Intrinsic Magnetism in Accretion Discs

Abstract

The generation of magnetic fields in accretion discs is addressed, together with the effects of large-scale magnetic fields on the disc structure. Standard mean-field dynamos are considered, which can generate large-scale magnetic fields in discs. Toroidal magnetic field is generated by the shearing of the radial field, while turbulence with Coriolis force and vertical stratification lead to the α-effect which converts toroidal field to poloidal field. The magnetorotational instability can act as a source of turbulence, since sub-thermal magnetic fields de-stabilize Keplerian disc flows, drawing energy from the differential rotation via the magnetic coupling of fluid elements. The validity of standard dynamo theory is discussed, in the context of this powerful instability. MHD dynamos are considered in connection with disc turbulence generated by the magnetorotational instability. An accretion disc model is then presented, with a large-scale, dynamo generated magnetic field of quadrupolar symmetry, including the determination of the radial and vertical disc structures. Turbulence plays a dual role of contributing to the α-effect in the dynamo and in partly driving the disc inflow via the outward viscous transport of angular momentum.