  We consider the free boundary problem for the 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, in
order to investigate the well-posedness of the problem, in particular in
relation with the electric field in vacuum. 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.
  Under suitable stability conditions satisfied at each point of the
plasma-vacuum interface, we derive a basic a priori estimate for solutions to
the linearized problem. The proof follows by a suitable secondary
symmetrization of the Maxwell equations in vacuum and the energy method.
  An interesting novelty is represented by the fact that the interface is
characteristic with variable multiplicity, so that the problem requires a
different number of boundary conditions, depending on the direction of the
front velocity (plasma expansion into vacuum or viceversa). To overcome this
difficulty, we recast the vacuum equations in terms of a new variable which
makes the interface characteristic of constant multiplicity. In particular, we
don't assume that plasma expands into vacuum.
