🔭 astrophysics

A view of the evolution of a CME and the associated wave-trains at high spatial and temporal resolution

This study utilizes high-resolution Metis/Solar Orbiter observations of an October 2022 CME to resolve fine substructures and internal plasma motions, revealing fast-propagating wave trains that offer new insights into coronal wave generation and energy transport.

Original authors: G. Russano, Y. De Leo, F. Frassati, G. Jerse, V. Andretta, H. Cremades, M. Temmer, S. Mancuso, L. Abbo, A. Burtovoi, F. Landini, M. Pancrazzi, M. Romoli, C. Sasso, R. Susino, M. Uslenghi

Published 2026-04-15

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CC BY 4.0

Original authors: G. Russano, Y. De Leo, F. Frassati, G. Jerse, V. Andretta, H. Cremades, M. Temmer, S. Mancuso, L. Abbo, A. Burtovoi, F. Landini, M. Pancrazzi, M. Romoli, C. Sasso, R. Susino, M. Uslenghi

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun as a massive, boiling pot of magnetic soup. Sometimes, this soup gets too turbulent, and a giant bubble of super-hot gas and magnetic fields bursts out into space. Scientists call this a Coronal Mass Ejection (CME). It's like a cosmic sneeze that can send billions of tons of material hurtling toward Earth, potentially messing up our satellites and power grids.

For decades, we've watched these sneezes from a distance (about 93 million miles away, or 1 Astronomical Unit). It's like trying to watch a movie on a tiny, grainy phone screen from the back of a theater. You can see the big shapes, but you miss all the fine details.

Enter the Solar Orbiter and its "Metis" camera.

This paper is about a specific event on October 8–9, 2022, where the Solar Orbiter flew much closer to the Sun (about 28 million miles away). The Metis camera on board acted like a high-definition, super-fast camera, zooming in on the Sun's atmosphere (the corona) with a resolution so sharp it could see details the size of a small city.

Here is what the scientists discovered, explained through simple analogies:

1. The "Slow-Motion" Movie of a Cosmic Sneeze

Usually, we see CMEs as a big, blurry blob expanding outward. But because Metis took pictures every 20 seconds with incredible clarity, the scientists could see the "insides" of the sneeze.

The Analogy: Imagine watching a firework explode. From far away, it's just a bright flash. But with Metis, they could see the individual sparks, the swirling smoke, and the specific trails of light. They saw that the CME wasn't just a smooth balloon; it was a complex structure with a bright front, a dark hollow center, and a dense core, all moving at different speeds.

2. The "Whirlpool" on the Side

As the CME expanded, the scientists noticed something strange happening on its western edge (the side). A giant swirl of plasma started spinning and falling back down, looking like a vortex.

The Analogy: Think of a river flowing fast next to a slow-moving patch of water. The friction between the two creates a whirlpool. This is called a Kelvin-Helmholtz Instability. It's like the wake behind a speedboat, but on a solar scale. This was a huge discovery because it's the first time we've seen such a massive "whirlpool" this high up in the Sun's atmosphere. It suggests the CME was rubbing against the surrounding solar wind, creating turbulence.

3. The "Ripples" in the Pond

The most exciting find was a series of circular waves moving ahead of the CME, like ripples spreading out when you drop a stone in a pond.

The Analogy: Usually, when we see waves on the Sun, they are linked to a massive solar flare (a huge explosion of energy). But in this case, there was no explosion. The CME was a "stealth" sneeze—quiet and low-energy. Yet, it still created waves.
The Mystery: It's as if a car drove by a puddle without splashing, yet the water still rippled. The scientists think the CME itself, just by expanding and stretching its magnetic "tethers," was shaking the solar atmosphere enough to create these waves. These waves were traveling at about 500 km/s (over 1 million mph!) and had a rhythmic beat, appearing roughly every 3 minutes.

Why Does This Matter?

Think of the Sun's atmosphere as a complex machine. If we only look at the big gears (the main CME), we miss the tiny springs and levers (the waves and vortices) that tell us how the machine actually works.

Better Forecasts: By understanding these "stealth" eruptions and the waves they create, we can get better at predicting space weather. Even a quiet sneeze can sometimes have a big impact on Earth.
New Physics: Seeing these waves without a flare suggests the Sun has other ways to generate energy and movement that we didn't fully understand before.

In a nutshell: This paper is like upgrading from a grainy black-and-white TV to a 4K IMAX screen. It showed us that even "quiet" solar eruptions are full of chaotic, beautiful, and complex dance moves—swirling vortices and rhythmic waves—that we've never been able to see so clearly before.

1. Problem Statement

Coronal Mass Ejections (CMEs) are large-scale expulsions of magnetized plasma that drive space weather. While historically studied by coronagraphs at 1 AU (e.g., SOHO/LASCO, STEREO), these instruments lack the spatial and temporal resolution to resolve fine-scale structures, internal dynamics, and early acceleration phases of CMEs.

The Gap: There is a need to observe CMEs in the "middle corona" (1.7–6.4 $R_\odot$ $R_{⊙}$ ) with high cadence to understand:
- The internal kinematics of the flux rope and plasma.
- The generation and propagation of coronal waves (often associated with shocks or reconnection).
- The evolution of "stealth" CMEs (events with weak low-coronal signatures).
Objective: To exploit the unique high-resolution capabilities of the Metis coronagraph on board the Solar Orbiter (SolO) to resolve substructures within a CME front and characterize associated wave-trains that were previously unresolvable.

