Distance–time diagrams of the zonal component of the magnetic field (B_X) around the line connecting the polarities (upper panel). The black solid lines indicate the positions of the strongest vertical field, and the dotted line represents the position of the polarity barycentres. The outlined lines represent the position of ±500 G vertical field iso-contour. In the bottom panel, we plot the time evolution of the same quantity evaluated along the solid black lines. A corresponding movie is available online.