Decoding the Pre-Eruptive Magnetic Structure of Solar Active Region 12860

A twisted magnetic flux tube, or magnetic flux rope (MFR), is thought to drive most solar eruptions. An ongoing debate is whether MFRs form prior to, or during the eruption. Here we study the magnetic structure in active region 12860 prior to a moderate-sized flare and coronal mass ejection. We use photospheric vector magnetograms and nonlinear force-free field extrapolations to model the coronal magnetic field. We use images of the corona to guide our interpretation. The model shows that the majority of field lines reconnecting at the onset of the event are only weakly twisted, a signature of the sheared magnetic “arcade” configuration. Therefore the MFR, if present, is likely formed during the eruption. We test the onset criteria for two common ideal magnetohydrodynamic instabilities, and discuss their possible role in the event. We estimate the pre-eruption magnetic free energy in the corona to be about 1.76×10³² erg, sufficient to power the eruption.