Haimin Wang and Chang Liu,
Space Weather Research Laboratory, New Jersey Institute of Technology, University Heights, Newark, NJ07102-1982, USA
Solar flares are often seen with two chromospheric ribbons, which lie in opposite magnetic polarities and run parallel to the magnetic polarity inversion line (PIL), evidencing the standard flare geometry. With high-resolution observations, flares with a closed circular-like ribbon have also been revealed (Ref. 1 and references therein). The associated surface magnetic field configuration usually consists of a parasitic region encompassed by the opposite-polarity field, forming a circular PIL. Besides the circular ribbon, an inner and an outer (remote) ribbons are usually found. Some flares also exhibit surge-like eruptions from the area of the circular ribbon. In interpreting these events, the fan-spine magnetic topology is involved, where the dome-shaped fan portrays the closed separatrix surface, and the inner and outer spine field lines in different connectivity domains pass through a coronal null point.
This nugget reports on rare observations of two closely spaced three-ribbon flares with GOES-class M1.3 and C9.2 in NOAA AR 11515 on 2012 July 6. The flaring region is characterized with two Delta spots p1-n and p2-n sharing the elongated strip n of negative polarity (Figures 1a and 1b). Three ribbons (labeled R1-R3) appear in both the M1.3 and C9.2 flares that occurred in succession in 0.5 hr, and they are located at either end of three bright “channels” seen in Ca II H (Figures 1d and 1e). The central ribbon R2 and the outer ribbons R1/R3 are in the negative and positive polarity regions, respectively. All ribbons run parallel to the PIL, which makes a closed turn in the southwest end. During the earlier M1.3 flare (Figure 1d), R1 and R3 undergo a seemingly sequential brightening in the counterclockwise direction, and H-alpha surges are seen to envelope the ribbons and converge toward a remote region in the northeast. The flaring magnetic structure is more clearly illuminated in the later C9.2 flare (Figures 1e, 1c, and 1f), in which two rows of semi-parallel arcade loops connect R1/R2 and R2/R3. Interestingly, the 12–25 keV hard X-ray sources first line up with the central R2 then shift to concentrate on the top of the higher (see below) branch of loops R2-R1. A chain of surges emerge from multiple cusp-shaped locations above R2, and a fountain-like spray also envelopes the loops R2–R1. Notably, there is a relatively thin thread of surge (jet 3) that shoots northwestward, which is in a nearly perfectly opposite direction of the main surge (jet 2).
Using a nonlinear force-free field (NLFFF) extrapolation model based on a preflare HMI vector magnetogram, we delineate in Figures 2a and 2b, the characteristic structure of the flare volume by tracing magnetic fields from the central negative polarity region n. Field lines landing at p1 and p2 are separated using different colors. The result evidently shows that the sheared fields stemming from n consist of closed field lines bifurcate cleanly into two semi-parallel rows of loops, forming a “fish-bone”-like structure consistent with observations (e.g., Figures 1a and 1f). The northern (red) and southern (blue) branches of loops show a distinct asymmetry in height, with the mean value of the former about double that of the latter. Intriguingly, the northern and southern branches of loops possess opposite magnetic twists (Figure 2c). As to connectivity in a large scale, the fish-bone structure is completely embedded under the overlying loops connecting from p1/p2 to the negative field region S in the northeast (Figure 2d). Open field lines (cyan in Figure 2a) are also found to originate from the western portion of the positive flux.
Based on high-resolution images and our NLFFF model, we speculate that this flaring region, in general, consists of the same cross section as that of the fan-spine magnetic topology, which repeats in the third dimension to produce a fish-bone-like structure. Accordingly, the single isolated coronal null-point in the 2D fan-spine fields would be prolonged to form a null line. This conjecture is schematically illustrated in Figure 3, where a semi-circular fan-spine element is also incorporated. Such fan-spine field lines in a translational symmetry was modeled before using MHD simulations, where magnetic reconnection can be induced along the null-line when the bottom photosphere is subject to perturbations from horizontal flows.
 Wang, H. & Liu, C. 2012, ApJ, 760, 101
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 Wang, H., Liu, C., Deng, N., Zeng, Z., Xu, Y., Jing, J. & Cao, W. 2014, ApJ Lett, 781, L23