  The eruptions of solar filaments often show rotational motion about their
rising direction, but it remains elusive what mechanism governs such rotation
and how the rotation is related to the initial morphology of the pre-eruptive
filament (and co-spatial sigmoid), filament chirality, and magnetic helicity.
The conventional view regarding the rotation as a result of a magnetic flux
rope (MFR) under-going the ideal kink instability still has confusion in
explaining these relationships. Here we proposed an alternative explanation for
the rotation during eruptions, by analyzing a magnetohydrodynamic simulation in
which magnetic reconnection initiates an eruption from a sheared arcade
configuration and an MFR is formed during eruption through the reconnection.
The simulation reproduces a reverse S-shaped MFR with dextral chirality, and
the axis of this MFR rotates counterclockwise while rising, which compares
favorably with a typical filament eruption observed from dual viewing angles.
By calculating the twist and writhe numbers of the modeled MFR during its
eruption, we found that accompanied with the rotation, the nonlocal writhe of
the MFR's axis decreases while the twist of its surrounding field lines
increases, and this is distinct from the kink instability, which converts
magnetic twist into writhe of the MFR axis.
