Isocontours of the magnetic field strength deduced from the MDI magnetogram taken at 16:03:02 UT over the TRACE 171 Å image taken at 16:30:15 UT. The green and blue contours indicate the positive and negative magnetic field at  ± 200 (solid line),  ± 1000 G (dashed line). The field of view is of 160 × 90 Mm². The arrow shows a small bright loop which disappears during the flare (see Fig. 13).