Large-scale Coronal Dimming Foreshadowing a Solar Eruption on 2011 October 1

Understanding the initiation of a large-scale solar eruption requires a detailed investigation of the evolution of the entire system. This includes both the magnetic environment surrounding and plausibly connected to the source region and a careful search for possible precursor activity during the approach to eruption. We combine observations from the two viewpoints of the SDO and STEREO-B spacecraft for several hours prior to a filament ejection, M1.2-class eruptive flare, and coronal mass ejection that originated in Active Region NOAA 11305 on 2011 October 1. Two different episodes of significant coronal dimming are identified during almost 10 hours before the eruption, and well to the southeast of the source region hosting the filament. The CME subsequently takes off with a substantial non-radial component of velocity toward the southeast, accompanied by very pronounced dimmings observed by SDO/AIA and STEREO-B/EUVI. A potential-field reconstruction of the magnetic environment of the eruption derived from SDO/HMI data reveals a null point near the nexus of the dimming regions. The magnetic flux system is quite complex, with AR 11305 connecting eastward to AR 11306, westward to AR 11302, and both northward and southward into plage regions of older active-region flux. We find that the increase in the depth and spatial extent of the precursor dimming is associated with clear southeastward rising motions of loops toward the inferred null point, and with more subtle northeastward and southwestward motions of loops in the transverse directions away from the null point. These loop motions and the co-spatial dimming are consistent with gradual magnetic reconnection occurring in the null-point region as it expands upward and to the southeast. The resultant weakening and removal of flux from the strapping field high above AR 11305 is a plausible cause of the subsequent explosive ejection of the filament toward the null point. We conclude that this spectacular event is fully consistent with the classic magnetic-breakout model for CMEs occurring in highly complex, multipolar topologies.