Title: Observation of a 3D magnetic null point,
Abstract: 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.