Representative field lines in the central portion of the active region for each NLFFF model listed in Table 1. The cubes shown here comprise the same 160 × 160 × 160 pixel subvolumes excerpted from the full 320 × 320 × 256 pixel computational domain. (The base of this subvolume is the region indicated by dotted lines in Figures  1(d) and (e).) The starting locations for the integration of the field lines are the same in each case, and form an array of regularly spaced grid points located near the lower boundary of the volume. Black field lines indicate (closed) lines that intersect the lower boundary twice, and red and green field lines represent field lines that leave the box through either the sides or top, with color indicative of polarity.