The topology and evolution of the Bastille Day flare

On 1998 July 14, a class M3 flare occurred at 12:55 UT in AR 8270 near disk center. Kitt Peak line-of-sight magnetograms show that the flare occurred in a delta spot. Mees vector magnetograms show a strong shear localized nea r a portion of the closed neutral line around the parasitic polarity of the 6 spot. Observations of the hare in 171, 195, and 1600 Angstrom have been obtained by TRACE, with similar or equal to 40 s temporal and 0." 5 spatial resolutions. They reveal that small-scale preflare loops above the sheared region expanded and disappeared for more than 1 hr before flare maximum. D uring the flare, bright loops anchored in bright ribbons form and grow. Thi s occurs while large-scale dimmings, associated with large expanding loops, develop on both sides of the active region. This suggests that the flare w as eruptive and was accompanied by a coronal mass ejection (CME). Magnetic field extrapolations reveal the presence of a null point in the corona, wit h its associated "spine" held line, and its "fan" surface surrounding the p arasitic polarity. We show that while the whole event occurs, the intersect ions of the "fan" and the "spine" with the photosphere brighten and move co ntinuously. The interpretation of the event shows that the magnetic evoluti on of the eruptive flare is strongly coupled with its surrounding complex t opology. We discuss evidence supporting a "magnetic breakout" process for t riggering this eruptive flare. We finally conclude that multipolar fields c annot be neglected in the study and modeling of the origin of CMEs in the c orona.