Comparison of pre-flare magnetic field structures involved in the first (1) and last (6) HXR peaks. 1a and 2a: HMI LOS magnetic field with contours of AIA 1600 Å enhancements during HXR peak 1 (left) and 6 (right). See Fig. 4, bottom panels for the intermediate HXR peaks. 1b and 2b: AIA 131 Å images taken during the two HXR peaks. 1c and 2c: selection of the most important field lines. 1d and 2d: side view of the same structure. 1e and 2e: same contours, but the field lines have been swapped. 3: comparison between selected field structures.