Yan, X. L.1,2, Wang, J. C. 1,3, Pan, G. M. 4, Kong, D. F. 1, Xue, Z. K. 1, Yang, L. H. 1, Li, Q. L. 1,3, Feng, X. S. 5
1. Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China
2. Sate Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing 100190, China
3. University of Chinese Academy of Sciences, Yuquan Road, Shijingshan Block, Beijing 100049, China
4. College of Mathematics Physics and Information Engineering, Jiaxing University, Jiaxing 314001, China
5. SIGMA Weather Group, State Key Laboratory for Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
Sunspot rotation was discovered about one century ago. Using high spatial and temporal resolution of recent ground-based and satellite-borne telescopes, more and more sunspots were found to rotate around the center of its umbra or another sunspot [2,3,4].
Recent investigations revealed that active regions hosting rotating sunspots have relatively high solar flare productivity, for example, NOAA 10486, NOAA 10930, and NOAA 11158 produced many flares and coronal mass ejections (CMEs). However, what is the relationship between evolution of rotating sunspots and solar eruptions is still unclear.
Figure 1| Active region NOAA 12673 seen in the continuum intensity image (a), line-of-sight magnetogram (b), and high resolution TiO images (c and d). The blue box in Figures 1(a) and 1(b) outlines the field of view of the Figure 2. The yellow box indicates the field of view of Figure 3.
In order to address this issue, we use HMI data to study the occurrence of two successive X-class flares and two coronal mass ejections (CMEs) triggered by shearing motion and sunspot rotation in active region NOAA 12673 on 2017 September 6. Figure 1 shows the continuum intensity image (a), line-of-sight magnetogram (b), and high resolution TiO images (c and d). The blue box in Figures 1(a) and 1(b) outlines the field of view of Figure 2, which includes the sunspots S1 and S2 with opposite polarities. From the evolution in the continuum intensity images and the line-of-sight magnetograms, it is obvious that there was a shearing motion between the sunspots S1 and S2 starting from September 5 and lasting until the second X-class flare on September 6. Moreover, the sunspot S1 with negative polarity rotated counter-clockwisely around its umbral center, and the sunspot S2 with positive polarity also exhibited a slow counter-clockwise rotation. The structure of sunspot S1 is very complex, as shown in high resolution TiO images in Figures 1(c) and 1(d). Figure 2(a) shows the vector magnetogram observed by HMI. The red and blue arrows indicate the transverse magnetic field in the negative and positive polarities in the photosphere. The transverse field of sunspot S1, as well as the velocity flow (Figure 2(b)) derived from vector magnetograms by using DAVE method, exhibits a swirling shape. Before the occurrence of the first X-class flare, we find that there was a flux rope forming between sunspots S1 and S2 (Figure 2(c)). The selected purple lines in Figure 2(c) show the structure of the flux rope. The S-shaped structure seen from the EUV observation corresponds to this flux rope. The first X-class flare and CME were associated with the eruption of the flux rope.
Figure 2| Vector magnetogram(a), velocity field(b), and selected magnetic field lines of the flux rope(c).
The sunspot with negative polarity at the northwest of active region also began to rotate counter-clockwise before the onset of the first X-class flare, which was related to the formation of the second S-shaped structure in the EUV observation. The continuum intensity image, vector magnetogram, and the velocity field of sunspot S3 were presented in Figure 3. The field of view of Figure 3 is marked by the yellow box in Figure 1(b). The red dotted line along the light bridge in Figure 3(a) was used to trace the rotation of the sunspot S3. The vector magnetogram and velocity field exhibit a swirling shape due to the rotation of sunspot S3. After the occurrence of the first flare, the second S-shaped structure was formed in the EUV observation. The eruption of the second S-shaped structure produced the second X-class flare and CME.
Figure 3| Continuum intensity image(a), vector magnetogram(b), and velocity field(c) of sunspot S3.
The successive formation and eruption of two S-shaped structures were closely related to the counter-clockwise rotation of three sunspots. The existence of a flux rope is found prior to the onset of two flares by using non-linear force free field extrapolation based on the vector magnetograms observed by SDO/HMI. These results suggest that shearing motion and sunspot rotation play an important role in the buildup of the free energy and the formation of flux ropes in the corona that produces solar flares and CMEs.
For more details of this work, please refer to our recent publication:
Yan, X.L., Wang, J.C., Pan, G.M., et al., 2018, ApJ, 856, 79
Evershed, J. 1910, MNRAS, 70, 217
Brown, D. S., Nightingale, R. W., Alexander, D., et al. 2003, Solar Phys., 216, 79
Yan, X. L., Qu, Z. Q., & Xu, C. L. 2008a, ApJL, 682, L65
Zhu, C., Alexander, D., Tian, L., 2012, Solar Phys., 278,112
Yan, X. L., Qu, Z. Q., & Kong, D.F. 2008b, MNRAS, 391, 1887
Does the major flare occur above the structure between two umbra of the sunspot in Figure 1c? The structure also looks like a light bridge.
Yes, you are right. The two X-class flares occurred just above the structure between two umbra of the left sunspot. The structure can be defined as a light bridge, which is formed due to convergence motion of the small sunspots.