  In ideal compressible hydrodynamics there is an isomorphism between spatially
one-dimensional unstea- dy and two-dimensional steady supersonic flow called
piston analogy [7]. This notice shows that this is also true for
non-equilibrium magnetosonic flow under alignment of undisturbed flow and
magnetic field in case of steady flow. An example for two generic problems,
i.e. the signal problem of radiation into a half space and steady flow along a
kinked wall bounding a half space, is given.
