Talk abstract

Coronal Mass Ejections (CMEs) are very energetic events (10^32 erg) initiated in the solar atmosphere, resulting in the expulsion of magnetized plasma clouds that propagate into interplanetary space. It has been proposed that CMEs can play an important role in shedding magnetic helicity, avoiding its endless accumulation in the corona. We therefore investigated the behavior of magnetic helicity accumulation in sites where the initiation of CMEs occurred, in order to determine whether and how changes in magnetic helicity accumulation are temporally correlated with CME occurrence. We used MDI magnetograms to calculate magnetic flux evolution and magnetic helicity injection in 10 active regions that gave rise to halo CMEs observed during the period February 2000 - June 2003. The results indicate that magnetic helicity injection does not have a unique trend in the events analyzed: in 40 % of the cases it shows a large sudden and abrupt change that is temporally correlated with CME occurrence, while in the other cases it shows a steady monotonic trend, with a slight change in magnetic helicity at CME occurrence. The results obtained indicate that major changes in magnetic helicity are observed in active regions generating halo CMEs associated with X-class flares or filament eruptions. Finally, we present the results obtained by some simulations carried out to determine how magnetic helicity can change in response to changes in the magnetic field. We considered different types of evolution for two polarities: flyby, cancellation, emergence, coalescence, and fragmentation. The accumulation of magnetic helicity seems to be largest in the case of flyby.