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We present the results of a series of radiation-MHD simulations of a localpatch of an accretion disk, with fixed vertical gravity profile but withdifferent surface mass densities and a broad range of radiation to gas pressureratios. Each simulation achieves a thermal equilibrium that lasts for manycooling times. After averaging over times long compared to a cooling time, wefind that the vertically integrated stress is approximately proportional to thevertically-averaged total thermal (gas plus radiation) pressure. We mapout--for the first time on the basis of explicit physics--the thermalequilibrium relation between stress and surface density: the stress decreases(increases) with increasing surface mass density when the simulation isradiation (gas) pressure dominated. The dependence of stress on surface massdensity in the radiation pressure dominated regime suggests the possibility ofa Lightman-Eardley inflow instability, but global simulations or shearing boxsimulations with much wider radial boxes will be necessary to confirm this anddetermine its nonlinear behavior.

Turbulent Stresses in Local Simulations of Radiation-Dominated Accretion Disks, and the Possibility of the LIghtman-Eardley Instability