Current understanding of accretion disc viscosity

We describe the alpha prescription of Shakura and Sunyaev and discuss the c onstraints placed on it by observable phenomena in rime varying discs. We e xplain why hydrodynamic turbulence alone cannot be the source of viscosity and why the introduction of a weak magnetic field changes this so that magn etohydrodynamic turbulence is effective. We discuss recent numerical simula tions that suggest that MHD turbulence might drop to zero during dwarf-nova quiescence and consider the consequences for dwarf-nova outbursts. We spec ulate on the part played by the secondary star's own magnetic held. (C) 200 0 Elsevier Science B.V. All rights reserved.