Abstract:
This paper studies the magnetic topology of successively erupting active regions (ARs)
11,429 and 12,371. Employing vector magnetic field observations from Helioseismic and
Magnetic Imager, the pre-eruptive magnetic structure is reconstructed by a model of nonlinear force-free field (NLFFF). For all the five CMEs from these ARs, the pre-eruptive
magnetic structure identifies an inverse-S sigmoid consistent with the coronal plasma
tracers in EUV observations. In all the eruption cases, the quasi-separatrix layers (QSLs) of
large Q values are continuously enclosing core field bipolar regions in which inverse-S
shaped flare ribbons are observed. These QSLs essentially represent the large connectivity
gradients between the domains of twisted core flux within the inner bipolar region and the
surrounding potential like arcade. It is consistent with the observed field structure largely
with the sheared arcade. The QSL maps in the chromosphere are compared with the flareribbons observed at the peak time of the flares. The flare ribbons are largely inverse-S
shape morphology with their continuity of visibility is missing in the observations. For the
CMEs in the AR 12371, the QSLs outline the flare ribbons as a combination of two inverse
J-shape sections with their straight parts being separated. These QSLs are typical with the
weakly twisted flux rope. Similarly, for the CMEs in the AR 11429, the QSLs are co-spatial
with the flare ribbons both in the middle of the PIL and in the hook sections. In the frame
work of standard model of eruptions, the observed flare ribbons are the characteristic of
the pre-eruptive magnetic structure being sigmoid which is reproduced by the NLFFF
model with a weakly twisted flux rope at the core
