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Analysis of Lyman-beta and Lyman-gamma Lines in a Pre-Eruptive and Eruptive Prominence with Solar Orbiter SPICE Observations

This study analyzes Solar Orbiter SPICE observations of a prominence's Lyman-beta and Lyman-gamma lines to characterize the dynamic changes in plasma parameters during its eruption and demonstrates a method for calculating radial velocity using 2D images.

Original authors: Yong Zhang (SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ UK), Nicolas Labrosse (SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ UK), Susann

Published 2026-04-16

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CC BY 4.0

Original authors: Yong Zhang (SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ UK), Nicolas Labrosse (SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ UK), Susanna Parenti (Institut d'Astrophysique Spatiale, Bat 121, Université Paris-Saclay/CNRS 91405 Orsay Cedex France), Therese A. Kucera (Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA)

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

The Big Picture: Catching a Solar "Sneeze" in Slow Motion

Imagine the Sun as a giant, fiery ball of gas. Sometimes, it sneezes. This "sneeze" is called a solar prominence eruption, where huge loops of super-hot, electrically charged gas (plasma) shoot out into space. These eruptions can mess with our satellites and power grids on Earth, so scientists want to understand exactly how they work.

On April 15, 2023, a space telescope called Solar Orbiter (specifically an instrument named SPICE) got a rare, front-row seat to watch a prominence before it exploded and while it was exploding.

This paper is like a detective report. The scientists used the light coming from this solar sneeze to figure out what was happening inside the gas cloud. They didn't just look at the shape; they looked at the "colors" (spectral lines) of the light to measure temperature, density, and speed.

The Tools: Looking at the Sun's "Fingerprint"

To understand the gas, the scientists looked at two specific "fingerprints" in the light: the Lyman Beta and Lyman Gamma lines.

The Analogy: Imagine the Sun is a choir. Every singer (atom) sings a specific note. Hydrogen atoms sing very high-pitched notes called the "Lyman series."
Lyman Beta and Gamma are two specific notes in that series. By listening to how loud these notes are (intensity) and how "fuzzy" or wide they sound (line width), the scientists can tell if the choir is singing in a hot room, a cold room, or if they are running around frantically.

What They Found

1. The "Before" and "After" Comparison

The team looked at the prominence in two states:

The Calm Before the Storm (Pre-eruptive): The gas was sitting quietly. The light from the prominence was dimmer than the light from the Sun's surface (the disk).
The Explosion (Eruptive): As the prominence shot upward, things changed dramatically.
- The Analogy: Think of a quiet campfire. Suddenly, someone throws a bucket of gasoline on it. The fire flares up, gets brighter, and the smoke gets thicker.
- The Result: In one part of the erupting prominence, the light became brighter than the Sun's surface itself. This told the scientists that the gas was getting super-heated and compressed, making it glow intensely. However, in another part of the eruption, the light actually got dimmer, suggesting that part of the gas was spreading out and becoming less dense (like smoke dissipating).

2. The "Fuzziness" of the Sound (Line Width)

The scientists measured how "wide" the spectral lines were.

The Analogy: If a singer holds a note perfectly still, the sound is a sharp, thin line. If the singer is shaking, vibrating, or running around while singing, the note gets "fuzzy" or wide.
The Result: Before the eruption, the "fuzziness" of the prominence was similar to the Sun's surface. But during the eruption, the lines got narrower in some spots. This suggests the gas was moving in a more organized, streamlined way, or perhaps the conditions changed so much that the "noise" of the plasma settled down.

3. The Speed of the Sneeze (Radial Velocity)

One of the trickiest parts of the paper is figuring out how fast the prominence was moving.

The Problem: Usually, scientists measure speed by looking for a "Doppler shift" (like the change in pitch of a passing siren). But the telescope wasn't sensitive enough to hear the "pitch change" clearly.
The Solution: The authors invented a clever new math trick. They took two photos of the eruption taken 20 minutes apart (like a flipbook).
The Analogy: Imagine you see a car in a photo at 1:00 PM and another photo at 1:20 PM. Even if you can't hear the engine, you can measure how far the car moved between the two pictures to guess its speed.
The Result: By tracking a specific knot of gas in the photos, they calculated the speed. They found the prominence was moving at about 30 to 40 kilometers per second (roughly 70,000 to 90,000 mph). That's fast, but not as fast as a bullet.

Why Does This Matter?

This paper is important for three main reasons:

It's a First: This was the first time a specific instrument (SPICE) got a dedicated, high-quality look at a prominence while it was exploding. It's like finally getting a high-definition video of a volcano erupting instead of just a blurry sketch.
It's a Diagnostic Tool: The scientists proved that by looking at these specific light lines, we can tell if a solar eruption is heating up, cooling down, or getting denser. This helps us predict how dangerous an eruption might be for Earth.
A New Speed Trick: They showed a new, easy way to calculate the speed of solar eruptions using just two pictures. This is a handy tool for future solar weather forecasting.

The Bottom Line

The Sun is a dynamic, chaotic place. When it "sneezes" a prominence, the gas inside doesn't just move; it heats up, compresses, and changes density in complex ways. By listening to the specific "notes" of hydrogen light, this team of scientists decoded the story of the eruption, showing us that these solar events are much more dramatic and varied than we previously thought.

In short: They used a space telescope to listen to the Sun's "voice" during a tantrum, figured out how fast it was moving, and learned that the gas gets incredibly hot and bright right before it flies off into space.

