Magnetic helicity balance during a filament eruption that occurred in active region NOAA 9682

Context. Photospheric shear plasma flows in active regions may be responsible for the magnetic helicity injection in the solar corona not only during the energy storage process before a solar eruption, but also during and after the release of the free magnetic energy caused by the eruption. Indeed, after a filament eruption or expansion the magnetic torque imbalance can induce shear flows that can be responsible for yet another injection of magnetic helicity into the corona. Aims. We investigated the magnetic helicity balance in an active region where a confined solar eruption occurred. This was done to verify a possible relationship between the filament expansion and the helicity transport at its footpoints. We aimed to verify if this variation in the helicity transport rate could be interpreted as a consequence of the magnetic torque imbalance caused by the tube expansion, as proposed by Chae et al. (2003, J. Kor. Astron. Soc., 36, 33). Methods. We used 171 angstrom TRACE data to measure some geometrical parameters of the new magnetic system produced by a filament eruption that occurred on 2001 November 1 in active region NOAA 9682. We used MDI full disk line-of-sight magnetogram data to measure the accumulation of magnetic helicity in the corona before and after the event. Results. From the measured expansion factor in the magnetic arcade, visible at 171 angstrom during the eruption, we estimated that the resulting torque imbalance at the photosphere ought to lead to the injection of negative helicity following the eruption. We compared this with measurements of the helicity injection using photospheric velocity and magnetogram data. Conclusions. In contradiction to the expectations from the Chae et al. model, the helicity injection after the eruption was positive. We offer the alternative interpretation that the helicity injection resulted from torque of the opposite sign, generated as the filament lost its negative helicity through magnetic reconnection with its surroundings.