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In this paper, we present observations and analysis of an interesting sigmoidformation, eruption and the associated flare that occurred on 2014 April 18using multi-wavelength data sets. We discuss the possible role of the sigmoideruption in triggering the flare, which consists of two different set ofribbons: parallel ribbons as well as a large-scale quasi-circular ribbon.Several observational evidence and nonlinear force-free field extrapolationresults show the existence of a large-scale fan-spine type magneticconfiguration with a sigmoid lying under a section of the fan dome. The eventcan be explained with the following two phases. During the pre-flare phase, weobserved the formation and appearance of sigmoid via tether-cuttingreconnection between the two sets of sheared fields under the fan dome. Thesecond, main flare phase, features the eruption of the sigmoid, the subsequentflare with parallel ribbons, and a quasi-circular ribbon. We propose thefollowing multi-stage successive reconnections scenario for the main flare.First, tether-cutting reconnection is responsible for the formation and theeruption of the sigmoid structure. Second, the reconnection occurred in thewake of the erupting sigmoid produces the parallel flare ribbons on the bothsides of the circular polarity inversion line. Third, the null-typereconnection higher in the corona, possibly triggered by the erupting sigmoid,leads to the formation of a large quasi-circular ribbon. For the first time wesuggest a mechanism for this type of flare consisting of a double set ofribbons triggered by an erupting sigmoid in a large scale fan-spine typemagnetic configuration.

The Role of Erupting Sigmoid in Triggering a Flare with Parallel and Large-Scale Quasi-Circular Ribbons