Magnetosonic waves traveling against a plasma wind

The points of a plasma flow where the velocity matches the wave velocity of either the slow or fast magnetohydrodynamics modes are known to be highly relevant e.g. in the study of stellar winds. We develop a geometric optics analysis of a simple flow geometry where slow or fast critical points are present. A first integral of the ray equations is useful for a qualitative study of the ray topology and the transport of wavefronts, which are illustrated with numerical examples. The evolution of perturbations traveling along rays and eventually becoming shocks is also analyzed, showing that whenever wavefronts are slowed either by a small wave velocity or the geometry of the ray, shocks are more likely to occur.