Z. Xu¹, K. Yang², Y. Guo², J. Zhao³, Z. J. Zhao¹, and L. Kashapova⁴

1. Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China
2. School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China
3. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
4. Institute of Solar-Terrestrial-Physics SB RAS, Irkutsk, Russia

A circular ribbon flare usually comprises a quasi-circular ribbon and an elongated inner ribbon enclosed. It is usually related to a fan-spine magnetic field topology (or a 3D null-point topology) in the corona¹. Such a topology could naturally form when a parasitic polarity is encompassed by a parent polarity in a solar active region. When a disturbance is imparted, the magnetic reconnection preferably takes place and develops at the coronal null, or generally speaking, in the quasi-separatrix layers (QSLs, ref.2). Several efforts have been made to understand the role played by different disturbances to trigger the null-point reconnection, such as photospheric shearing motion, continuously twisting motion at the boundary within fan lines, and emergence of twisted flux ropes. In this nugget, we report two homologous circular-ribbon flares associated with pre-brightening features and two filament eruptions on 2014 March 5. They were well observed by the New Vacuum Solar Telescope (NVST) and the Solar Dynamics Observatory (SDO), as shown in Figure 1. We aim to understand the cause-and-consequence in these events.

Figure 1| (a-b) NVST observations in the Hα line center at different evolution times. A total of four flare ribbons are distinguished and are referred as R1, R2, R3, and R4. (c) Magnetogram observed by SDO. Different flare ribbons are denoted by contours in different colors. (d) AIA 1600 Å images displaying flare ribbons before saturations. (e-f) AIA 171 Å images prior to and during the flare stages. RHESSI X-ray contours observed simultaneously are given.

We investigate the magnetic field topology based on a field extrapolation from the observed photospheric field with a nonlinear force-free field assumption³, and calculate the squashing degree factor, Q (ref. 4). Firstly, several QSLs are revealed in the photosphere where log₁₀Q is larger than 5 (Figure 2a). Besides a circular-like QSL, there exist multiple inner QSL structures along the polarity inversion line, which are consistent with the Hα filaments seen in Hα images in locations and morphologies (see Figure 2b and 2c). Meanwhile, a typical fan–spine topology can be seen in the lateral view (e.g., Figure 2d or Figure 2e): a dome-like surface whose apex height reaches up to 7 arc-seconds. A QSL resembling the inner spine is present in the middle of the dome-like structure, while the Q values residing on its two sides show several concentric structures that concentrate on the high Q values found at the photospheric boundary. In addition, for filament F1 (located on the right side), the critical height to trigger the torus instability⁵ is relatively low and under the dome-like surface.

Figure 2| Spatial distribution of the magnetic squashing degree Q. (a) 2D maps of log₁₀Q on the photospheric layer with the NLFFF approximations. (b-c) Hα images before and during the flare superposed with the log₁₀Q contours (with the level of log₁₀Q > 5). (d-e) 2D maps of log₁₀Q on vertical slices, whose intersections with the boundary are denoted by two horizontal lines in panel (a).

In addition, the magnetogram evolution shows that the parasitic polarity has exhibited a continuous rotation since its first appearance (Figure 3a) and the surrounding magnetic field is strongly twisted. Besides, the electric current grows almost simultaneously as the magnetic flux increases with time (Figure 3c and 3d). Consequently, we conjecture that this event involves the emergence of magnetic flux ropes into a pre-existing polarity area, which yields the formation of a 3D null-point topology in the corona. The free energy stored in the twisted flux is continuously transported into the higher atmosphere, which may drive a breakout-type reconnection occurring high in the corona, supported by the pre-flare brightening in Hα. This initiation reconnection could release the constraint on the flux rope and trigger the MHD instability to first make filament F1 lose its equilibrium. The subsequent more violent magnetic reconnection with the overlying flux is driven during the filament rising. In return, the eruption of filament F2 is further facilitated by the reduction of the magnetic tension force above it. These two processes form a positive feedback to each other to cause the energetic mass eruption and flare.

Figure 3| (a) Proper motion on the photosphere, derived from the HMI magnetograms. (b) Vector magnetic fields about 1 hr before the flare. (c) Time evolution of the positive magnetic flux (blue crosses) and vertical currents (red diamonds) integrated over the parasitic polarity area. (d) Scattering plot of total positive magnetic flux and total currents.

References

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