Left: schematic diagram of wave propagation at the oblique shock, in the shock rest frame. Upstream (left), plasma moves toward the shock front with velocity v ₁, carrying with it magnetic field B ₁ at angle θ ₁ to v ₁ (or the shock normal). Post-shock, the magnetic field is B ₂ at angle θ ₂ to the shock normal, and the flow velocity is v ₂. In the preshock medium, waves are assumed to be parallel propagating along B ₁. Post-shock, the magnetosonic waves refract and travel at an angle θ _2W to the shock normal. Alfvén waves refract at a slightly different angle, but this plays no role in their reflection and transmission coefficients. Right: contour plot of the first critical fast Mach number in the plasma β–θ ₁ plane for ﹩{E}_{\mathrm{CR}}/{\rho }_{1}{v}_{1| | }^{2}=0﹩ (see Figure 4(c) in Edmiston & Kennel 1984).