Results from the mpole potential field extrapolations. Top: samples of extrapolated potential magnetic field lines (white), viewed from above, overlaid on the HMI line-of-sight magnetogram produced before (left), during (middle), and after (right) the collapse of the 3D null point. Blue filled circles lie on the locations of NRH centroids, and the colours match the correlation to frequency given in Fig. 2. Overplotted onto each frame as a black dashed line is the location of the separator that passes closest to the NRH centroids. The three times were chosen to match those shown in Fig. 2. Bottom: the same magnetic field lines and radio sources shown in the top row are shown again, viewed at an angle to represent the 3D structure of the field. The dashed line again refers to the separator. Radio source heights are estimated directly from the one-fold Newkirk model (Newkirk 1961). An animated version of this figure is available online.