  An unexpected strong geomagnetic storm occurred on 2018 August 26, which was
caused by a slow coronal mass ejection (CME) from a gradual eruption of a large
quiet-region filament. We investigate the eruption and propagation
characteristics of this CME in relation to the strong geomagnetic storm with
remote sensing and in situ observations. Coronal magnetic fields around the
filament are extrapolated and compared with EUV observations. We determine the
propagation direction and tilt angle of the CME flux rope near the Sun using a
graduated cylindrical shell (GCS) model and the Sun-to-Earth kinematics of the
CME with wide-angle imaging observations from STEREO A. We reconstruct the
flux-rope structure using a Grad-Shafranov technique based on the in situ
measurements at the Earth and compare it with those from solar observations and
the GCS results. Our conclusions are as follows: (1) the eruption of the
filament was unusually slow and occurred in the regions with relatively low
critical heights of the coronal field decay index; (2) the axis of the CME flux
rope rotated in the corona as well as in interplanetary space, which tended to
be aligned with the local heliospheric current sheet; (3) the CME was bracketed
between slow and fast solar winds, which enhanced the magnetic field inside the
CME at 1 AU; (4) the geomagnetic storm was caused by the enhanced magnetic
field and a southward orientation of the flux rope at 1 AU from the rotation of
the flux rope.
