The influence of large-scale magnetic fields on the structure of accretion
disks is studied. The magnetic field is obtained by a self-consistent nonli
near dynamo model with magnetic pressure strongly influencing the density s
tratification which itself feeds back to the field generation. The resultin
g magnetic field geometry is discussed in relation to the accretion disk wi
nd theory.
Regarding new results of MHD turbulence simulations, both possible signs of
the oc-effect are allowed (Brandenburg & Donner 1997). In the canonical ca
se of positive alpha the resulting field is of quadrupolar symmetry. The fi
eld strength is about 50% of the value for dynamo models nonlinearly limite
d by alpha-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 alpha, 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 i
s more flat. The magnetic torque becomes of the same order as the radial vi
scous torque. The inclination angle of the poloidal field exceeds 30 degree
s even for a magnetic Prandtl number of order unity, and also the criterion
for poloidal collimation after Spruit et al. (1997) is fulfilled. The dyna
mo-generated magnetic field configuration thus supports the magnetic wind l
aunching concept for accretion disks for realistic turbulent magnetic Prand
tl numbers.