Lateral Deformation of Large-scale Coronal Mass Ejections during the Transition from Nonradial to Radial Propagation

Many coronal mass ejections (CMEs) initially propagate nonradially, and then transition to radial propagation in the corona. This directional transition is a significant process that determines a CME’s space weather effects but remains poorly understood. Based on multiwavelength observations, we investigate the transition from nonradial to radial propagation in the low corona for two large-scale CMEs from the same active region on the solar limb. In the beginning, both CMEs move in a nonradial direction, beneath a system of overlying loops that are roughly parallel to the flux-rope axis. The CMEs laterally deform by bulging their upper flanks in the nonradial stage toward the higher corona, which results in the transition to a radial propagation direction approximately 25° away from the eruption site. After the directional transition, the nonradial-stage upper flank becomes the leading edge in the radial stage. Although the overlying loops do not strap the flux rope, their strong magnetic tension force constrains the radial expansion of part of the CME during the transition by acting on the flux-rope legs. A major portion of the filament is displaced to the southern part of a CME in the radial stage, which implies the complexity of observational CME features. This study presents the first observational investigation of the lateral deformation during the transition of CMEs from nonradial to radial in the low corona, and makes an essential contribution to the complete CME evolution picture.