Three-phase evolution of aspect ratio in fast and slow CMEs from the Sun to 1 au

ABSTRACT Coronal mass ejections (CMEs) undergo significant geometric evolution as they propagate from the Sun to 1 au, influencing their radial size, expansion, and space weather impact. We investigate the evolution of CME aspect ratio ($\kappa$) and expansion dynamics for four fast and four slow Earth-directed CMEs. Using multipoint coronagraphic observations with the graduated cylindrical shell model and corrected in situ measurements of associated magnetic clouds at 1 au, we track the evolution of $\kappa$ from the low-middle corona to interplanetary space. We find that $\kappa$ does not remain constant but exhibits a systematic three-phase evolution: a rise phase in the low-middle corona ($\lesssim 10$–$15\, \mathrm{ R}_{\odot }$), a saturation phase at intermediate heights, and then a decline phase in the interplanetary space. The ratio of radial expansion speed to leading-edge speed ($V_{\rm exp}/V_{\rm LE}$) decreases substantially from the corona to 1 au, indicating a reduction in radial expansion efficiency during interplanetary propagation. The consistent evolution of $\kappa$ and $V_{\rm exp}/V_{\rm LE}$ suggests a transition from magnetically dominated expansion in the corona to a regime increasingly controlled by the heliospheric environment. We note that fast CMEs show stronger early expansion and evolve into larger, more radially extended structures, whereas slow CMEs exhibit a more gradual rise and a steeper decline. These results demonstrate that CME geometry evolves significantly during propagation and highlight the need to incorporate aspect ratio evolution in models to improve predictions of CME size, arrival time, and geoeffectiveness.