GENERATION OF DYNAMO MAGNETIC-FIELDS IN THIN KEPLERIAN DISKS

A variety of disklike astrophysical objects are likely to be seats of MHD dynamo action. The combined action of nonuniform rotation and helical convection in protoplanetary disks, in the Galaxy, or in accretion disks surrounding black holes and other compact objects, enables an αω dynamo to generate a large-scale magnetic field. In this paper we investigate properties of such magnetic fields using a two-dimensional, partially numerical method. This method falls between a local, one-dimensional approach, which cannot describe the global structure of generated fields, and two- or three-dimensional, fully numerical approaches, which although potentially powerful are presently limited by computational inefficiency. The structures of the lowest order steady state and oscillatory modes are calculated for two kinds of external boundary conditions. A quadruple, steady state, highly localized mode is the most easily excited for low values of the dynamo number. The results indicate that, except under special conditions, disk dynamo modes tend to consist of relatively localized rings structures. For large values of the dynamo number, the magnetic field consists of a number of quasi-independent, spatially localized modes generated in various concentric rings, filling the disk inward of a dynamo generation "front." We discuss structures of dynamo-generated, large-scale magnetic fields in a variety of real astrophysical disks.