[P-EM08] 宇宙天気・宇宙気候

[PEM08-17] Onset Mechanism of the Successive M-class Solar Flares in the Solar Active Region 12673 Based on a Nonlinear Force-Free Modeling

*山崎 大輝¹、井上 諭²、石井 貴子¹、浅井 歩¹、永田 伸一¹、一本 潔¹ (1.京都大学大学院理学研究科附属天文台、2.名古屋大学宇宙地球環境研究所)

キーワード：太陽フレア、太陽磁場、磁気流体力学

Active region (AR) 12673 successively produced M5.5 and M4.2 flares, between 2017 September 4th 20:30UT and September 5th 01:30UT. On one hand, in both flares, the initial brightenings and the flare ribbons were observed in 1600 angstrom at the similar location. On the other hand, coronal mass ejection (CME) was only observed just after the peak time of the M5.5 flare. The purpose of this study is to answer the following questions; ”How the magnetic free energy of the M4.2 flare was build up in just 5 hours?”, ”Whether the onset mechanisms of these two flares were same or not?”, and ”Why only the M5.5 flare was accompanied with CME?”. According to the analyses of the photospheric magnetic field before the flares and the extreme-ultraviolet images during the flares, we found that the small-scale magnetic flux evolved at the polarity inversion line and the flare ribbons were observed outside of it. Thus, we suggest that the evolution of the small-scale magnetic flux played an important role in the rapid formation of the magnetic flux rope. We extrapolated the three-dimensional (3D) coronal magnetic field by using a nonlinear force-free field (NLFFF) extrapolation method before and after the time when the brightening on the small-scale magnetic flux was observed. As a result, we found that strongly twisted lines are formed compared to the NLFFF just after the M5.5 flare. Furthermore, we compared the 3D coronal magnetic field before the two flares. We found that, in both of the flares, the magnetic null point exists in the overlying field lines. We suggest that, in both cases, the null point reconnection would play an important role in the initiation of the flares. Eventually, we will discuss the generation mechanism of the CME in terms of a relationship between the flare-triggering process and global magnetic field covering the twisted magnetic field lines.