Vertical Structure of Gas Pressure–dominated Accretion Disks with Local Dissipation of Turbulence and Radiative Transport

(shortened) We calculate the vertical structure of a local patch of anaccretion disk in which heating by dissipation of MRI-driven MHD turbulence isbalanced by radiative cooling. Heating, radiative transport, and cooling arecomputed self-consistently with the structure by solving the equations ofradiation MHD in the shearing-box approximation. Using a fully 3-d andenergy-conserving code, we compute the structure of this disk segment over aspan of more than five cooling times. After a brief relaxation period, astatistically steady-state develops. Measuring height above the midplane inunits of the scale-height H predicted by a Shakura-Sunyaev model, we find thatthe disk atmosphere stretches upward, with the photosphere rising to about 7H,in contrast to the approximately 3H predicted by conventional analytic models.This more extended structure, as well as fluctuations in the height of thephotosphere, may lead to departures from Planckian form in the emergentspectra. Dissipation is distributed across the region within roughly 3H of themidplane, but is very weak at greater altitudes. Because fluctuations in thedissipation are particularly strong away from the midplane, the emergentradiation flux can track dissipation fluctuations with a lag that is only0.1--0.2 times the mean cooling time of the disk. Long timescale asymmetries inthe dissipation distribution can also cause significant asymmetry in the fluxemerging from the top and bottom surfaces of the disk. Radiative diffusiondominates Poynting flux in the vertical energy flow throughout the disk.Comment: accepted by Ap