A New Parameter of the Photospheric Magnetic Field to Distinguish Eruptive-flare Producing Solar Active Regions

Solar flares and coronal mass ejections (CMEs) are eruptive phenomena caused by coronal magnetic fields. In particular, large eruptive events originate in active regions (AR) with strong surface magnetic fields. However, it is still unclear what determines the capability of an AR to specifically produce eruptive flares and CMEs, and this hinders our knowledge of the initiation mechanism for the eruptive component of these phenomena. In this study, we propose a new parameter r _m to measure the possibility that a flare that occurs in an AR can be eruptive and produce a CME. The parameter r _m is defined by the ratio of the magnetic flux of twist higher than a threshold T _c to the surrounding-and specifically, the overlying-magnetic flux. The value of r _m for each AR can be estimated using nonlinear force-free field extrapolation models of the coronal magnetic field. Based on the data obtained by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we calculated the values of r _m for 29 ARs at 51 times prior to flares larger than M5.0 class. We find that the footpoints of field lines with twist higher than 0.2 can represent the subsequent flare ribbons well, and field lines that overlie and "fence in" the highly twisted region will work to confine the eruption, generating confined flares. Discriminant function analysis is used to show that r _m is moderately well able to distinguish ARs that have the capability of producing eruptive flares.