Log ratio of the amplified magnetic field in the immediate upstream relative to the field of a simulation without any shell interactions, shown as the solid lines show. The vertical dashed red lines mark from left to right the times of the first shock-shell interaction, the time of when the shock reached the peak density of the shell, and the time when the shock is propagating in the free wind again for the simulation with the earliest shell interaction (strong line). The pale lines show the behavior for later shell interactins.