Evolution of the persistent fan-spine magnetic topology (selected field lines plotted in red) and the associated magnetic null (blue cross, at a height of about 500 km above z = 0), surrounding the observed UV burst. In each panel, the gray scale image shows the line-of-sight magnetic field map saturated at ±250 G. It covers the spatial extent outlined by the yellow box in Fig. 7. Panel a is obtained at 20:37 UT, and panel b at 21:00 UT. The field lines are plotted only up to a height of 2 Mm. The z-axis is stretched for display purposes. See Sects. 3.2 and 3.3.