  We report on observations of conversion of bright filament strands into flare
loops during 2012 August 31 filament eruption. Prior to the eruption,
individual bright strands composing one of the legs of the filament were
observed in the 171 A filter channel data of the Atmospheric Imaging Assembly.
After the onset of the eruption, one of the hooked ribbons started to propagate
and contract, sweeping footpoints of the bright filament strands as well as
coronal loops located close by. Later on, hot flare loops appeared in regions
swept by the hook, where the filament strands were rooted. Timing and
localization of these phenomena suggest that they are caused by reconnection of
field lines composing the filament at the hook, which, to our knowledge, has
not been observed before. This process is not included in the standard flare
model (CSHKP), as it does not address footpoints of erupting flux ropes and
ribbon hooks. It has, however, been predicted using the recent
three-dimensional extensions to the standard flare model. There, the erupting
flux rope can reconnect with surrounding coronal arcades as the hooked
extensions of current ribbons sweep its footpoints. This process results in
formation of flare loops rooted in previous footpoints of the flux rope. Our
observations of sweeping of filament footpoints are well described by this
scenario. In all observed cases, all of the footpoints of the erupting filament
became footpoints of flare loops. This process was observed to last for about
150 minutes, throughout the whole eruption.
