  The solar active region NOAA 12887 produced a strong X1.0 flare on 2021
October 28, which exhibits X-shaped flare ribbons and a circle-shaped erupting
filament. To understand the eruption process with these characteristics, we
conducted a data-constrained magnetohydrodynamics simulation using a nonlinear
force-free field of the active region about an hour before the flare as the
initial condition. Our simulation reproduces the filament eruption observed in
the Ha images of GONG and the 304 angstrom images of SDO/AIA and suggests that
two mechanisms can possibly contribute to the magnetic eruption. One is the
torus instability of the pre-existing magnetic flux rope (MFR), and the other
is upward pushing by magnetic loops newly formed below the MFR via continuous
magnetic reconnection between two sheared magnetic arcades. The presence of
this reconnection is evidenced by the SDO/AIA observations of the 1600 angstrom
brightening in the footpoints of the sheared arcades at the flare onset. To
clarify which process is more essential for the eruption, we performed an
experimental simulation in which the reconnection between the sheared field
lines is suppressed. In this case too, the MFR could erupt, but at a much
reduced rising speed. We interpret this result as indicating that the eruption
is not only driven by the torus instability, but additionally accelerated by
newly formed and rising magnetic loops under continuous reconnection.
