= Solution
Resolve the upstream uniform field $\mathbf B_0=B_0\mathbf e_z$ into components normal and tangential to the spherical shock:
$$
B_{r,1}=B_0\cos\theta,
\qquad
B_{\theta,1}=-B_0\sin\theta,
\qquad B_{\phi,1}=0.
$$
The <ideal magnetohydrodynamic shock conditions> preserve the normal magnetic component and multiply the tangential component of a weak passive field by the gas compression ratio. Part (b) therefore gives
$$
\boxed{B_r(R,\theta,t)=B_0\cos\theta,
\qquad
B_\theta(R,\theta,t)=-4B_0\sin\theta,
\qquad
B_\phi=0}.
$$
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