Chirality and Magnetic Configuration Associated with Two-Ribbon Solar Flares.

The magnetic configuration of flare-bearing active regions (ARs) is one key aspect for understanding the initiation of solar flares. In this paper, we perform a comparative analysis on the chiral characteristics and the magnetic configurations of two X-class two-ribbon flares happening in AR 10930 and AR 11158, whose photospheric magnetic fields were observed by the Hinode and SDO satellites, respectively. The corresponding coronal magnetic fields were calculated based on the nonlinear force-free field model. It is found that both the flares were initiated in local areas with extremely strong electric current density. The chirality of the magnetic field (indicated by the sign of force-free factor $\alpha$) along the main polarity inversion line (PIL) is opposite for the two ARs, that is, left-handed ($\alpha 0$) for AR 11158. The analysis shows that, for both flare events, prominent magnetic connectivity (indicated by both high $\alpha$ and strong current distributions) was formed above the main PIL before the flare and was totally broken after the flare eruption. Yet the two branches of broken magnetic connectivity, combined with the isolated electric current at the magnetic connectivity breaking site, compose the opposite magnetic configurations for the two ARs owing to their opposite chiral characteristics. That is, Z-shaped configuration for AR 10930 with negative $\alpha$ and left-handed chirality, and inverse Z-shaped configuration for AR 11158 with positive $\alpha$ and right-handed chirality. We speculate that two-ribbon flares can be generally classified to these two magnetic configurations by chirality ($\alpha$ sign) of ARs.