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

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Number:	522
1st Author:	Huidong HU
2nd Author:
Published:	April 13, 2026
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Introduction

Solar coronal mass ejections (CMEs) are large-scale expulsions of plasma and magnetic fields from the solar atmosphere. Their speed can be up to 3000 km/s near the Sun, and their mass could be as large as 10¹⁴ kg. When the ejecta reach the Earth, it can disrupt the Earth's magnetic fields and inject a large amount of energy and energetic particles into the magnetosphere, causing a geomagnetic storm.

The propagation direction of a CME is an important factor in its space weather effects because it determines whether and which part of the CME impacts the Earth. However, a significant fraction of CMEs initially propagate nonradially in the corona, and then transition to radial direction that is often about a dozen degrees away from the source position (e.g., Ref. [1,2]). This transition from nonradial to radial propagation is an important process in the evolution of such CMEs, yet it remains poorly understood.

Obervations and Results

We investigated the lateral deformation of two large-scale CMEs from the same active region on the solar limb (Ref. [3]). Based on multiwavelength observations, we present a detailed analysis of the transition from nonradial to radial propagation of CMEs in the low corona.

The two CMEs erupted on 2023 March 3 and 4, respectively, from the solar west limb. In the beginning, both CMEs move in a nonradial direction (indicated by arrows in Figures 1(a)(c) for the CME on March 3), beneath a system of overlying loops that are roughly parallel to the flux-rope axis. During the nonradial stage, the CMEs laterally deform by bulging their upper flanks toward the higher corona (indicated by rightward white arrows in Figures 1(e)(h)). This process eventually leads to a transition to a radial propagation direction approximately 25^° away from the eruption site (see Figure 1(i)). After the directional transition, the nonradial-stage upper flank becomes the leading edge in the radial stage.

Figure 1: Multiwavelength observations of the 2023 March 3 CME transitioning from nonradial to radial propagation. (a)--(c) The erupting filament observed by SDO/AIA? 304 Å and SATech-01/SUTRI 465 Å moving along a nonradial direction; (d)--(h) GOES/SUVI 284 Å running-difference images showing the CME transitioning from nonradial to radial propagation through lateral deformation (bulging of the upper flank); (i) SOHO/LASCO C2 image showing the CME finally propagating radially. Animations are available online in Ref. [3].

The system of overlying loops is roughly parallel to the axis of the flux rope and thus does not strap over it. However, these loops are generally perpendicular to the flux-rope legs, and their strong magnetic tension force can constrain the radial expansion of part of the CME by acting on the legs during the transition. The interaction between the expanding CME and the overlying loops causes transverse oscillations of the loops (see Figure 2(c)+(f)).

Furthermore, the study found that the motion direction of a major portion of the filament lagged behind the transition of the overall CME structure in the March 3 event. This resulted in the filament being displaced to the southern part of the CME, moving away from the ecliptic plane (as indicated by the arrow in Figure 1(i)). This highlights the complexity of CME features; for instance, in situ observations near the ecliptic plane might not detect the filament material.

Figure 2: (a)+(d) PFSS-extrapolated magnetic loops (cyan curves) overlying the CME source regions. (b)+(e) SDO/AIA 171 Å images showing the overlying loops and the nonradial morphology of the two CMEs in the early stage. (c)+(f) Time-distance profiles along the loops showing oscillations caused by the interaction between the CMEs and the loops.

Summary

This study presents the first observational investigation of the lateral deformation of CMEs during the transition from nonradial to radial propagation in the low corona, contributing significantly to a comprehensive understanding of the complete CME evolution picture. More details are in Ref. [3].

References

[1] "Quantitative Analysis of CME Deflections in the Corona"

[2] "Global Trends of CME Deflections Based on CME and Solar Parameters"

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