  For the free boundary problem of the plasma-vacuum interface to ideal
incompressible magnetohydrodynamics (MHD) in two-dimensional space, the a
priori estimates of solutions are proved in Sobolev norms by adopting a
geometrical point of view. In the vacuum region, the magnetic field is
described by the div-curl system of pre-Maxwell dynamics, while at the
interface the total pressure is continuous and the magnetic field is tangent to
the boundary. We prove that the $L^2$ norms of any order covariant derivatives
of the magnetic field in vacuum and on the boundaries are bounded in terms of
initial data and the second fundamental forms of the free interface and the
rigid wall.
