An unconventional accretion disc dynamo

Abstract

In the light of recent results from numerical simulations of accretion disc MHD turbulence, we revisit the problem of the configuration of large-scale magnetic fields resulting from an αΩ dynamo operating in a thin accretion disc. In particular, we analyse the consequences of the peculiar sign of the α-effect suggested by numerical simulations. We determine the symmetry of the fastest-growing modes in the kinematic dynamo approximation and, in the framework of an ‘α-quenched’ dynamo model, study the evolution of the magnetic field. We find that the resulting field for this negative polarity of the α-effect generally has dipole symmetry with respect to the disc midplane, although the existence of an equilibrium configuration depends on the properties of the turbulence. The role of magnetic field dragging is discussed and, finally, the presence of an external uniform magnetic field is included to address the issue of magneto centrifugal wind launching from accretion discs.