= Parallel magnetohydrodynamic shock
In a planar <magnetohydrodynamic shock> with both <velocity> and <magnetic field> normal to the front, the normal <magnetic field> is continuous. Its normal <Maxwell stress tensor> contribution is consequently the same on both sides, and the <Poynting vector> vanishes because $\mathbf u\times\mathbf B=0$. The remaining jump conditions are the ordinary <Rankine-Hugoniot conditions for a perfect gas>. This is the purely parallel branch; an oblique or switch-on shock has additional transverse fields and is not covered by this reduction.
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