Upper row shows the active region pre-flare state in continuum intensity (left) and vertical field strength (right). Green contours overlaid on both images indicate Ca ii H flare brightening observed at flare peak. The  ~ 50″ × 40″ sub-region indicated by the black box in the upper row is shown in the lower row as preprocessed vertical field strength, at increasing scan time from left to right. A region of interest, thresholded at  − 750 G, is defined by the blue contours, and the purple line indicates the time of flaring between scans 3 and 4.