Structure functions of the starlight polarization (orange), submillimetre polarization along the lines of sight to stars, (blue), and submillimetre polarization over the whole map, (magenta).In the lower plot of each panel we present the differences (dark green), (light green), and the corresponding bias corrections, (dashed gray) and (solid gray). The vertical dashed line indicates the effective size of the Planck beam. The horizontal dashed line in the upper plot of each panel indicates  rad ≈52°, which corresponds to a random distribution of orientations.