Local radiative hydrodynamic and magnetohydrodynamic instabilities in optically thick media

We examine the local conditions for radiative damping and driving of short-wavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves in static, optically thick, stratified equilibria. We show that so-called strange modes in stellar oscillation theory and magnetic photon bubbles are intimately related and are both fundamentally driven by the background radiation flux acting on compressible waves. We identify the necessary criteria for unstable driving of these waves and show that this driving can exist in both gas pressure- and radiation pressure-dominated media, as well as pure Thomson scattering media in the MHD case. The equilibrium flux acting on opacity fluctuations can drive both hydrodynamic acoustic waves and magnetosonic waves unstable. In addition, magnetosonic waves can be driven unstable by a combination of the equilibrium flux acting on density fluctuations and changes in the background radiation pressure along fluid displacements. We briefly describe the conditions under which these instabilities might be manifested in both main-sequence stellar envelopes and accretion disks.