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

Characteristics of a Gradual Filament Eruption and Subsequent CME Propagation in Relation to a Strong Geomagnetic Storm