  A photon or neutrino gas--semi-contained by a baryonic species through
scattering--comprises a rather peculiar MHD fluid where the magnetic field is
truly frozen only to the co-moving volume associated with the mass density.
Although radiative diffusion precludes an adiabatic treatment of compressive
perturbations, we show that the energy equation may be cast in
"quasi-adiabatic" form for exponentially growing non-propagating wave modes.
Defining a generalized quasi-adiabatic index leads to a relatively
straightforward dispersion relation for non-axisymmetric magnetorotational
modes in the horizontal regime when an accretion disk has comparable stress
contributions from diffusive and non-diffusive particle species. This analysis
is generally applicable to optically thick, neutrino-cooled disks since the
pressure contributions from photons, pairs and neutrinos, all have the same
temperature dependence whereas only the neutrino component has radiative heat
conduction properties on the time and length scales of the instability. We
discuss the energy deposition process and the temporal and spatial properties
of the ensuing turbulent disk structure on the basis of the derived dispersion
relation.
