Regions used to compute the HRO diagram and the shape parameter shown in Fig. 4. The yellow surfaces are density isocountours and the dark arrows represent the magnetic field. The top-left panel corresponds to gas with density n ∈ [10, 30] cm⁻³, top-right panel to n ∈ [30, 100] cm⁻³, bottom-left to n ∈ [100, 300] cm⁻³, and bottom-right to n ∈ [300, 1000] cm⁻³. It can be seen that for the four density intervals shown here, the magnetic field is noticeably parallel to the density structures. However, the magnetic field shows a deviation from this trend for the highest density interval in the bottom right panel. The associated movie is available online.