  Solar filaments often erupt partially. Although how they split remains
elusive, the splitting process has the potential of revealing the filament
structure and eruption mechanism. Here we investigate the pre-eruption
splitting of an apparently single filament and its subsequent partial eruption
on 2012 September 27. The evolution is characterized by three stages with
distinct dynamics. During the quasi-static stage, the splitting proceeds
gradually for about 1.5 hrs, with the upper branch rising at a few kilometers
per second and displaying swirling motions about its axis. During the precursor
stage that lasts for about 10 min, the upper branch rises at tens of kilometers
per second, with a pair of conjugated dimming regions starting to develop at
its footpoints; with the swirling motions turning chaotic, the axis of the
upper branch whips southward, which drives an arc-shaped EUV front propagating
in the similar direction. During the eruption stage, the upper branch erupts
with the onset of a C3.7-class two-ribbon flare, while the lower branch remains
stable. Judging from the well separated footpoints of the upper branch from
those of the lower one, we suggest that the pre-eruption filament processes a
double-decker structure composed of two distinct flux bundles, whose formation
is associated with gradual magnetic flux cancellations and converging
photospheric flows around the polarity inversion line.
