Xudong Sun¹, Monica Bobra¹, Todd Hoeksema¹, Yang Liu¹, Yan Li², Chenglong Shen³, Sebastien Couvidat¹, Aimee Norton¹, & George Fisher²

1. W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305
2. Space Science Laboratory, University of California, Berkeley, CA 94720
3. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China

AR 12192 of October 2014 harbors the largest sunspot group in 24 years, and is so far the most flare-productive site of Cycle 24. During its disk passage, it produced six X-class flares, but none was associated with a CME. This behavior clearly deviates from the flare-CME association statistics, and quickly raised interest from the community ^[1].

To explore the cause and consequence of these major confined events, we inspect HMI vector observations around the largest flare from AR 12192 (X3.1; SOL2014-10-24T21:41). We compare them with two other large eruptive flares: one from AR 11429 (X5.4; SOL2012-03-07T00:24), the other from AR 11158 (X2.2; SOL2011-02-15T01:56)^[2]. All three flares showed clear double ribbons. The sunspots in the latter two underwent strong shearing, while in AR 12192 they mainly separated from each other (Figure 1(a)-(c)).

Figure 1. | Comparison of magnetic conditions of three ARs. (a)-(c) Vertical field (Bz) maps of the core region. The yellow shaded regions demarcate the AR core field. (d)-(f) Maps of vertically integrated current density over the lower 11 Mm based on a nonlinear force-free field (NLFFF) extrapolation. (g)-(i) Height profile of horizontal field Bh (black) and decay index n (green) above the core field based on a potential field extrapolation.

We evaluate various indices from the vector magnetograms that have been shown to be useful for flare/CME prediction^[3,4]. AR 12192 exhibits the weakest relative non-potentiality amongst the three. In the core field, all of the following parameters are significantly weaker: current density (Figure 1(d)-(f)), twist parameter, current helicity, and ratio of the modeled free magnetic energy to the potential field energy. AR 12192 also has the strongest overlying field straddling the polarity inversion line (Figure 1(g)-(i)).

It is interesting to note that the absolute amount of electric current and free magnetic energy in AR 12192 are comparable to or greater than the other two ARs, which are enough to power multiple X-class flares. This is likely related to its extraordinary sunspot area (~4300 μHem) and magnetic flux (~1.6×10²³ Mx). These results support the notion that AR eruptiveness is controlled by the relative non-potentiality over the overlying field restriction, rather than global, extensive-type measures that scale with the AR size. That is, flares and CMEs may have controlling factors that are different in nature.

Figure 2. | Magnetic changes over 1 hr in three events. (a)-(c) Differenced horizontal field maps. Contours show the core field region. (d)-(f) Squashing factor Q on a vertical cut in the NLFFF model, before (left) and after (right) each event. The location of the cut is marked by a short double line in (a)-(c). (g)-(i) Selective modeled field lines demonstrating the connectivity change in the AR core.

We also compare the pre- and post-flare AR configurations. There is much less photospheric field change (i.e. increase of horizontal field) in AR 12192, in contrast to the other two eruptive cases (Figure 2(a)-(c)). This is consistent with previous theoretical work, which argues that the photospheric field change correlates with the Lorentz force impulse provided to CME ejecta^[5]. Correspondingly, the modeled coronal magnetic topology changes much less for AR 12192 too (Figure 2(d)-(i)). It remains to be seen whether this is a general feature for confined events.

References

[1] Freeland, S., & Slater, G. 2014, RHESSI Science Nugget #239 (http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/A_Record-Setting_CMEless_Flare)
[2] Sun, X., Bobra, M. G., Hoeksema, J. T., Liu, Y., Li, Y., Shen, C., Couvidat, S., Norton, A. A., & Fisher, G. H. 2015, ApJL, in press (http://arxiv.org/abs/1502.06950)
[3] Bobra, M. G., Couvidat, S. 2014, ApJ, 798, 135
[4] Falconer, D. A., Moore, R. L., & Gary, G. A. 2006, ApJ, 644, 1258
[5] Fisher, G. H., Bercik, D. J., Welsch, B. T., Hudson, H. S. 2012, SoPh, 277, 59