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

Based on multi-wavelength observations of two large-scale CMEs, this study reveals that lateral deformation of upper flanks beneath overlying magnetic loops drives the transition from non-radial to radial propagation in the low corona, fundamentally shaping the CME's final structure and space weather impact.

The Gist

The Big Picture: A Solar "U-Turn"

Imagine the Sun as a giant, active playground. Sometimes, it sneezes out massive clouds of hot gas and magnetic fields called Coronal Mass Ejections (CMEs). Usually, we expect these clouds to shoot straight out into space like a rocket launching from a pad.

However, this paper studies two specific "sneezes" from the Sun that didn't go straight. Instead, they started by shooting sideways, skimming along the Sun's surface, before suddenly curving back to shoot straight out into space. The researchers wanted to figure out how and why these clouds managed to make such a dramatic U-turn.

The Scene: The "Overhanging Branches"

To understand the turn, you have to look at the neighborhood where the explosion happened. Above the spot where the CMEs erupted, there was a system of giant, looping magnetic fields.

Think of these magnetic loops like low-hanging tree branches or a tall, arched trellis hanging over a garden path.

• The Eruption: The CMEs started underneath these "branches."
• The Sideways Move: Because of the shape of the magnetic fields, the CMEs couldn't go straight up immediately. Instead, they were forced to slide sideways, moving almost parallel to the Sun's surface, like a car driving under a low bridge.

The Twist: The "Bulging" Maneuver

Here is the most interesting part of the discovery. As the CMEs tried to escape from under these magnetic branches, they didn't just turn like a rigid car making a sharp turn. They deformed.

Imagine a soft, water-filled balloon being pushed sideways under a low ceiling. As it tries to get out, the top part of the balloon (the part furthest from the ground) bulges upward and squeezes through the gap, while the bottom part is still stuck or moving slowly.

• The Bulge: The top edge of the CME cloud swelled upward, breaking free from the magnetic "branches."
• The Turn: Once that top edge broke free, it became the new "front" of the cloud. The whole structure then straightened out and began shooting radially (straight out) into space.
• The Result: The part of the cloud that was originally the "side" (the bulging top) became the new "nose" leading the way.

The "Strapping" Effect

The paper explains that these magnetic loops weren't just sitting there; they were acting like elastic straps.

• Even though the loops were running parallel to the CME (like a roof over a hallway), they were still holding down the "legs" of the magnetic rope that made up the CME.
• Think of it like trying to run through a doorway while someone is holding a bungee cord attached to your ankles. You can move forward, but your legs are pulled back, forcing your upper body to lean or bulge forward to get through.
• This magnetic "strap" held the bottom of the CME back, forcing the top to bulge out and change direction.

The Surprise: The "Passenger" Got Left Behind

The paper also noticed something odd about the "cargo" inside the CME. Inside these magnetic clouds, there is often a dense knot of cooler gas called a filament (think of it as a heavy passenger sitting in the back seat of a car).

• When the CME made its sharp turn from sideways to straight, the heavy filament didn't turn as easily as the rest of the cloud.
• Because of its weight (inertia), the filament kept moving in its original sideways direction for a while.
• The Result: By the time the CME was flying straight out into space, the heavy filament had been left behind, drifting to the "south" side of the cloud. It was like a passenger sliding to the side of the car during a sharp turn.

Why This Matters

This study is important because it shows that CMEs aren't rigid, unchanging objects. They are flexible and can change their shape and direction significantly as they leave the Sun.

• The "Where" vs. The "Where To": Just because we see a CME start in one direction doesn't mean it will hit Earth from that same angle. It can change its path by up to 25 degrees (a significant distance in space terms) just by bulging and reshaping itself.
• The Forecasting Challenge: This makes it harder to predict space weather. If we only look at the start of the eruption, we might think the CME is heading one way, but it might actually curve and hit us from a different angle later on.

In short, the Sun's magnetic fields act like a complex obstacle course, forcing these massive clouds to twist, bulge, and reshape themselves before they can escape into space.

Technical Summary

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

Problem Statement
Solar coronal mass ejections (CMEs) frequently initiate with non-radial propagation in the low corona before transitioning to a radial trajectory. While the deflection of CMEs due to background magnetic pressure imbalances is a known phenomenon, the specific physical mechanisms governing the transition from non-radial to radial propagation remain poorly understood. This directional shift is critical for space weather forecasting, as it determines the final impact geometry of a CME relative to Earth. Existing literature notes that CMEs often leave the Sun in a direction roughly a dozen degrees away from their source, but the structural evolution and deformation processes enabling this redirection have not been thoroughly investigated.

