Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 314 / 3 / d / ii / Solution

If $B_{y2}=0$ but $B_{y1}\ne 0$, tangential momentum continuity in the formforces $A_1^2=1$. Since the normal components are positive,so the upstream velocity equals the upstream Alfvén speed. This is the limiting switch-off shock condition.