Onset of a Large Ejective Solar Eruption from a Typical Coronal-jet-base Field Configuration

Utilizing multiwavelength observations and magnetic field data from the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), SDO/Helioseismic and Magnetic Imager (HMI), the Geostationary Operational Environmental Satellite (GOES), and RHESSI, we investigate a large-scale ejective solar eruption of 2014 December 18 from active region NOAA 12241. This event produced a distinctive "three-ribbon" flare, having two parallel ribbons corresponding to the ribbons of a standard two-ribbon flare, and a larger-scale third quasi-circular ribbon offset from the other two. There are two components to this eruptive event. First, a flux rope forms above a strong-field polarity inversion line and erupts and grows as the parallel ribbons turn on, grow, and spread apart from that polarity inversion line; this evolution is consistent with the mechanism of tether-cutting reconnection for eruptions. Second, the eruption of the arcade that has the erupting flux rope in its core undergoes magnetic reconnection at the null point of a fan dome that envelops the erupting arcade, resulting in formation of the quasi-circular ribbon; this is consistent with the breakout reconnection mechanism for eruptions. We find that the parallel ribbons begin well before (similar to 12 minutes) the onset of the circular ribbon, indicating that tether-cutting reconnection (or a non-ideal MHD instability) initiated this event, rather than breakout reconnection. The overall setup for this large-scale eruption (diameter of the circular ribbon similar to 10(5) km) is analogous to that of coronal jets (base size similar to 10(4) km), many of which, according to recent findings, result from eruptions of small-scale "minifilaments." Thus these findings confirm that eruptions of sheared-core magnetic arcades seated in fan-spine null-point magnetic topology happen on a wide range of size scales on the Sun.