The Variability of Active Galactic Nuclei and the Radial Transport of Vertical Magnetic Flux

We consider the radial buoyancy of vertical magnetic field lines in radiation- and gas-pressure–dominated accretion disks. We find that in addition 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 magnetic field will move outward at a rate comparable to its inward advection. On the other hand, in a radiation-pressure--dominated disk the poloidal magnetic field will usually move outward faster than it is advected inward. This implies that the fields in disks in active galactic nuclei are generated at small radii by an internal disk dynamo. This conclusion can be avoided if the external field imposes a supersonic Alfvén speed within the disk without giving rise to interchange instabilities. 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.