Density structure and magnetic field lines of the region R1 are shown after 5 Myr of evolution. The magnetic field lines are colored by the Alfvénic Mach number. We note that at this evolution time, magnetic field lines are almost perpendicular to their original orientation, along the x axis and the change from super-Alfvénic to trans-Alfvénic of magnetic field lines across the shock. The dark-shaded regions at both sides of the center are the shock fronts where magnetic field lines start to bend. The associated movie is available online.