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

Abstract:

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 extreme ultraviolet
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.