Average distribution of the magnetic field densities in coronal holes (a). Imbalance of the magnetic field at a given magnetic level l (b), and total unbalanced magnetic flux arising from pixels at a magnetic level l (c). The solid lines are the measured distributions and the dashed lines correspond to the fitted background magnetic field. In panel c, the dotted lines give the remaining unbalanced magnetic flux of the quiet coronal hole regions, which is not explained by a background magnetic field.