Current Helicity in Response to Coronal Mass Ejections

Abstract Coronal mass ejections (CMEs), powerful solar eruptions with massive plasma ejected into the interplanetary space, are caused by the release of the magnetic free energy stored in coronal electric currents. Photospheric current helicity, defined as the integral of the product of the vertical electric current density and vertical magnetic field ( H c = ∫ j z B z dS ), serves as a key parameter in understanding the eruptions. Using a 3D magnetohydrodynamic model, we identified a current helicity reversal pattern associated with the eruption: a pre-eruption decrease and a post-eruption increase. This helicity reversal is attributed to the redistribution of electric currents: before the eruption, currents concentrate toward the polarity inversion line (PIL); after the eruption they move away from the PIL, consistent with flare ribbon separation, which is caused by the upward progression of the reconnection site. To validate this pattern, we conducted an observational analysis of 50 ≥M5.0 eruptive flares. The results reveal that 58% of cases exhibit a pre-eruption decrease and 92% show a post-eruption increase in current helicity. Detailed analysis of two cases with this reversal suggests that they share the same current redistribution pattern, consistent with the mechanism identified in the simulations. Moreover, a pre-eruption decrease could be observed clearly even in the long-term evolution of the two cases. Current helicity can serve as an indicator of when electric currents are built up for the subsequent eruption, and it has the potential to predict CMEs to some extent.