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. Axisymmetric, two-dimensional models corresponding to the so-called standard alpha-disc and to the fiducial protoplanetary disc powered by turbulent viscous stress are calculated. The geometry of the total field lacks equatorial symmetry as the generated field, having a quadrupole symmetry, is superposed on an external field having a dipole symmetry. The degree of this asymmetry depends on the magnitude of the generated field relative to the externally imposed field, The existence of an external field makes it possible for some otherwise closed field Lines to open. Owing to the intrinsic asymmetry of the solution, open field Lines emerging from the top and the bottom surfaces of a disc are inclined in a different sense with respect to the vertical. This may be pertinent to the problem of asymmetric or one-sided jets seen among AGNs and protostars.