Observational study of the formation of homologous confined circular ribbon flares

When several solar flares with comparable classes occur successively at the same location and exhibit similar morphological features, they are called homologous flares. From May 8 to May 10, 2012, five M-class homologous circular ribbon flares associated with no coronal mass ejection occurred in the active region (AR) 11476. The formation process of these homologous confined flares, particularly the homologous aspect, is unclear and inconclusive. This paper is dedicated to studying how the energy for this series of flares was accumulated and whether there existed null points responsible for the flare energy release. With the multiwavelength images and vector magnetograms from the Solar Dynamics Observatory, we study the formation process of these homologous confined circular ribbon flares. Using nonlinear force-free field modeling, the three-dimensional coronal magnetic structures are reconstructed. Before and during the five flares, the sunspots with opposite polarities sheared against each other and also rotated individually. Before each flare, the magnetic fields at the polarity inversion line were highly sheared and there existed a magnetic flux rope overlain by arch-shaped loops. For the first four flares, we find magnetic null points in the fan-spine topology, situated at about 3.8 Mm, 5.7 Mm, 3.4 Mm, and 2.6 Mm above the photosphere, respectively. For the fifth flare, no null point is detected. However, in the (extreme-) ultraviolet images, the evolution behaviors of all the flares were almost identical. Therefore, we speculate that a null point responsible for the occurrence of the fifth flare may have existed. These results reveal that, for these homologous flares in AR 11476, the sunspot rotation and shearing motion play important roles in energy accumulation, the null point of the fan-spine topology is crucial for energy release through magnetic reconnection therein, and large-scale magnetic loops prevent the erupting material from escaping the Sun, and thus form the observed homologous confined major circular ribbon flares. This study provides clear evidence of the drivers of successive, homologous flares as well as the nature of confined events.