During the rising phase of Solar Cycle 24 tremendous activity occurred on the
Sun with fast and compact emergence of magnetic flux leading to bursts of
flares (C to M and even X-class). We investigate the violent events occurring
in the cluster of two active regions (ARs), NOAA numbers 11121 and 11123,
observed in November 2010 with instruments onboard the {\it Solar Dynamics
Observatory} and from Earth. Within one day the total magnetic flux increased
by 70% with the emergence of new groups of bipoles in AR 11123. From all the
events on 11 November, we study, in particular, the ones starting at around
07:16 UT in GOES soft X-ray data and the brightenings preceding them. A
magnetic-field topological analysis indicates the presence of null points,
associated separatrices and quasi-separatrix layers (QSLs) where magnetic
reconnection is prone to occur. The presence of null points is confirmed by a
linear and a non-linear force-free magnetic-field model. Their locations and
general characteristics are similar in both modelling approaches, which
supports their robustness. However, in order to explain the full extension of
the analysed event brightenings, which are not restricted to the photospheric
traces of the null separatrices, we compute the locations of QSLs. Based on
this more complete topological analysis, we propose a scenario to explain the
origin of a low-energy event preceding a filament eruption, which is
accompanied by a two-ribbon flare, and a consecutive confined flare in AR
11123. The results of our topology computation can also explain the locations
of flare ribbons in two other events, one preceding and one following the ones
at 07:16 UT. Finally, this study provides further examples where flare-ribbon
locations can be explained when compared to QSLs and only, partially, when
using separatrices.Comment: 42 pages, 15 figure
