Observation of a 3D magnetic null point

We describe high resolution observations of a GOES B-class flare characterized by a circular ribbon at chromospheric level, corresponding to the network at photospheric level. We interpret the flare as a consequence of a magnetic reconnection event occurred at a three-dimensional (3D) coronal null point located above the supergranular cell. The potential field extrapolation of the photospheric magnetic field indicates that the circular chromospheric ribbon is cospatial with the fan footpoints, while the ribbons of the inner and outer spines look like compact kernels. We found new interesting observational aspects that need to be explained by models: 1) a loop corresponding to the outer spine became brighter a few minutes before the onset of the flare; 2) the circular ribbon was formed by several adjacent compact kernels characterized by a size of 1"-2"; 3) the kernels with stronger intensity emission were located at the outer footpoint of the darker filaments departing radially from the center of the supergranular cell; 4) these kernels start to brighten sequentially in clockwise direction; 5) the site of the 3D null point and the shape of the outer spine were detected by RHESSI in the low energy channel between 6.0 and 12.0 keV. Taking into account all these features and the length scales of the magnetic systems involved by the event we argued that the low intensity of the flare may be ascribed to the low amount of magnetic flux and to its symmetric configuration.