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
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-Alfvenic and sub-Alfvenic flows. Since the propagation speed of
fronts is slow in the ISM, it is the sub-Alfvenic 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.
