  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
Alfv\'en 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.
