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Solar flare ribbons provide an important clue to the magnetic reconnectionprocess and associated magnetic field topology in the solar corona. We detecteda large-scale secondary flare ribbon of a circular shape that developed inassociation with two successive M-class flares and one CME. The ribbon revealedinteresting properties such as 1) a quasi-circular shape and enclosing thecentral active region; 2) the size as large as 500\arcsec\, by 650\arcsec\,, 3)successive brightenings in the clockwise direction at a speed of \kms{160}starting from the nearest position to the flaring sunspots, 4) radialcontraction and expansion in the northern and the southern part, respectivelyat speeds of $\leq$ \kms{10}. Using multi-wavelength data from \textit{SDO},\textit{RHESSI}, XRT, and Nobeyama, along with magnetic field extrapolations,we found that: 1) the secondary ribbon location is consistent with the fieldline footpoints of a fan-shaped magnetic structure that connects the flaringregion and the ambient decaying field; 2) the second M2.6 flare occurred whenthe expanding coronal loops driven by the first M2.0 flare encountered thebackground decayed field. 3) Immediately after the second flare, the secondaryribbon developed along with dimming regions. Based on our findings, we suggestthat interaction between the expanding sigmoid field and the overlyingfan-shaped field triggered the secondary reconnection that resulted in thefield opening and formation of the quasi-circular secondary ribbon. We thusconclude that interaction between the active region and the ambient large-scalefields should be taken into account to fully understand the entire eruptionprocess.

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