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Accretion disc dynamos in the presence of a weak external magnetic field

M. Reyes-Ruiz tex2html_wrap_inline_super941 and T.F. Stepinski tex2html_wrap_inline_super943

tex2html_wrap_inline_super945 Department of Space Physics and Astronomy, Rice University, Houston, TX 77051, USA
tex2html_wrap_inline_super947 Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA

Abstract:

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 tex2html_wrap_inline949 -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 relative magnitude of the generated field to the externally imposed field. The existence of an external field makes it possible for some otherwise closed field lines to open. Due 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.