DYNAMO GENERATION OF MAGNETIC-FIELDS IN ACCRETION DISKS

We consider the problem of dynamo generation of magnetic fields in acc retion disks of compact binary systems, young stellar objects, and act ive galactic nuclei (AGNs). This study of the structure and evolution of magnetic fields has relevance to acceleration mechanisms for outflo ws present in these objects. In this work we improve upon previous tre atment of alpha-omega dynamos in accretion disks by including the effe cts of advection, and by adopting known spatial forms for resistivity, shear, and radial drift. In addition, we calculate the external field s by employing realistic boundary conditions to connect to nonzero vac uum or force-free fields. While no dipole modes were found, we find th at the fundamental steady quadrupole mode corresponding to a dynamo nu mber 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 expansio ns of the external field, are done numerically. We show that coronal b oundary conditions are favorable for bipolar flows. From an approximat e 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.