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NLFFF modeling of a circular-ribbon flare

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On October 22, 2011, a circular ribbon, shortly preceded by localized inner and remote compact brightening, was observed in AR11324 using SDO /AIA. The global evolution of such peculiarly shaped flares is believed to be caused by slipping reconnection at a coronal null point, with the circular ribbon being the footprint of the dome-shaped fan, and the inner and remote brightening tracing the two photospheric anchoring of the spine. The high spatial resolution and time cadence of AIA observations give the unique opportunity to explore the details of this flare event, which is globally agreeing with, but not entirely explained by, the null/slipping reconnection model. In particular, pre-flare brightening and a three-phase time evolution of EUV signals do not have straightforward interpretations. Also, the non-eruptive character of the flare makes standard interpretations inadequate.

In this talk I will briefly introduce the nonlinar-force-free field (NLFFF) extrapolaiton technique, and apply it to the interpretatin of that event. In particular, I will show how the combination of the NLFFF extrapolation with a topological analysis based on quasi-separatrix layers (QSL) allows to interpret the AIA observation of the dynamic of the flare. In addition to the expected topological features of a standard circular-ribbon flare model, we find a flux rope and a complex QSL internal to the dome, which are connected with the brightening locations. Guided by the AIA observation, we are able to provide a coherent description of the specific dynamic of this event that, because of its non-eruptive nature, has a peculiar reconnection pattern. The different peaks in the EUV signals are then explained by the cooling of specific sets of reconnected field lines that are matching both the reconnection process evinced by our modeling and the multi-wavelengths observations.

The combination of high-resolution observations with powerful modeling and analysis techniques allow us to interpret very complex observations with realistic models of the coronal field, helping us to deepen the theoretical understanding of flare dynamics.

This talk is part of the DAMTP Astro Mondays series.

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