Magnus M. Woods1,2,3,4, Satoshi Inoue2, Louise K. Harra1, Sarah A. Matthews1, and Kanya Kusano2
1. Mullard Space Science Laboratory, University College London, Dorking, Surrey, RH5 6NT, UK
2. Institute for Space-Earth Environmental Research (ISEE)/Nagoya University Furo-cho, Nagoya, Aichi 464-8601, Japan
3. Bay Area Environmental Research Institute (BAERI), P.O. Box 25 Moffett Field, CA 94035, USA
4. Lockheed Martin Solar and Astrophysics Lab, Palo Alto, CA 94304, USA
Understanding the mechanisms that lead to the triggering of solar flares is of great importance to better understand the physics of flaring. Studying serial flaring within one active region can provide an excellent way to determine whether these flares are triggered by the same mechanism, and if not, why?
We present an analysis of three sequential flares of GOES class M1, C and C8, respectively, in AR 12087 using IRIS spectroscopic data and non-linear force free field (NLFFF) extrapolations, and seek to determine why only the final flare in this sequence was eruptive.
Figure 1| (a) AIA 193Å context image with IRIS FOV overlayed. (b) IRIS Si Ⅳ 1403Å intensity raster. (c) Intensity light curve in the marked region in panel (b). (d) Averaged spectra within overlayed region for 2 hours around each flare time.
The three flares in this study occurred within a 4-hour window on 2014 June 12-13, and exhibited homologous ribbon structures (shown in Figure 1a). We analyzed IRIS Si Ⅳ (1403 Å) observations of a recurring brightening region (black box in Figure 1b to investigate whether there were any clear spectral differences between the events. We observe intensity enhancements between 10-20 mins prior to flaring in all three flares. The average spectral profiles within this region for each flare are shown in Figure 1d. Comparing the over-plotted representative spectra, all three events show broad profiles, where the profiles prior to flares 1 and 2 are largely redshifted, and the profile prior to the eruptive flare 3 exhibits a more Lorentzian-like profile.
Figure 2| (a) The central portion of AR 12087, with the overlayed box marking the region in which the evolution of magnetic flux was calculated. (b) Evolution of positive and negative magnetic flux between 19:00UT of June 12 and 01:30UT of June 13.
The evolution of the observed magnetic field during the time of these flares is also investigated. Figure 2a shows the HMI LOS magnetic field in the central portion of the AR in the region of the recurring bright point and flare ribbons. Figure 2b shows the evolution of positive and negative magnetic flux within the over-plotted box in Figure 2a. There are sharp drops in both positive and negative flux prior to the non-eruptive flares. This is highly indicative of flux cancellation, whilst during the eruptive third flare evidence of flux cancellation is not so apparent.
To investigate the three-dimensional structure and evolution of the magnetic field within this active region, we produce a series of NLFFF extrapolations (Figure 3). These extrapolations take observed HMI SHARPS vector magnetograms as their lower boundary condition following the method of Ref.  and clearly reproduce the structure of the active region (Figure 3e). The timings of the boundary magnetograms are chosen so that we have an extrapolation for the pre- and post-flare magnetic configuration, allowing us to investigate changes caused by each successive flare. A comparison of Figures 3a-d show that while there are some small variations, the magnetic structure does not exhibit strong morphological changes even after the eruptions. These extrapolations also reveal that the overlying magnetic field exhibit a fan-spine topology. Additionally, we chart the evolution of various quantities derived from the extrapolations, such as twist and decay index. We find that neither twist nor decay index change greatly between the flares. The twist values are found to be below the threshold for kink instability, and the twisted magnetic structure (flux rope) lies within a region where it is stable to torus instability.
In examining the results of our observations in the context of existing studies of pre-flare activity, we note that while we do see pre-flare intensity enhancements and evidence of flux cancellation within AR 12087, it seems unlikely that this is the result of the scenario of flare and eruption triggering presented in Ref.  and . However, our NLFFF extrapolations have confirmed the conclusion of Ref.  of the presence of a fan-spine topology in the overlying magnetic field. The fan-spine topology raises the possibility that the overlying magnetic field was weakened by breakout reconnection during flares 1 and 2, and this weakening in the overlying magnetic field could then have led to the eruption during flare 3.
The sequence of flares described in this work demonstrates the complexity of flare and eruption triggering. Superficially, the three flares are morphologically similar but show different eruptive outcomes. Also, when compared to other events such as in Ref. , we again find similar but very different results. This makes it clear that while there have been many advances in the studies of flare and eruption triggering in recent years, each event is in many ways unique and highly complex.
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