DYNAMO GENERATION OF MAGNETIC-FIELDS IN ACCRETION DISKS

We consider the problem of dynamo generation of magnetic fields in accretion disks of compact binary systems, young stellar objects, and active galactic nuclei (AGNs). This study of the structure and evolution of magnetic fields has relevance to acceleration mechanisms for outflows present in these objects. In this work we improve upon previous treatment of alpha-omega dynamos in accretion disks by including the effects of advection, and by adopting known spatial forms for resistivity, shear, and radial drift. In addition, we calculate the external fields by employing realistic boundary conditions to connect to nonzero vacuum or force-free fields. While no dipole modes were found, we find that the fundamental steady quadrupole mode corresponding to a dynamo number of 6.5 has a maximum near the disk surface, which is contrary to the assumption usually made. While the solution to the global equation is obtained analytically, the boundary conditions, involving expansions of the external field, are done numerically. We show that coronal boundary conditions are favorable for bipolar flows. From an approximate treatment of the time-dependent problem, we estimate that fields in AGN disks can be built up in timescales of order 10(5)-10(6) yr.