a) Hα image of NOAA 9682 acquired at INAF-Catania Astrophysical Observatory on Nov. 1 at 10:55 UT. The white arrow indicates the filament involved in the eruption. b) Line-of-sight magnetogram taken by MDI on Nov. 1 at 11:11 UT. The saturation levels are ±300 G. The black line indicates the filament location. The contours A and B correspond to the negative patches of the helicity density at filament footpoints (see also Fig. 6). The field of view is ~280 × 280 Mm² and is corrected for the projection effects by applying a standard differential rotation rate (Howard et al. 1990). North is at the top and west to the right.