Comparisons between the extrapolation structures and the XRT observations. The first row: the XRT observation at 20:30 on 2006 December 12 before the X3.4 flare (panels a), b)) and at 04:30 UT on 2006 December 13 after the flare (panel c)). The white rectangle in panel a) shows the field of view of the seven other panels. Second row: visualizations of the sample closed field lines (red) and the iso-surface of the current density at the level of 6.96   mA   m^-2 (green) within the NLFFF model volume (Wheatland 2007) at 20:30 UT on December 12 and at 04:30 UT on December 13, shown against the corresponding map of Hinode/SP B_z at the photosphere (panel d)) and at a height of z = 1371   km (panels e) and f)). The last row shows the 3D images of the middle row. The transverse CS (yellow arrows) and the vertical one (white arrows) can be clearly identified in panel h).