Dynamo Driven Accretion Discs and Dwarf Nova Eruptions

We explore the consequences of a magnetic dynamo origin for the viscosity in accretion discs, for the structure and evolution of discs in dwarf nova systems. We propose that the rapid cooling that sets in at the end of a dwarf nova eruption acts to inhibit the Balbus-Hawley instability, and thereby to quench dynamo action and so reduce disc viscosity. We demonstrate that a modi ed disc instability model can reproduce the basic properties of dwarf nova eruptions, as well as some properties of quiescent discs. We also discuss some observational consequences of our model. Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA.