A solar eruption triggered by the interaction between two magnetic flux systems with opposite magnetic helicity

Context. In recent years the accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions has been considered to play an important role in the destabilization processes that lead to eruptive phenomena occurring in the solar atmosphere. Aims. In this paper we want to highlight a specific aspect of magnetic helicity accumulation, providing new observational evidence of the role played by the interaction of magnetic fields characterized by opposite magnetic helicity signs in triggering solar eruption. Methods. We used 171 Å TRACE data to describe a filament eruption on 2001 Nov. 1 in active region NOAA 9682 and MDI full disk line-of-sight magnetograms to measure the accumulation of magnetic helicity in corona before the event. We used the local correlation tracking (LCT) and the differential affine velocity estimator (DAVE) techniques to determine the horizontal velocities and two methods for estimating the magnetic helicity flux. Results. The chirality signatures of the filament involved in the eruption were ambiguous, and the overlying arcade visible during the main phase of the event was characterized by a mixing of helicity signs. However, the measures of the magnetic helicity flux allowed us to deduce that the magnetic helicity was positive in the whole active region where the event took place, while it was negative near the magnetic inversion line where the filament footpoints were located. Conclusions. These results suggest that the filament eruption may be caused by magnetic reconnection between two magnetic field systems characterized by opposite signs of magnetic helicity. We also find that only the DAVE method allowed us to obtain the crucial information on the horizontal velocity field near the magnetic inversion line.