MHD Stability of ISM Phase Transition Layers I: Magnetic Field Orthogonal to Front

We consider the scenario of a magnetic field orthogonal to a front separating two media of different temperatures and densities, such as cold and warm neutral interstellar gas, in a 2-D plane-parallel geometry. A linear stability analysis is performed to assess the behavior of both evaporation and condensation fronts when subject to incompressible, corrugational perturbations with wavelengths larger than the thickness of the front. We discuss the behavior of fronts in both super-Alfvénic and sub-Alfvénic flows. Since the propagation speed of fronts is slow in the ISM, it is the subAlfvénic regime that is relevant, and magnetic fields are a significant influence on front dynamics . In this case we find that evaporation fronts, which are unstable in the hydrodynamic regime, are stabilized. Condensation fronts are unstable, but for parameters typical of the neutral ISM the growth rates are so slow that steady state fronts are effectively stable. However, the instability may become important if condensation proceeds at a sufficiently fast rate. This paper is the first in a series exploring the linear and nonlinear effects of magnetic field strength and orientation on the corrugational instability, with the ultimate goal of addressing outstanding questions about small-scale ISM structure. Subject headings: instabilities—ISM: structure—MHD