Block-induced Complex Structures Building the Flare-productive Solar Active Region 12673

Abstract

Solar active region (AR) 12673 produced 4 X-class, 27 M-class, and numerous lower class flares during its passage across the visible solar disk in September 2017. Our study is to answer the questions why this AR was so flare-productive and how the X9.3 flare, the largest one of the last decade, took place. We find that there was a sunspot in the initial several days, and then two bipolar regions emerged nearby it successively. Due to the standing of the pre-existing sunspot, the movement of the bipoles was blocked, while the pre-existing sunspot maintained its quasi-circular shaped umbra only with the disappearance of a part of penumbra. Thus, the bipolar patches were significantly distorted, and the opposite polarities formed two semi-circular shaped structures. After that, two sequences of new bipolar regions emerged within the narrow semi-circular zone, and the bipolar patches separated along the curved channel. The new bipoles sheared and interacted with the previous ones, forming a complex topological system, during which numerous flares occurred. At the highly sheared region, a great deal of free energy was accumulated. On September 6, one negative patch near the polarity inversion line began to rapidly rotate and shear with the surrounding positive fields, and consequently the X9.3 flare erupted. Our results reveal that the block-induced complex structures built the flareproductive AR and the X9.3 flare was triggered by an erupting filament due to the kink instability. To better illustrate this process, a block-induced eruption model is proposed for the first time.

Very Fast Helicity Injection Leading to Critically Stable State and Large Eruptive Activity in Solar Active Region NOAA 12673

“…The unique non-potential characteristics found in this study are in agreement with previous studies and interpretations. Studies of this AR 12673 by Yang et al (2017) showed that the flux rope eruption is triggered by kinkinstability because of the exceeding critical twist. They had proposed a block-induced complex structure for a flare-productive AR.…”

“…AR 12673 starts emerging from September 2 at disk position E11S08 into pre-existing positive sunspot polarity (SP). As described by Yang et al (2017, Figure 1), the emergence on September 3 is through two bipolar regions (Hou et al 2018). The proper and shear motion of these emerging patches was interacted by the pre-existing spot and formed a large sheared main PIL.…”

Predictability of the Solar Cycle Over One Cycle

“…Hence it keeps almost stable. The two notable plumes mainly result from two big ARs, i.e., AR12674 and AR12673 (Yang et al 2017;Yan et al 2018). They occurred on the solar disc with large tilts around 2017 September 5th located at northern and southern hemispheres, respectively.…”

Peculiarities of the Dynamics of Solar NOAA Active Region 12673

“…Solar NOAA active region (AR) 12673, which was observed in September 2017, was remarkable for its complex structure and extraordinary flare productivity-the highest in Solar Cycle 24 (Yang et al 2017;Attie et al 2018;Hou et al 2018). It produced 4 flares above X1 class and 8 flares above M3 class; moreover, its X-9.3 flare (on 2017 September 6, 11:53 UT) was the most intense one since 2005 (see, e.g., Sun & Norton 2017).…”