A study of the kinematic and volumetric co-evolution of Earth-directed CMEs

TL;DR

This study investigates the co-evolution of CME volume and kinematics using multi-viewpoint observations, revealing a strong correlation between volumetric changes, CME acceleration, and flare activity, and identifying a multi-phase expansion pattern.

Contribution

It is the first to link CME volumetric evolution with flare timing and to analyze the multi-phase expansion behavior using 3D reconstructions from multiple viewpoints.

Findings

CME volume follows a power-law dependence on height.

Different structural components expand at different rates.

Volumetric evolution correlates with CME acceleration and flare X-ray flux.

Abstract

While flare-associated CMEs generally show a strong association between flare X-ray flux and CME kinematics, their volumetric evolution and its link to both kinematics and flare activity remains less explored. In this study, we investigate the volumetric and kinematic co-evolution of ten Earth-directed, flare-associated CMEs using multi-viewpoint observations from STEREO-A, STEREO-B, and SOHO. We perform 3D reconstructions of the CME flux ropes with the Graduated Cylindrical Shell (GCS) model and derive their geometrical parameters. We find that the total CME volume follows a power-law dependence on the leading edge height, and that different structural components expand at different rates, with the ellipsoidal front expanding faster than the conical legs. Furthermore, the volumetric evolution follows a multi-phase pattern: initial overexpansion, a gradual reduction in the expansion rate, and finally saturation at a higher heliocentric distance. This is similar to the well-established three-phase evolution of the CME kinematics. Notably, the second-order derivative of volume with time shows a strong temporal correlation with both CME acceleration and the GOES soft X-ray flux of the associated flare. This is the first study to report such a correspondence between volumetric evolution and flare timing, highlighting the role of flare energy release in governing CME expansion dynamics. Our findings motivate further studies into the coupling between magnetic reconnection and CME volumetric evolution in the corona.