Salvo Luigi Guglielmino, Mariachiara Falco, Mariarita Murabito

We describe high-resolution observations of a GOES B-class flare observed at the Dunn Solar Telescope with IBIS and ROSA instruments. The flare was characterized by a circular ribbon at chromospheric level and was interpreted as a consequence of a magnetic reconnection event that occurred at a three-dimensional (3D) coronal null point located above a supergranular cell. We highlight some interesting observational aspects that need to be explained by models. We observe a bundle of loops corresponding to the outer spine that becomes brighter a few minutes before the onset of the flare. The circular ribbon was formed by several adjacent compact kernels with a size of 1″–2″ and brightening sequentially in clockwise direction. We note that the kernels with a stronger intensity emission were located at the outer footpoint of the darker filaments, departing radially from the center of the supergranular cell. The site of the 3D null point and the shape of the outer spine were also detected by RHESSI in the low-energy channel between 6.0 and 12.0 keV. We ascribe the low intensity of the flare to the low amount of the involved magnetic flux and to its symmetric magnetic configuration.