2. Methodology

The study focuses on a CME event occurring between October 8 and 9, 2022, observed while Solar Orbiter was at a heliocentric distance of 0.3 AU.

Instrumentation & Data:
- Metis (Solar Orbiter): Operated in two modes:
  - WIND mode: Standard cadence (16 min VL, 8 min UV).
  - FLUCTS-TBF mode: High-cadence mode (20 s cadence in Visible Light) for 40 minutes (03:30–04:10 UT), providing a spatial resolution of $\sim$ 4.4 $\times$ 10 $^3$ km/pixel.
- Multi-viewpoint Context: Data was integrated from SolO/EUI-FSI (disk imaging), GOES-R/SUVI, SDO/AIA, SDO/HMI, and STEREO-A/COR2 to reconstruct the 3D geometry and source region.
Data Processing:
- Enhancement: A normalization-based running difference algorithm was applied to Metis images to suppress background noise and highlight transient structures.
- 3D Reconstruction: The Graduated Cylindrical Shell (GCS) model was used to fit the CME flux rope geometry, determining the propagation direction and deprojecting velocities.
- Kinematic Analysis: "J-maps" (height-time diagrams) were constructed along specific radial segments to track features. Second-order polynomial fits were applied to estimate speeds and accelerations.
- Wave Analysis: Time-series frequency decomposition (Wavelet analysis and Empirical Mode Decomposition) was used to identify periodicities in the wave-trains.

3. Key Contributions

Unprecedented Resolution: The study demonstrates the first high-cadence (20s) imaging of a CME in the middle corona ( $\sim$ 2–3 $R_\odot$ ) with sufficient resolution to track individual plasma elements and wavefronts.
Discovery of Wave-Trains: It provides the first direct white-light imaging of quasi-periodic, circular coronal wave-trains propagating laterally from a CME flank in the middle corona, distinct from the low-corona EUV waves.
Vortex Detection: It identifies a large-scale, descending vortex-like structure along the CME flank, interpreted as a potential Kelvin-Helmholtz instability (KHi).
Stealth CME Characterization: It offers a detailed case study of a "stealth" CME (lacking a strong flare signature) to understand its initiation and evolution.

4. Key Results

A. CME Morphology and Kinematics

Source Region: The eruption originated from a filament channel along a polarity inversion line (PIL) in the southern hemisphere. Magnetic reconnection was confirmed by the formation of a Post-Eruptive Arcade (PEA), though no impulsive flare was detected.
3D Geometry: The CME propagated at a Carrington longitude of 339° and latitude -32°. The propagation direction was nearly aligned with the Solar Orbiter's plane of sky, minimizing projection effects.
Velocity Profile:
- Low Corona: Initial speed $\sim$ 16 km/s.
- Middle Corona (Metis): The CME front accelerated to 251 $\pm$ 22 km/s (deprojected) with an acceleration of 23 $\pm$ 13 m/s $^2$ .
- UV Dark Ring: A dark ring structure in the UV channel moved at 61 $\pm$ 12 km/s, indicating a slower, denser front component.

B. Vortex-Like Feature (Feature B)

Observation: A large, descending plasma structure resembling a vortex was observed along the western flank of the CME.
Kinematics:
- Upflow: $\sim$ 212 km/s (coupled to the CME).
- Downflow: $\sim$ -138 km/s with high acceleration ( $\sim$ 102 m/s $^2$ ).
Interpretation: The structure is likely a Kelvin-Helmholtz Instability (KHi) driven by velocity shear between the expanding CME and the ambient corona. This would represent one of the first detections of a large-scale KHi at intermediate coronal heights.

C. Quasi-Periodic Wave-Trains (Feature C)

Observation: Nearly circular, concentric wavefronts propagated laterally from the CME flank, independent of the main eruption front.
Kinematics:
- Speed: $\sim$ 482 km/s (nearly twice the CME speed).
- Deceleration: Strong deceleration observed (-111 to -398 m/s $^2$ ).
- Periodicity: A dominant period of 2.7–3 minutes (5–6 mHz).
Interpretation: These are identified as Quasi-Periodic Fast-mode (QFP) waves. Unlike typical QFPs associated with flares, these were flare-independent, suggesting they were driven by the lateral expansion of the CME envelope or sequential magnetic reconnection within the flux rope.

5. Significance

New Diagnostic Window: The study proves that Metis can resolve fine-scale plasma dynamics and wave phenomena in the middle corona, a region previously difficult to observe with such detail.
Wave Generation Mechanisms: It challenges the paradigm that QFP waves require flare drivers, proposing that CME expansion and magnetic reconfiguration can independently generate magnetosonic waves.
Space Weather Implications: Understanding "stealth" CMEs and their associated instabilities (like KHi) and wave generation is crucial for improving space weather forecasting, as these events can still be geoeffective despite lacking classical low-coronal precursors.
Future Modeling: The findings provide critical observational constraints for Magnetohydrodynamic (MHD) simulations regarding CME-flank stability and wave excitation in the extended corona.

In conclusion, this paper establishes the Solar Orbiter/Metis as a transformative tool for solar physics, enabling the transition from statistical CME studies to detailed, high-resolution kinematic and dynamic analyses of eruptive events and their interaction with the solar corona.