  We present observations of the eruption and then reformation of a filament
caused by its nearby emerging magnetic flux. Driven by the emerging magnetic
flux, the emerged positive fluxes moved toward and cancelled with its nearby
negative fluxes, where the negative ends of a filament channel beneath the
filament and a bundle of left-skewed coronal loops overlying the filament were
anchored. Complemented by the nonlinear force-free field extrapolation, we find
that the coronal magnetic field lines associated with the filament channel and
the emerging magnetic fields consist of sheared field lines. Prior to the
filament eruption, unambiguous observational evidence indicates that multiple
interactions occurred between the emerging magnetic fields and the left-skewed
coronal loops, implying a tether-weakening reconnection. Specifically, during
the final episode of the tether-weakening reconnection, a remarkable sigmoid
structure was formed and lifted up together with the filament. Accordingly, we
speculate that the tether-weakening reconnection probably destabilized the
filament system and triggered its rise. Subsequently, the filament and the
sigmoid structure erupted together and produced a CME. After the eruption, the
emerging magnetic fields continued to reconnect with the remaining filament
channel, leading to the reformation of the filament. This observation strongly
supports the idea that emerging magnetic flux plays an important role in
triggering the filament to erupt, and the filament is reformed by magnetic
reconnection between the emerging magnetic fields and its nearby filament
channel.
