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Context. The accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions is considered an important tool for understanding the processes that lead to eruptive phenomena. Aims. We highlight a specific aspect of the magnetic helicity accumulation, providing new observational evidence of the role played by the interaction of magnetic field systems that are characterized by opposite signs of the magnetic helicity flux in triggering solar eruptions. Methods. The amount of magnetic helicity injected into the corona through the photosphere in a sample of active regions (ARs) during their passage across the solar disk was measured by inferring the apparent motion of photospheric footpoints of magnetic field lines from a time series of MDI full-disk line-of-sight magnetograms. The temporal variation of the maps of magnetic helicity flux was analysed by measuring the fragmentation of the patches that are characterized by the flux of magnetic helicity. The temporal correlation between the number of these patches and the flare and coronal mass ejection (CME) occurrence has also been studied. Results. The fragmentation of the patches singled out in the maps of the magnetic helicity flux provides a useful indication of the evolution of the AR complexity. The more fragmented the maps of the magnetic helicity flux are, the higher is the flare and CME frequency. Moreover, most of the events occur for low values (~3 ÷ 17) of the difference of the number of patches with opposite signs of magnetic helicity flux. Conclusions. These results indicate that not only the accumulation of magnetic helicity in the corona, but also its positive and negative fragmentation and distribution should be taken into account to provide a more confident indication of AR complexity and flare/CME productivity. In particular, the interaction of magnetic systems characterized by opposite sign of magnetic helicity flux may be responsible for many observed eruptions.

Flare occurrence and the spatial distribution of the magnetic helicity flux