MHD turbulence is generally believed to have two important functions in accretion disks: it transports angular momentum outward, and the energy in its shortest wavelength modes is dissipated into the heat that the disks radiate. In this paper we examine a pair of mechanisms which may play an important role in regulating the amplitude and spectrum of this turbulence: photon diffusion and viscosity. We demonstrate that in radiation pressure-dominated disks, photon damping of compressive MHD waves is so rapid that it likely dominates all other dissipation mechanisms.
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