Panel a) detected magnetic structures (in black) on a typical binarized SOHO/MDI magnetogram. The threshold is equal to 3σ. Panel b) circles obtained by recursively defining the local DF maxima. Panel c) circles packed together to generate the voids. All the circles pertaining to the same void structure are labeled with the same index, i.e., color (in this figure we highlight voids with a white boundary). The panels show a portion of the MDI FOV for the sake of visualization.