tokens: observed Galactic clouds the full spectrum of PRS shapes found in our simulations is recovered: PRS curves which show (i) local minima, (ii) an increase towards the highest N , (iii) and random or perpendicular orientation over the entire column density range. In the context of the results presented here, the clouds in these observations might still have a preferentially perpendicular orientation of the magnetic field and density structures in 3D, though not observed in 2D due to projection effects. This could in particular explain observations of Soler et al. ( 2017 ) and Soler ( 2019 ), which report significant variations in the PRS when considering different subregions of the same molecular cloud complex. Finally, we note that radiative transfer effects and imperfect dust alignment are unlikely to contribute significantly to these projection effects: As we have shown in Seifried et al. ( 2019 ), the observed magnetic field traces well the mass-weighted, LOS-integrated magnetic field. To further support this, we also calculated the PRS using this mass-weighted, LOS-integrated magnetic field instead of that inferred from the polarization maps. We find that these PRS do not differ significantly from those shown in Fig. 7 . 5.2 A critical magnetic field strength Overall, our findings of a change from a parallel to a perpendicular orientation of magnetic fields with respect to dense structures with increasing field strength is in good agreement with previous theoretical works (e.g. Heitsch et al. 2001a ; Ostriker et al. 2001 ; Li Nakamura 2004 ; Nakamura Li 2008 ; Collins et al. 2011 ; Hennebelle 2013 ; Soler et al. 2013 ; Chen Ostriker 2015 ; Li et al. 2015 ; Chen et al. 2016 ; Zamora-Avilés et al. 2017 ; Mocz Burkhart 2018 ; Chen et al. 2020 ). As pointed out by Soler Hennebelle ( 2017 ), this change of relative orientation is an indicator of compressive motions, i.e. ∇ v 0, coupled with a dynamically important magnetic field. The compressive motions could be created either by converging flows or gravitational collapse. As indicated in the Figs 1 and 3 , a stronger magnetic field results in (i) more guided motions towards the central collision interface, (ii) suppressing turbulent motions perpendicular to it, (iii) a stronger gravitation collapse (see also Heitsch et al. 2009 ; Zamora-Avilés et al. 2018 ; Iwasaki et al. 2019 ), and (iv) consequently a more pronounced perpendicular relative orientation between magnetic fields and the density structures, in good agreement with other theoretical works (Soler et al. 2013 ; Chen et al. 2016 ; Soler Hennebelle 2017 ; and Girichidis et al. (submitted)). For the CF runs, we find a critical field strength of ∼ 5 μ G above which we observe a flip in field orientation. The SILCC-Zoom simulations have an initial magnetic field strength of 3 μ G, which is close to this critical value. As demonstrated in Section 4.2 , the observed trend of ζ in the SILCC-Zoom runs supports the idea of a critical field
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