ACCRETION DISC DYNAMOS IN THE PRESENCE OF A WEAK EXTERNAL MAGNETIC-FIELD

We consider the problem of an MHD dynamo operating in a thin accretion disc vertically threaded by a uniform external magnetic field. Axisym metric, two-dimensional models corresponding to the so-called standard alpha-disc and to the fiducial protoplanetary disc powered by turbule nt viscous stress are calculated. The geometry of the total field lack s equatorial symmetry as the generated field, having a quadrupole symm etry, is superposed on an external field having a dipole symmetry. The degree of this asymmetry depends on the magnitude of the generated fi eld relative to the externally imposed field, The existence of an exte rnal field makes it possible for some otherwise closed field Lines to open. Owing to the intrinsic asymmetry of the solution, open field Lin es emerging from the top and the bottom surfaces of a disc are incline d in a different sense with respect to the vertical. This may be perti nent to the problem of asymmetric or one-sided jets seen among AGNs an d protostars.