Radial component of the magnetic field in NOAA active region AR 12673 (SHARP 7115). This region produced multiple M- and X-class flares during its rotation across the solar disk in September 2017. (a) Initial emergence of the field at 0000 UT on 1 September 2017; (b) Complex evolution at 0900 UT on 5 September 2017, approximately 24 h before producing an X17 flare, the largest flare of solar cycle 24; and (c) at 1000 UT on 7 September 2017, during an M-class flare.