Shin Toriumi1, Carolus J. Schrijver2, Louise K. Harra3, Hugh Hudson4, & Kaori Nagashima5
1 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
2 Lockheed Martin Advanced Technology Center, Palo Alto, CA 94304, USA
3 UCL-Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
4 SUPA School of Physics and Astronomy, University of Glasgow, UK
5 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
We studied the magnetic properties of sunspot regions that produce large flares and eruptions by statistically analyzing the SDO data of all major flare events1. Comparison of the statistics and historical observations of gigantic sunspots suggests that “superflares” could occur on the Sun.
Figure 1 | Sample flare data. GOES X3.1-class event in NOAA 12192. (a) HMI continuum image showing the entire active region. (b) HMI magnetogram showing the magnetic fields (white: positive, black: negative). (c) Flare ribbons observed by AIA’s near UV band image. (d) Temporal evolution of soft X-ray flux (arrow indicates the flare duration).
A subset of active regions with large sunspot groups are the source regions of strong flares and coronal mass ejections (CMEs). With the aim of understanding the magnetic properties of such regions, we surveyed all flare events with GOES levels ≥ M5 within 45 degrees from the center of the solar disk for 6 years from May 2010. For each event, we analyzed SDO/HMI continuum images and magnetograms, SDO/AIA 1600 Å images, and GOES soft X-ray data and referred to the SOHO/LASCO CME catalog. Fig. 1 shows a sample flare event, summarizing the observational data that we analyzed. We measured various parameters such as sunspot area, total unsigned magnetic flux of the whole region, area of the flare ribbons (elongated intensity enhancements observed in the chromosphere), and magnetic flux in the ribbons, and compared them with flare durations obtained from the GOES light curves.
Figure 2 | Two representative statistical results. (Left) Scatter plot of the flare duration (vertical) versus the ribbon distance (horizontal). The duration is linearly proportional to the ribbon distance. Similar results were obtained for the ribbon area and the total magnetic flux inside the ribbon. (Right) Histograms of the ratio of ribbon area normalized by sunspot area for the CME eruptive (black) and non-eruptive (red) events. Vertical lines indicate the means of the log values. The non-eruptive cases show relatively small flare ribbons. Similar results were obtained for the ratio of total magnetic flux.
Two selected results from the statistical analysis are shown in Fig. 2. The first one is that the flare duration is linearly related with the distance between the two flare ribbons associated with the positive and negative polarities. The correlation between the two parameters is quite strong with the correlation coefficient being 0.83. This relation can be explained by assuming (1) that the distance between the ribbons represents the footpoint separation of the post-flare loops (reconnected field lines) and (2) that the flare duration is dictated by the time scale of magnetic reconnection2, which may be a direct function of the loop length.
The second result, which is shown in the right panel of Fig. 2, is that the CME-poor events generally have smaller flare ribbons. What we can see from this figure is that the ribbon area normalized by the spot area for the CME non-eruptive events is on average smaller than that for the eruptive cases. This statistical result may indicate that in the non-eruptive active regions, large over-lying arcade fields may inhibit the CME eruptions, which is well in line with preceding event studies, e.g., on the flare-rich but CME-poor active region NOAA 12192 (ref. 3).
Figure 3 | Great flare event in July 25, 1946, in RGO 14585, the 4th largest sunspot group since the late 19th century. (Left) Sunspots observed in Ca II K1v. (Right) flare ribbons observed in H-alpha. Images courtesy of Paris Observatory.
We also explored the possibilities that gigantic flares (the so-called superflares) could occur on the Sun. Fig. 3 displays the observation of perhaps the largest-ever-imaged sunspot-related flare ribbons. This great sunspot group (RGO 14585) appeared in July 1946 and ranks 4th in size since the late 19th century. The comparison with the SDO statistical results suggests that this active region contains a total magnetic flux of 1.5 × 1023 Mx and that the magnetic energy contributing to the flare eruption (not the released energy) amounts to 8 × 1033 erg. If we suppose that the largest sunspot group in history (RGO 14886, ref. 4), which appeared in April 1947 and was not flare-active, causes a whole-region-scale flare eruption, which may not be unrealistic, the estimated magnetic energy reaches 1034 erg.
 Toriumi, S., Schrijver, C. J., Harra, L. K., Hudson, H., & Nagashima, K., 2017, ApJ, 834, 56 (arXiv:1611.05047)
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