Y. Li1,2, J. Qiu2, D. W. Longcope2, M. D. Ding1, K. Yang1
1 School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China
2 Department of Physics, Montana State University, Bozeman, MT 59717, USA
Magnetic reconnection refers to the topological reconfiguration of magnetic field, which is responsible for solar flares. In the 2D standard flare model, magnetic reconnection takes place at a null point between anti-parallel coplanar field lines. In a more realistic 3D picture, magnetic reconnection occurs preferentially at a separator (intersection of two separatrices) or its generalization, a hyperbolic flux tube (intersection of two quasi-separatrix layers). The 3D magnetic reconnection is manifest in many observations of two-ribbon flares and also circular-ribbon flares.
Figure 1 | Evolution of the footpoint brightenings observed in SJI 1330 Å in the full view (left) and zoomed-in view (right; black box in the left). Contours mark the positive (pink) and negative (orange) magnetic field at 500 and −500 G, respectively. The X symbol denotes the X-point.
In this nugget, we report a quite different scenario from those above: an atypical X-shaped ribbon flare with its four chromospheric ribbons (footpoints of reconnection loops) intersecting at the center (an X-point), revealed by the high-resolution UV images from IRIS (Interface region Imaging Spectrograph1). In addition, using HMI magnetograms, we construct the magnetic topology of the active region, which suggests the presence of a separator connected to the X-point. These present the first evidence, as far as we know, for a special scenario of 3D magnetic reconnection at a separator.
Figure 2 | Topological magnetic model superimposed on imaging data. The upper left panel shows a line-of-sight HMI magnetogram with contours representing the positive (pink) and negative (orange) polarities at 500 and −500 G, respectively. The violet and blue lines show traces of the negative separatrix, and the green and red curves are traces of the positive separatrix. The cyan lines show the separators lying within the fan surface of the null point and connecting to null points within the negative ribbon. The upper right panel shows an AIA 211 Å image along with the magnetic skeleton. The lower panels show some of the separatrix traces (red and blue) as well as representative field lines from several connections (cyan, yellow, and magenta), over AIA 171 and 94 Å images.
The X-shaped M2.3 flare on 2014 November 9 was observed by IRIS in slit-jaw 1330 Å images (SJI 1330; formed in the upper chromosphere and lower transition region). It shows an unusual ribbon pattern. The four flare ribbons spread toward each other and cross at an X-point around the flare peak, when the ribbons exhibit an X-shape (Fig. 1). Then, all the ribbons display an outward motion. Furthermore, we see evident post-flare loops in AIA images, which appear to converge toward the X-point (Fig. 2). The inward motion of four flare ribbons as well as converging post-flare loops illustrate sequential 3D reconnection between two sets of non-coplanar loops that approach laterally. A movie is also available here.
Figure 3 | Sketch of the 3D separator reconnection. The main stage is dominated by an inward ribbon motion (blue arrows) to the X-point due to the reconnection site moving downward (red arrow), which is perhaps triggered by a perturbed flux rope. Two sets of non-coplanar inflowing loops approach laterally and reconnect at the current sheet. In the last stage, the outward ribbon motion dominates with the reconnection site moving upward, generating outflowing (post-flare) loops.
The HMI magnetogram (upper left panel of Fig. 2) shows that, near the X-point, the negative and positive regions are separate, distinct sources. Such a magnetic feature tends to form a null point (thus the X-point). We apply the Magnetic Charge Topology (MCT2) method and linear force-free field (LFFF) extrapolation to study the magnetic topology. In the MCT model, the separatrices whose intersection forms the separator cross at a photospheric null point located close to the observed X-point. The LFFF extrapolation also reveals a null point slightly closer to the X-point. In addition, a nonlinear force-free field (NLFFF) extrapolation shows a similar null point, still closer to the actual location (X. Sun 2016, private communication).
In the LFFF model, the separator connecting to the X-point is shown as cyan curve in the upper panels of Fig. 2. In the lower panels, some representative field lines resulting from reconnection around the separator are marked by yellow lines (cyan lines are from the pre-reconnection domains).
The configuration of 3D separator reconnection and evolution in the flare is sketched in Fig. 3. More details of this work can be found in Ref. 3.
 De Pontieu, B., Title, A. M., Lemen, J. R., et al., 2014, SoPh, 289, 2733
 Longcope, D. W., 2005, LRSP, 2, 7
 Li, Y., Qiu, J., Longcope, D. W., Ding, M. D., & Yang, K., 2016, ApJL, 823, L13