks the geometry of the disks is affected as well presents disk-dynamo models with moving upper and lower boundaries as the only nonlinear feedback possibility. For given α and η T-effects the disk thickness is the only adjustable eigenvalue of such dynamo models which – for the given parameters – exceeds the nonmagnetic standard accretion disk thickness. The difference should be produced by the magnetic pressure, which to this end needs such high values that the magnetic transfer of angular momentum also reaches remarkable efficiency with important consequences. This is all done using …