SOL2013-11-10 Eruptive Circular-ribbon Flare with Extended Remote Brightenings
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|1st Author:||Chang LIU|
|2nd Author:||et al.|
|Published:||31 July 2020|
|Next Nugget:||Flare energy partitioning|
|Previous Nugget:||Extreme-Ultraviolet Late Phase of Solar Flares|
H-α observations have long shown that some solar flares have circular-shaped ribbons, rather than the usual (ideally) straight and parallel ones. This geometry does not match the standard CSHKP picture, either in 2D or 3D representations. After these circular-ribbon events were found in TRACE EUV images and later in Big Bear (BBSO) digitized solar film images, they have been extensively studied in the SDO era (e.g., in the previous Nugget!. This kind of flare suggests magnetic reconnection at a coronal null point (NP) and in the quasi-separatrix layers that embed the fan and spine field lines. With this fan-spine-NP magnetic structure, the resulting events must become confined flares or jets, depending on whether the outer-spine fields return to the surface or open upward. Intriguingly, CMEs can also be produced if there is a filament lying under the fan dome and it becomes unstable. How the eruptive filament interacts with fan and spine fields is a geometrically and physically interesting subject. Due to their simplicity and well-constrained topology, circular-ribbon flares can challenge our understanding of flare initiation and the occurrence of successful or failed eruptions (Ref. ). Here we presented analyses of the SOL2013-11-10T05:14 X1.1 circular-ribbon flare/CME event spawned by NOAA AR 11890 (Ref. ).
Non-force-free Field Model
Figure 1 shows the preflare coronal field structure of the AR reconstructed using a non-force-free field (Ref. ). The NFFF extrapolation method used here is a novel technique based on the principle of minimum (energy) dissipation rate where the coronal magnetic field solution is expressed as the superposition of one potential field and two distinct linear force-free fields (Ref. ). We see that under the magnetic dome, two mildly twisted sheared arcades (labeled SA1 and SA2) lie close to each other. Around their inner two footpoints, pronounced shearing and converging flows are found from flow-tracking the photospheric motions. Also, the arcade SA2 approaching the null point (NP) could be in the state of a torus-unstable regime. In large scale, the outer spine-like loops that envelop the dome constitute a magnetic curtain, hanging above and rooted in the extended arc-like remote brightening regions.
Asymmetric Eruption and Remote Brightenings
We divide the whole event into two stages. In stage I, SA1 and SA2 undergo a tether-cutting reconnection to trigger the event. The southern portion of the newly-formed flux rope (FR) close to the null point NP subsequently rises upward due to the torus instability. When the FR reaches the fan surface, null-point reconnection occurs to brighten up the circular ribbon. Energy also flows outward primarily along the high-Q surfaces to ignite the initial remote brightening in the west (Figure 2). In stage II, the southern end of the FR detaches from the surface and rapidly erupts in a whipping fashion to become the CME. This opens the dome and the outer spine-like loops sequentially, which results in fast-moving remote brightenings spanning a large range at the footpoints of the magnetic curtain and the formation of the cospatial coronal dimmings.
This event represents a good example of an eruptive circular-ribbon flare, in which the full evolution of a filament (from its formation to become a successful asymmetric eruption breaking through the fan dome) has all been well observed. The outer curtain-shaped loops are opened sequentially, as evidenced by the fast-moving extended remote brightenings and the NFFF model.
Avijeet Prasad, Jeongwoo Lee, and Haimin Wang contributed to the paper (Ref. ) and to this nugget.