THE VARIABILITY OF ACTIVE GALACTIC NUCLEI AND THE RADIAL TRANSPORT OFVERTICAL MAGNETIC-FLUX

We consider the radial buoyancy of vertical magnetic held lines in rad iation- and gas-pressure-dominated accretion disks. We find that in ad dition to radial drift driven by turbulent diffusion and biased by the global field geometry, there are buoyancy effects which tend to move magnetic flux outward. In gas-pressure-dominated disks the poloidal ma gnetic held will move outward at a rate comparable to its inward advec tion, On the other hand, in a radiation-pressure-dominated disk the po loidal magnetic field will usually move outward faster than it is adve cted inward. This implies that the fields in disks in active galactic nuclei are generated at small radii by an internal disk dynamo. This c onclusion can be avoided if the external field imposes a supersonic Al fven speed within the disk without giving rise to interchange instabil ities. In any case, we note that variations in the mass transfer rate will lead directly to a modulation of the nonthermal emission from the disk system.