Why could a new-born active region produce coronal mass ejections?

Abstract

Solar active regions (ARs) are the main sources of flares and coronal mass ejections (CMEs). NOAA AR 12089, which emerged on 2014 June 10, produced two C-class flares accompanied by CMEs within five hours after its emergence. When producing the two eruptive flares, the total unsigned magnetic flux (${\Phi }_{\text{AR}}$) and magnetic free energy ($E_f$) of the AR are much smaller than the common CME-producing ARs. Why can this extremely small AR produce eruptive flares so early? We compare the AR magnetic environment for the eruptive flares to that for the largest confined flare from the AR. Besides the ${\Phi }_{\text{AR}}$ and $E_f$, we calculate the ratio between the mean characteristic twist parameter (${\alpha }_{\text{FPIL}}$) within the flaring polarity inversion line (FPIL) region and ${\Phi }_{\text{AR}}$, a parameter considering both background magnetic field constraint and non-potentiality of the core region, for the three flares. We find higher ${\alpha }_{\text{FPIL}}/{\Phi }_{\text{AR}}$ values during the eruptive flares than during the confined flare. Furthermore, we compute the decay index along the polarity inversion line, revealing values of 1.69, 3.45, and 0.98 before the two eruptive and the confined flares, respectively. Finally, nonlinear force-free field extrapolation indicates that a flux rope was repeatedly formed along the FPIL before eruptive flares, which ejected out and produced CMEs. No flux rope was found before the confined flare. Our research suggests that even a newly emerged, extremely small AR can produce eruptive flares if it has sufficiently weak background field constraint and strong non-potentiality in the core region.

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