Mean spectrum and circular statistics used for the study of the orientation of the HI filamentary structures across velocity channels toward the region presented in Fig. 1. Top: mean HI intensity (I). Middle top: mean resulting vector (r), which indicates if the distribution of orientation angles is flat (r ≈ 0) or sharply unimodal (r ≈ 1). Middle bottom: projected Rayleigh statistic (V), which indicates if the distribution of orientation angles is clearly peaked around 0° (V ≫ 0) or 90° (V ≪ 0). Bottom: circular mean () and standard deviation (σ_V) of the orientation angles, represented by the line and the shaded area, respectively. The error bars correspond to the observation uncertainties and the error propagation based on the Monte Carlo sampling introduced in Appendix A.1. The dashed horizontal lines in each panel indicate the values of those quantities in the 1.4 GHz noise maps used to characterize the effect of the side lobes and continuum sources in the HI maps. The vertical gray lines correspond to the velocity channels shown in Fig. 1.