  We consider the free boundary problem for the two-dimensional plasma-vacuum
interface in ideal compressible magnetohydrodynamics (MHD). In the plasma
region, the flow is governed by the usual compressible MHD equations, while in
the vacuum region we consider the Maxwell system for the electric and the
magnetic fields. At the free interface, driven by the plasma velocity, the
total pressure is continuous and the magnetic field on both sides is tangent to
the boundary.
  We study the linear stability of rectilinear plasma-vacuum interfaces by
computing the Kreiss-Lopatinskii determinant of an associated linearized
boundary value problem. Apart from possible resonances, we obtain that the
piecewise constant plasma-vacuum interfaces are always weakly linearly stable,
independently of the size of tangential velocity, magnetic and electric fields
on both sides of the characteristic discontinuity.
  We also prove that solutions to the linearized problem obey an energy
estimate with a loss of regularity with respect to the source terms, both in
the interior domain and on the boundary, due to the failure of the uniform
Kreiss-Lopatinskii condition, as the Kreiss-Lopatinskii determinant associated
with this linearized boundary value problem has roots on the boundary of the
frequency space. In the proof of the a priori estimates, a crucial part is
played by the construction of symmetrizers for a reduced differential system,
which has poles at which the Kreiss-Lopatinskii condition may fail
simultaneously.
