Observations of an X-Shaped Ribbon Flare in the Sun and Its Three-Dimensional Magnetic Reconnection

Abstract

We report evolution of an atypical X-shaped flare ribbon which provides novel observational evidence of three-dimensional (3D) magnetic reconnection at a separator. The flare occurred on 2014 November 9. High-resolution slit-jaw 1330 A images from the Interface Region Imaging Spectrograph reveal four chromospheric flare ribbons that converge and form an X-shape. Flare brightening in the upper chromosphere spreads along the ribbons toward the center of the "X" (the X-point), and then spreads outward in a direction more perpendicular to the ribbons. These four ribbons are located in a quadrupolar magnetic field. Reconstruction of magnetic topology in the active region suggests the presence of a separator connecting to the X-point outlined by the ribbons. The inward motion of flare ribbons in the early stage therefore indicates 3D magnetic reconnection between two sets of non-coplanar loops that approach laterally, and reconnection proceeds downward along a section of vertical current sheet. Coronal loops are also observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory confirming the reconnection morphology illustrated by ribbon evolution.

Spectroscopic Observations of Magnetic Reconnection and Chromospheric Evaporation in an X-shaped Solar Flare

“…In short, the four chromospheric ribbons approach each other and converge at the X-point around the flare peak time; then they move outward, with the two ribbons on the right separating away from the polarity inversion line (the ribbon motion pattern 7 is also shown in Figure 3(a)). The observed ribbon motions as well as the reconstructed magnetic topology 8 (see Figure 4 and more details in Li et al 2016) suggest that magnetic reconnection takes place at a separator connecting to the X-point. More specifically, the inward and outward motions of ribbon brightenings illustrate that the reconnection occurs along a curved separator (or current sheet), which consists of a vertical part above the X-point and a horizontal part extending to the right (see the sketched Figure4 in Li et al 2016).…”

“…The observed ribbon motions as well as the reconstructed magnetic topology 8 (see Figure 4 and more details in Li et al 2016) suggest that magnetic reconnection takes place at a separator connecting to the X-point. More specifically, the inward and outward motions of ribbon brightenings illustrate that the reconnection occurs along a curved separator (or current sheet), which consists of a vertical part above the X-point and a horizontal part extending to the right (see the sketched Figure4 in Li et al 2016). The IRISslit cut across parts of the flare ribbons near the X-point (see Figure 3 and the online animated Figure 1).…”

“…The morphological evolution of this X-shaped flare is described in Li et al (2016). In short, the four chromospheric ribbons approach each other and converge at the X-point around the flare peak time; then they move outward, with the two ribbons on the right separating away from the polarity inversion line (the ribbon motion pattern 7 is also shown in Figure 3(a)).…”

“…One can see that the north ribbon (where R1 is located) spreads up toward the north as time evolves (Figure 5(c)). This can also 7 For the ribbon brightenings in SJI 1330 Å, we select the flaring pixels whose intensity is enhanced to be more than 25 times the intensity of the quiescent Sun for more than 2 minutes (see Li et al 2016 for more details). 8 The method of Magnetic Charge Topology (Longcope 2005) is used to produce the topological model, which has an uncertainty of ∼15% in the connectivity of extrapolated field lines.…”

Disintegration of an eruptive filament via interactions with quasi-separatrix layers

“…This is expected to result in much more intense heating than kinetic impacting, the latter of which carries an energy of ∼ 10 27 erg [15] as small as a micro-flare. Magnetic reconnection has been reported to occur between an eruptive filament and its ambient field taking the forms of various structures, e.g., a coronal hole [21], coronal loops [22], chromospheric fibrils [23], or another filament [24]. The reconnection between an eruptive filament and its overlying (quasi-)separatrix surfaces is well expected but has not been studied in the literature.…”