1. Problem Statement

Solar prominences are large-scale plasma structures suspended in the solar corona, often associated with Coronal Mass Ejections (CMEs) that impact space weather. While the physical conditions (temperature, density, velocity) within prominences are critical to understanding solar dynamics, diagnosing these parameters during the transition from a quiescent state to an eruption remains challenging. Specifically, there is a need to:

Characterize the evolution of hydrogen Lyman series lines ( $\beta$ at 102.57 nm and $\gamma$ at 97.25 nm) during the pre-eruptive and eruptive phases.
Understand how eruption dynamics (heating, compression, mass motion) alter line profiles, intensities, and widths.
Determine radial velocities of erupting filaments to assess Doppler effects on spectral lines, which is crucial for Non-LTE radiative transfer modeling.

2. Methodology

The study utilizes multi-instrument observations of a prominence event on April 15, 2023:

Primary Instrument: SPICE (Spectral Imaging of the Coronal Environment) on board Solar Orbiter.
- Observations: Captured the prominence in the Long Wavelength (LW) channel (96.0–105.8 nm) during two phases: pre-eruption (07:03–08:09 UT) and eruption (08:13–11:58 UT).
- Data Processing: Integrated intensities were calculated by integrating spectral intensity over 0.17 nm around the line centers. Line widths were measured using a quantile method (CDF=0.12 to CDF=0.88) rather than Gaussian fitting, as the lines are optically thick and asymmetric. Instrumental broadening ( $L_{inst} \approx 0.0775$ nm) was accounted for to derive intrinsic widths.
Supporting Instruments:
- GONG H $\alpha$ (El Teide): Used to track the filament's footpoints and eruption timing (starting ~10:00 UT).
- STEREO/EUVI-A: Used for cross-validation of the eruption direction and velocity.
Velocity Estimation Technique:
- The authors developed a geometric method to estimate radial velocity ( $v_r$ ) using two 2D images separated by a time interval.
- Assuming a radial eruption (supported by statistical studies) and uniform acceleration, they projected the observed 2D motion onto a spherical coordinate system to derive the true radial velocity.

3. Key Contributions

First Dedicated SPICE Observation: This is the first dedicated analysis of an off-limb eruptive prominence using SPICE data, demonstrating the instrument's capability to diagnose prominence plasma via the Lyman series.
Novel Velocity Estimation: Introduction of a geometric method to calculate the radial velocity of an eruptive filament using a pair of 2D images from a single viewpoint, bypassing the need for direct Doppler shift measurements which were below the instrument's resolution limit.
Comparative Diagnostics: A comprehensive comparison of Lyman $\beta$ and $\gamma$ properties across the solar disk, quiet prominence, and eruptive prominence regions, highlighting the impact of eruption dynamics on radiative transfer.

4. Key Results

A. Integrated Intensity and Spatial Variation

Pre-Eruption: The ratio of integrated Lyman $\beta$ to Lyman $\gamma$ intensity in the prominence and disk regions was stable at approximately 2.5, suggesting similar radiative transfer conditions (likely resonance scattering).
Eruption Phase:
- Region 1: Showed a substantial increase in Lyman $\beta$ intensity (up to 1.07 W m $^{-2}$ sr $^{-1}$ ), occasionally exceeding the solar disk intensity. This indicates enhanced heating, increased hydrogen column density, or compression.
- Region 2: Showed lower intensity and narrower line widths compared to the pre-eruption phase, suggesting a low-density region.
Corona: The intensity ratio in the corona was highly variable due to low signal-to-noise ratios.

B. Line Widths (Thermal and Non-Thermal Broadening)

Pre-Eruption: Intrinsic line widths were similar for the prominence and disk regions (Lyman $\beta$ : ~0.10 nm; Lyman $\gamma$ : ~0.16 nm).
Eruption: The intrinsic line widths for both regions converged to approximately 0.05 nm.
Comparison: These eruptive widths are significantly narrower than some previous quiescent prominence studies (e.g., Heinzel et al. 2001 reported ~0.05–0.06 nm but without instrumental correction), suggesting that plasma conditions close to eruption differ significantly from stable states, or that instrumental uncertainties play a role.

C. Line Profiles and Doppler Shifts

Spatial/Temporal Evolution: The eruption enhanced spatial and temporal variations in line intensity.
Doppler Shifts: Most calculated line-of-sight velocities were below the SPICE resolution limit (~30 km s $^{-1}$ ). No significant Doppler shifts were detected in the spectral profiles.

D. Radial Velocity Estimation

Using the new geometric method on GONG H $\alpha$ images (09:57 UT and 10:17 UT), the radial velocity was estimated at 31 ± 7 km s $^{-1}$ at 10:07 UT.
Cross-validation with STEREO/EUVI-A yielded 44 ± 5 km s $^{-1}$ .
Implication: The estimated Doppler shift for Lyman $\beta$ (~0.015 nm) is comparable to the line width, suggesting that resonance scattering intensity could be reduced (Doppler dimming), though the shift is small enough to be consistent with the lack of significant shifts in the SPICE profiles.

5. Significance and Future Work

Diagnostic Potential: The study confirms that SPICE observations provide unique insights into the dynamic changes of plasma conditions (density, temperature, optical thickness) during prominence eruptions.
Modeling Constraints: The derived radial velocity limits and line profile characteristics provide essential constraints for future Non-LTE radiative transfer modeling.
Future Directions: The authors plan to apply Non-LTE modeling to reproduce the observed Lyman $\beta$ and $\gamma$ profiles to further constrain plasma parameters and refine the understanding of the physical processes governing prominence evolution.

In summary, this paper establishes a baseline for using SPICE Lyman line diagnostics to study eruptive prominences, offering a new geometric approach to velocity estimation and revealing distinct plasma behaviors between pre-eruptive and eruptive phases.