  How filaments form and erupt are topics about which solar researchers have
wondered since more than a century and that are still open to debate. We
present observations of a filament formation, its failed eruption, and the
associated flare (SOL2019-05-09T05:51) that occurred in active region (AR)
12740 using data from SDO, STEREO-A, IRIS, and NSO/GONG. AR 12740 was a
decaying region formed by a very disperse following polarity and a strong
leading spot, surrounded by a highly dynamic zone where moving magnetic
features (MMFs) were seen constantly diverging from the spot. Our analysis
indicates that the filament was formed by the convergence of fibrils at a
location where magnetic flux cancellation was observed. Furthermore, we
conclude that its destabilization was also related to flux cancellation
associated to the constant shuffling of the MMFs. A two-ribbon flare occurred
associated to the filament eruption; however, because the large-scale magnetic
configuration of the AR was quadrupolar, two additional flare ribbons developed
far from the two main ones. We model the magnetic configuration of the AR using
a force-free field approach at the AR scale size. This local model is
complemented by a global potential-field source-surface one. Based on the local
model, we propose a scenario in which the filament failed eruption and flare
are due to two reconnection processes, one occurring below the erupting
filament, leading to the two-ribbon flare, and another one above it between the
filament flux-rope configuration and the large-scale closed loops. Our
computation of the reconnected magnetic flux added to the erupting flux rope,
compared to that of the large-scale field overlying it, lets us conclude that
the latter was large enough to prevent the filament eruption. A similar
conjecture can be drawn from the computation of the magnetic tension derived
from the global field model.
