The Eruption of Outer Spine-like Loops Leading to a Double-stage Circular-ribbon Flare

Circular-ribbon flares occur in a confined magnetic structure, but can also be associated with coronal mass ejections (CMEs) when a filament embedded under the fan erupts. Here we study an M8.7 circular-ribbon flare (SOL2014-12-17T04:51), which is accompanied by a CME yet without a clear indication of filament eruption. Using a nonlinear force-free field model, we find that the outer spine-like loops form a magnetic flux rope (FR1) rooted at the edge of the fan, and that there is another flux rope (FR2) at the main magnetic polarity inversion line (PIL) under a fan-like flux rope FR3. We divide the event evolution into two stages by combining modeling results with EUV observations. The onset stage is featured with bidirectional jets that occurred between a filament and FR1, immediately followed by an upward motion of the latter. During this first stage, the inner/outer spine-related ribbons and the circular ribbon begin to brighten up. After about 10 minutes, another ejection stems from the main PIL region. In this second stage, all ribbons are significantly enhanced, and the twist of FR2 footpoints is decreased. We discuss these results in favor of a scenario where the initial reconnection between the filament and FR1 activates the latter to reconnect with FR3 with opposite twist. This produces larger scale erupting loops and consequently causes a weakening of FR3, which induces another eruption of FR2 from below. This event thus represents a new type of eruptive circular-ribbon flare caused by unstable outer spine-like loops.