Observation of a Large-scale Quasi-circular Secondary Ribbon Associated with Successive Flares and a Halo CME

Solar flare ribbons provide an important clue to the magnetic reconnection process and associated magnetic field topology in the solar corona. We detected a large-scale secondary flare ribbon of a circular shape that developed in association with two successive M-class flares and one coronal mass ejection. The ribbon revealed interesting properties such as (1) a quasi-circular shape and enclosing the central active region (AR); (2) the size as large as 500″ by 650″; (3) successive brightenings in the clockwise direction at a speed of 160 km s^-1 starting from the nearest position to the flaring sunspots; (4) radial contraction and expansion in the northern and the southern part, respectively, at speeds of ≤10 km s^-1. Using multi-wavelength data from Solar Dynamics Observatory, RHESSI, XRT, and Nobeyama, along with magnetic field extrapolations, we found that: (1) the secondary ribbon location is consistent with those of the field line footpoints of a fan-shaped magnetic structure that connects the flaring region and the ambient decaying field; (2) the second M2.6 flare occurred when the expanding coronal loops driven by the first M2.0 flare encountered the background decayed field; (3) immediately after the second flare, the secondary ribbon developed along with dimming regions. Based on our findings, we suggest that interaction between the expanding sigmoid field and the overlying fan-shaped field triggered the secondary reconnection that resulted in the field opening and formation of the quasi-circular secondary ribbon. We thus conclude that interaction between the AR and the ambient large-scale fields should be taken into account to fully understand the entire eruption process.