Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........35s&link_type=abstract
Ph.D dissertation, 2004. 134 pages; United States -- California: University of California, Santa Barbara; 2004. Publication Num
Other
Accretion Disks, Magnetohydrodynamics, Radiation, Magnetic Fields
Scientific paper
Topics in radiation magnetohydrodynamics, with a particular emphasis on stars and accretion disks, are studied. We consider the local linear stability of stratified, magnetized, highly conducting, optically thick atmospheres that are in local thermodynamic equilibrium. Special attention is given to short wavelength compressible motion. When the radiation diffusion time is short compared to the mode crossing time, the presence of an equilibrium magnetic field allows the background radiative flux to secularly drive acoustic waves as long as damping due to radiative diffusion is overcome. Ultimately, driving results from the lack of equality between the equilibrium radiative flux's separate interaction with density fluctuations and fluid displacements. Dynamical growth rates are possible even for atmospheres that are gas pressure dominated and/or weakly magnetized.
The interaction between the violent turbulent motions generated by the magnetorotational instability and the intense thermal radiation field in cool radiation pressure dominated relativistic accretion disks is considered as well. If the turbulent accretion stress scales with the total pressure, the random motions of the electrons are primarily derived from the turbulence, rather than the thermal energy. It follows that thermally generated disk photons stand to gain energy directly from the turbulence via bulk Comptonization or in other words, turbulent Comptonization. To lowest order, the resultant spectral deformation is identical to that created by thermal Comptonization, but in this case, the cutoff temperature is determined by the square of the turbulent velocities rather than the electron temperature. Turbulent Comptonization provides a natural way for generating energetic X-rays directly from cool optically thick luminous accretion flows.
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