AXISYMMETRICAL 2-DIMENSIONAL COMPUTATION OF MAGNETIC-FIELD DRAGGING IN ACCRETION DISKS

In this paper we model a geometrically thin accretion disk interacting with an externally imposed, uniform, vertical magnetic field. The acc retion flow in the disk drags and distorts field lines, amplifying the magnetic field in the process. Inside the disk the radial component o f the field is sheared into a toroidal component. The aim of this work is to establish the character of the resultant magnetic field and its dependence on the disk's parameters. We concentrate on alpha-disks dr iven by turbulent viscosity. Axisymmetric, two-dimensional solutions a re obtained without taking into account the back-reaction of the magne tic field on the structure of the disk. The character of the magnetic field depends strongly on the magnitude of the magnetic Prandtl number , P. We present two illustrative examples of viscous disks: a so-calle d ''standard'' steady state model of a disk around a compact star (e.g ., cataclysmic variable), and a steady state model of a proto-planetar y disk. In both cases, P=1, P=10(-1), and P=10(-2) scenarios are calcu lated. Significant bending and magnification of the magnetic field is possible only for disks characterized by P of the order of 10(-2) In s uch a case, the field lines are bent sufficiently to allow the develop ment of a centrifugally driven wind. Inside the disk the held is domin ated by its toroidal component. We also investigate the dragging of th e magnetic field by a nonviscous protoplanetary disk described by a ph enomenological model. This scenario leads to large distortion and magn ification of the magnetic field.