Streamlines at the circled star from Fig. 3, with zoom-in on the shock and super-subsonic-transitions (left). The same streamlines are shown as in Fig. 12, with a limited range of azimuthal distances for clarity. We note the asymmetry in the vertical, giving the shock an overall concave shape. Ratios of pre- to post-shock variables are shown for the same vertical position on the shock (right). The shock jump values are important for explaining both the shape of the shock and the occurrence of the free-fall region. In the upper parts of the shock, the ram-pressure of the incoming flow is insufficient to overwhelm the shock static pressure of the spiral arm, and the flow is forced to become subsonic (seen where M_post < 1). Just below the flow separation at z_sep = 0.41r_H, as the density increases, the flow ram pressure becomes sufficient to overcome the static shock pressure. This happens below z = 0.38r_H. In this region, the pre-shock supersonic flow also remains supersonic post-shock, albeit slowed down. The flow then continues to advect material from the post-shock volume, leading to the formation of the free-fall region. The region below z = 0.03r_H encounters in its first post-shock cell material coming from above, along the shock curvature. Those colliding flows lead to compressional heating at the “foot” of the shock, increasing the density and temperature contrast. This foot region is important as it delivers mass to the midplane, keeping the midplane density jump alive. Along the entire shock, the adiabatic ratio [u² ∕2 +h] = 1 holds, revealing that radiative effects play only a minor role for the structure of this shock.