Left: columm density map, log ₁₀(N_H/ cm^-2), overlaid with magnetic field pseudo-vectors whose orientations are inferred from the Planck 353 GHz polarization observations. The length of the pseudo-vectors is normalized so does not reflect the polarization fraction. In this first group, the regions analysed are, from top to bottom, Taurus, Ophiuchus, Lupus, Chamaeleon-Musca, and CrA. Right: HROs for the lowest, an intermediate, and the highest N_H bin (black, blue, and red, respectively). For a given region, bins have equal numbers of selected pixels (see Sect. 4.1.1 and Appendix A) within the N_H ranges labelled. The intermediate bin corresponds to selected pixels near the blue contours in the column density images. The horizontal dashed line corresponds to the average per angle bin of 15°. The widths of the shaded areas for each histogram correspond to the ± 1σ uncertainties related to the histogram binning operation. Histograms peaking at 0° correspond to B_⊥ predominantly aligned with iso-N_H contours. Histograms peaking at 90° and/or − 90° correspond to B_⊥ predominantly perpendicular to iso-N_H contours.