Time evolution of the BP1 case. Top row shows the evolution of the magnetogram and the corresponding potential magnetic field extrapolation. The magnetogram is scaled to ±200 G, where blue represent negative polarity and red represents positive polarity. The grey isosurface indicates the location of the magnetic null point, with the field lines being traced from the vicinity of the null region. The field line colour represents the magnetic field strength; red represents the strong field and green-blue represents the weak field. The lower row shows the same magnetic fields, while the bottom boundary represents the XRT observations shown on a linear scale, with the red being low values and purple representing the peak values. The images correspond to the times 06:38:13 UT, 08:38:05 UT, 10:38:08 UT, and 11:59:35 UT (left to right) on 2007 November 9. The included online animation shows the time evolution.