a) Grayscale map of the vertical component of the vector magnetic field in the domain used for the nonlinear force-free field (NLFFF) extrapolation (the field strength is indicated in the vertical bar on the right). The field strength shown in the figure is corrected from the projection effects but has not yet been subjected to preprocessing. The solid box marks the region used in later sections to compute the bald patches and to plot magnetic field lines. b) Vertical and horizontal components of the vector magnetic field. The direction and length of the arrows indicate the direction and strength of the horizontal magnetic field, respectively. For the conversion from arrow length to field strength, a sample arrow is shown in the upper right corner.