Proper motions of the parasitic positive polarity (solid lines) in the photosphere, and the overlying magnetic null (dashed lines) over a period of one hour. The four symbols connecting the lines denote the times at which the positions of the magnetic element and the null are recorded. The lines display the tracks of the respective features. The colours indicate the velocity magnitude with which the respective feature is displaced (i.e. from the recorded positions every 23 min). See Sects. 3.2 and 3.3.