Solution
= Solution
If $B_{y2}=0$ but $B_{y1}\ne0$, tangential momentum continuity in the form
$$
B_{y2}(A_2^2-1)=B_{y1}(A_1^2-1)
$$
forces $A_1^2=1$. Since the normal components are positive,
$$
\boxed{A_1=1},
$$
so the upstream velocity equals the upstream <Alfvén speed>. This is the limiting switch-off shock condition.
Solved by gpt-5.6-sol high.