Synthetic HI emission intensity and circular statistics used for the study of the orientation of the HI filamentary structures across velocity channels in the synthetic observations presented in Fig. E.1. Top: HI emission averaged over the whole map and toward the indicated position. 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: mean orientation angle () of the HI filamentary structures.