v 1 1 2 A ug 1 99 3 Does Dissipation in AGN Disks Couple to the Total Pressure ?

Recent work on the transport of angular momentum in accretion disks suggests that the Velikhov-Chandrasekhar instability, in which a large scale magnetic field generates small scale eddys in a shearing environment, may be ultimately responsible for this process. Although there is considerable controversy about the origin and maintenance of this field in accretion disks, it turns out that it is possible to argue, quite generally, using scaling arguments, that this process is sensitive to the total pressure in an AGN disk, rather than the pressure contributed by gas alone. We conclude that the resolution of the conceptual difficulties implied by the presence of strong thermal and viscous instabilities in radiation pressure and electron scattering dominated does not lie in models that couple the total dissipation rate to the gas pressure alone, or to some weighted mean of the gas and radiation pressures. Subject headings: AGN