Distribution of the surface magnetic field underlying the burst. Left: sequence of HMI line-of-sight magnetic field maps, nearly simultaneous with the SuFI images in Fig. 2, saturated at ±100 G. The FOV here is marked by the solid white box in the IMaX map in Fig. 1. The contour is the same as the outer contour in Fig. 2 that outlines the ribbon-like feature observed in SuFI. Right: grayscale image is a sub-FOV of the IMaX LOS magnetic field (boxed region from Fig. 1). Selected magnetic field extrapolations are plotted in blue (open/spine) and yellow (closed/dome) to show the magnetic topology surrounding the burst. See Sect. 6 for details.