Methodology
The authors conducted a multi-wavelength observational study of two large-scale CMEs originating from the same active region (NOAA AR 13234) near the solar limb on March 3 and March 4, 2023. The study utilized data from a suite of solar observatories to bridge the gap between the low corona and the inner heliosphere:

• Low Corona Imaging: SDO/AIA (304 Å, 131 Å, 211 Å, 171 Å), SATech-01/SUTRI (465 Å), and GOES/SUVI (284 Å) were used to track filament eruptions and CME morphology in the low corona.
• Coronal Imaging: STEREO-A (EUVI, COR1, COR2) and SOHO/LASCO (C2) provided continuous tracking of the CMEs as they propagated outward, filling the field-of-view gaps between instruments.
• Magnetic Field Modeling: The Potential Field Source Surface (PFSS) model was applied to synoptic magnetograms (SDO/HMI) to reconstruct the ambient magnetic field, specifically the overlying loops.
• Quantitative Analysis: The Graduated Cylindrical Shell (GCS) model was employed to fit the CME geometry and determine propagation directions. Additionally, the study calculated the decay index ($n$) to assess torus instability conditions and analyzed the distributions of magnetic pressure ($P_B$) and the radial component of magnetic tension force ($T_{B,r}$).

Key Results

1. Lateral Deformation Mechanism: Both CMEs initiated with non-radial, nearly horizontal motion beneath a system of overlying magnetic loops. During the transition phase, the CMEs underwent significant lateral deformation. Specifically, the "upper flank" (the outermost edge on the greater-height side during the non-radial stage) bulged toward the higher corona.
2. Structural Reconfiguration: This bulging resulted in the upper flank of the non-radial stage becoming the leading edge in the subsequent radial stage. Consequently, the original leading edge of the non-radial phase transformed into the southern flank of the radial CME.
3. Filament Displacement: In the March 3 event, a major portion of the erupted filament was displaced to the southern part of the CME structure after the directional transition. The filament largely maintained its initial non-radial trajectory due to inertia and lack of full ionization, lagging behind the main CME body which redirected radially. This displacement explains the observation of the filament as a "Core 2" in the southern sector of the CME in coronagraph images.
4. Role of Overlying Loops: The overlying loops were oriented roughly parallel to the flux-rope axis, rather than strapping over the apex. However, the study found that the strong magnetic tension force ($T_{B,r}$) from these loops acted on the flux-rope legs (which were perpendicular to the loops in the non-radial stage). This tension constrained the radial expansion of the portion of the CME containing the legs, forcing the upper flank to bulge outward to escape the confinement. Once the CME moved beyond the high-tension region, it expanded laterally and transitioned to radial propagation.
5. Directional Shift: The final propagation direction of both CMEs was approximately 24°–26° away from the eruption site, aligning more closely with the solar equatorial plane.

Key Contributions

• First Detailed Analysis: This study presents the first detailed investigation of the lateral deformation occurring specifically during the transition of CMEs from non-radial to radial propagation in the low corona.
• Mechanism Elucidation: It identifies the bulging of the upper flank as the primary mechanism for directional transition, driven by the interplay between the CME's expansion and the constraining magnetic tension of parallel overlying loops.
• Structural Evolution: The work demonstrates that CMEs do not evolve as self-similar "solid bodies" during this phase; instead, their geometry changes dramatically, with the leading edge in the radial stage being a different structural component than in the non-radial stage.
• Filament-CME Decoupling: The study provides observational evidence that filaments can be significantly displaced relative to the CME leading edge during non-radial-to-radial transitions, offering a potential explanation for why some interplanetary CMEs lack in-situ filament signatures despite originating from filament eruptions.

Significance
The paper claims that understanding this lateral deformation is essential for constructing a complete picture of CME evolution. The findings highlight that the final space weather impact of a CME can differ significantly from its source region (up to ~25° away), posing challenges for forecasting based solely on low-corona observations. Furthermore, the study suggests that non-radial CMEs may interact with the solar atmosphere differently than radial ones, potentially driving distinct shock and wave properties (e.g., Moreton-Ramsey waves). The research underscores the complexity of CME kinematics, indicating that magnetic tension forces from non-strapping loops can play a decisive role in constraining and redirecting CME propagation.