The Origin of a Preeruptive Magnetic Structure with Significant Nonneutralized Electric Current

Abstract The buildup of the preeruptive magnetic field configuration and the eruption onset mechanism are critical yet poorly understood in solar eruptions. Coronal structures like sigmoids and filaments have been identified as preeruptive magnetic structures; their associated preflare motions as well as preflare brightenings have been identified as precursor signatures, yet none of these definitively lead to eruptions, and the cause and effect is contentious. Most importantly, how the preeruptive coronal structure evolves with the photospheric magnetic field is elusive. Here we report the development of the photospheric nonneutralized electric current associated with the buildup of a preeruptive coronal structure observed as a bundle of hot low-lying coronal loops collectively taking a sigmoidal shape. The significant nonneutralized electric current appeared several hours ahead of the formation of the preeruptive structure. The buildup of the preeruptive structure in the corona was simultaneous with the gradual extension of the nonneutralized electric current in the photosphere. The synchronous evolution seemed to stop when intermittent brightening occurred along the preeruptive structure in the corona. The preflare brightening lasted for about 4 hr, with two ribbon-like structures matching the nonneutralized electric current. Eventually, the preeruptive structure evolved into a magnetic flux rope (MFR) and erupted. Quantitative measurements indicate that the significant nonneutralized electric current also flows through the footpoints of the erupting MFR, which are well identified by a pair of conjugate dimmings. The evolution of the photospheric nonneutralized electric current is demonstrated to signal the buildup of the preeruptive structure and the imminent eruption.