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

Many coronal mass ejections (CMEs) initially propagate non-radially, 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 multi-wavelength observations, we investigate the transition from non-radial 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 non-radial 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 non-radial stage toward the higher corona, which results in the transition to a radial propagation direction approximately 25$^\circ$ away from the eruption site. After the directional transition, the non-radial-stage upper flank becomes the leading edge in the radial stage. Although the overlying loops do not strap over 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 investigation of the lateral deformation during the transition of CMEs from non-radial to radial in the low corona, and makes an essential contribution to the complete CME evolution picture.

💡 Research Summary

The authors present a comprehensive observational study of two large‑scale coronal mass ejections (CMEs) that erupted from the same active region (NOAA AR 13234) on 3 and 4 March 2023. Using a suite of multi‑wavelength instruments—including SDO/AIA (304 Å), SUTRI (465 Å), GOES‑SUVI (284 Å), STEREO‑A/SECCHI (EUVI 195 Å, COR1, COR2), and SOHO/LASCO—the paper tracks each CME from its low‑corona origin through the transition from an initial non‑radial trajectory to a final radial propagation.

In the early phase, both CMEs rise beneath a system of overlying coronal loops that run roughly parallel to the flux‑rope axis. The loops exert a strong magnetic tension that constrains the expansion of the flux‑rope legs, effectively pinning the lower part of the eruption. As a result, the upper flank of each CME bulges upward, forming a “lateral deformation” that reshapes the structure. This bulging upper flank becomes the new leading edge, while the original non‑radial front is relegated to the southern flank in the later radial stage. The deformation causes the CME propagation direction to shift by about 25° away from the source longitude, a change that is gradual rather than abrupt, as confirmed by Graduated Cylindrical Shell (GCS) fitting of SOHO and STEREO‑A coronagraph data (latitude changes from +9° to –1°).

A striking finding is the behavior of the associated filament material. While the CME body reorients, the filament largely retains its original non‑radial direction and, after the transition, is displaced to the southern part of the CME. This explains the appearance of two distinct white‑light cores (“Core 1” and “Core 2”) in the coronagraph images: Core 1 corresponds to the leading portion of the filament, whereas Core 2 represents the trailing portion that has migrated southward. The authors argue that the traditional identification of a single bright core with the filament is oversimplified for such events.

The study emphasizes that the overlying loops do not physically strap the flux rope; instead, their magnetic tension acts on the rope’s legs, limiting radial expansion during the deformation phase. Consequently, the CME does not rotate as a rigid body but reshapes through asymmetric expansion. This lateral deformation mechanism provides a physically grounded explanation for the frequent observation that many CMEs, initially deflected by background magnetic pressure gradients or coronal holes, eventually become nearly radial before leaving the low corona (typically below 10 R⊙).

By documenting the full sequence—from filament eruption, through loop‑constrained lateral bulging, to the emergence of a radially propagating CME—the paper fills a notable gap in CME evolution research. The findings have direct implications for space‑weather forecasting: early‑stage lateral deformation can alter the CME’s trajectory by tens of degrees, affecting whether the ejecta will intersect Earth’s magnetosphere. The authors call for high‑resolution 3‑D MHD simulations and larger statistical samples to quantify the role of overlying loop tension and lateral deformation in CME propagation models.