Sigmoid Formation, Filament Destabilization, and Initiation of Weak Flare by Tether Cutting Reconnection

Abstract We have studied a B-class solar flare and an associated filament eruption through multiwavelength observations. The flare is triggered at 16:24 UT on 2017 June 7 from Active Region(AR) 12661, and it maximizes at 16:54 UT. Magnetic flux cancellation occurs near the polarity inversion line preceding the flare, and ultraviolet (UV) brightenings occur in the preflare phase at the flux cancellation sites, suggesting reconnection occurs in the lower atmosphere, initially. An S-shaped sigmoid forms through successive steps in the corona, i.e., small-scale brightenings, helical/twisted field lines, bright patches, and finally, a developed sigmoid. This observation justifies that runaway reconnection within the coronal arcades forms the sigmoid within the filament. Differential emission measure analysis reveals the existence of plasma at a temperature of more than 10 MK within the sigmoid. The initial magnetic reconnection reorganizes the field overlying the filament as per the tether cutting model. Therefore, it enables the filament to rise slowly, and around 16:41 UT, the eruption phase of the filament begins. The filament eruption removes the overlying coronal field, including the sigmoid. During the eruption phase, we have found the intersecting/crossing of coronal loops and jet-like structures far away from the sigmoid–filament system. In conclusion, all the observational findings (e.g., magnetic flux convergence, cancellation, UV brightenings, and spatial and temporal correlation between formation/evolution of the sigmoid and rise/eruption of the filament) suggest that the formation of a solar flare and the eruption of the filament are consistent with the tether cutting model of solar eruptions.