Horizontal positions of the null points in the initial potential magnetic field relative to the frame centre. The colour coding of the points indicate the heights of the nulls points. The square-root of the height is shown to compress the scale. The contour lines (full and dashed) show the locations of the strong magnetic flux concentrations (±500 G). Top: 74 null points above 0.54 Mm. Bottom: the 13 central null points listed in Table 2.