INTERCHANGE INSTABILITY IN AN ACCRETION DISC WITH A POLOIDAL MAGNETIC-FIELD

We investigate the stability to non-axisymmetric perturbations of an a ccretion disc in which a poloidal magnetic held provides part of the r adial support against gravity. Interchange instability due to radial g radients in the magnetic held are strongly stabilized by the shear how in the disc. For smooth field distributions this instability is restr icted to discs in which the magnetic energy is comparable to the gravi tational energy. An incompressible model for the instability akin to t he Boussinesq approximation for convection is given which predicts the behaviour of the instability accurately. Global axisymmetric disturba nces are also considered and found to be stable for a certain class of models. The results indicate that accretion discs may be able to supp ort poloidal fields that are strong enough to suppress other forms of magnetic instability. These strong and stable field distributions are likely to be well suited for the magnetic acceleration of jets and win ds.