Abstract:
Magnetic helicity (MH) is a measure of twist and shear of magnetic field. MH is injected in the active region (AR) corona through photospheric footpoint motions causing twisted and sheared magnetic fields. From the conservation property of the helicity, it was conjectured that an already twisted flux rope (FR) with continuous injection of MH inevitably erupts to remove the excess accumulated coronal helicity. Therefore, understanding the nature and evolution of the photospheric helicity flux transfer is crucial to reveal the intensity of the flare/CME activity. Using the time-sequence vector-magnetograms of Helioseismic Magnetic Imager, we study the evolution of MH injection in emerging AR 12257. The photospheric flux motions in this AR inject positive helicity in the first 2.5 days followed by negative helicity later. This successive injection of opposite helicity is consistent with the sign of mean force-free twist parameter (αav), orientation of magnetic-tongues. Also, the extrapolated AR magnetic structure exhibits transformation of global-shear without a twisted FR in the core of the AR. No CMEs are launched from this AR but C-class flaring activity is observed predominantly in the second half of the evolution period. The ARs with sign reversal of the MH injection are not favorable to twisted FR formation with excess coronal helicity and therefore are important to identify CME-less ARs readily. A possible scenario in these ARs is that when one sign of helicity flux is replaced by opposite sign, the magnetic field of different connectivity with opposite shear undergoes reconnection at different scales giving rise to both intermittent flares and enhanced coronal heating.
