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Aditya-L1/VELC observations of CME associated broadening of 5303Ã coronal emission line

Using Aditya-L1/VELC observations, this study demonstrates that the broadening of the 5303 Å coronal emission line before and after CMEs is primarily driven by turbulence, with further line width increases post-CME attributed to enhanced turbulence from coronal dimmings and magnetic field reconfigurations.

Original authors: K. Sasikumar Raja, V. Muthu Priyal, R. Ramesh, Jagdev Singh, Prasad Subramanian

Published 2026-07-07

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CC BY 4.0

Original authors: K. Sasikumar Raja, V. Muthu Priyal, R. Ramesh, Jagdev Singh, Prasad Subramanian

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's outer atmosphere, the corona, as a giant, invisible ocean of super-hot gas. Usually, this gas is relatively calm, but sometimes, massive bubbles of this gas burst outward into space. These are called Coronal Mass Ejections (CMEs). Think of a CME like a massive underwater landslide or a tsunami wave crashing through the solar atmosphere.

This paper is a report from India's first space mission dedicated to watching the Sun, called Aditya-L1. Specifically, it focuses on a special camera on board called VELC. You can think of VELC as a high-tech "stethoscope" for the Sun. Instead of listening to heartbeats, it listens to the "voice" of the Sun's gas by measuring a specific color of light (green light, or 5303Å) that the corona emits.

Here is what the scientists found, explained simply:

1. The "Sit and Stare" Strategy

Most cameras take a quick snapshot and move on. But VELC has a special mode called "Sit & Stare." Imagine a photographer who sets up a camera on a tripod and watches a specific spot on the Sun for 10 hours straight, without moving. This allowed the team to watch the Sun's atmosphere evolve in real-time during two specific days: July 16, 2024, and August 5, 2024.

2. The "Blurry" Light

When the scientists looked at the light coming from the Sun, they measured the width of the green light line.

The Analogy: Imagine a laser pointer. If the beam is perfectly sharp and thin, the gas is calm. If the beam is fuzzy, wide, or blurry, it means the gas is moving chaotically.
The Finding: Even before the big explosions (CMEs) happened, the light was already a bit blurry (wider than expected for just heat). This suggested the solar atmosphere was already "turbulent," like a pot of water that is already simmering before it boils.

3. The Explosion Makes it Worse

When the CMEs happened (the solar "landslides"), the light got even blurrier.

On July 16, the blur increased by about 15%.
On August 5, it increased by about 7%.

The scientists noticed that right after the explosion, the area where the CME started became darker (a "coronal dimming"). It's like when a crowd of people suddenly runs away from a spot, leaving it empty and dark. The scientists found that as this darkening happened, the "chaos" (turbulence) in the gas got much stronger.

4. The "Fingerprint" of Chaos

To prove this was actually turbulence and not just random noise, the scientists used a mathematical tool called Power Spectral Density (PSD).

The Analogy: Think of a forest. If you listen to the wind, you hear a specific pattern of rustling leaves. If you listen to a storm, the pattern changes. Scientists have a "rulebook" for how wind and water move. One famous rule, called the Kolmogorov slope, predicts exactly how energy moves in a turbulent fluid (like water in a river or gas in the Sun).
The Result: The "fingerprint" of the Sun's gas movement matched this rulebook almost perfectly. Whether it was before or after the explosion, the pattern of the chaos followed the same mathematical rule. This confirmed that the blurriness was indeed caused by turbulence.

5. The Big Picture

The main conclusion is that the Sun's atmosphere is naturally turbulent. However, when a massive bubble of gas (CME) explodes, it causes a "rearrangement" of the Sun's magnetic fields (like untangling a knot of headphones). This rearrangement creates even more chaos and turbulence, making the gas move faster and more wildly.

In short: The Aditya-L1 mission watched the Sun's "green light" like a traffic camera. They saw that the Sun's atmosphere is always a bit chaotic, but when a solar explosion happens, the chaos gets significantly worse, creating a "turbulent storm" in the solar wind. This helps us understand how the Sun behaves, which is crucial for understanding space weather.

Technical Summary: Aditya-L1/VELC Observations of CME-Associated Broadening of the 5303Å Coronal Emission Line

Problem Statement
Understanding the mechanisms driving non-thermal broadening in solar coronal emission lines is critical for diagnosing plasma turbulence and magnetic reconnection processes. While previous studies have identified turbulence in post-CME current sheets and flare regions using instruments like UVCS and IRIS, there is a need for high-cadence, in-situ spectroscopic observations of the inner corona (1.05–1.5 $R_\odot$ ) to characterize the spectral properties of turbulence associated with Coronal Mass Ejections (CMEs) and the resulting coronal dimmings. Specifically, the paper investigates whether the observed broadening of the 5303Å (green line) emission line before and after CMEs is driven by turbulence and how the energy distribution of these fluctuations changes during these events.

Methodology
The study utilizes spectroscopic data from the Visible Emission Line Coronagraph (VELC), the primary payload of the Aditya-L1 mission. Observations were conducted in "sit & stare" mode on two specific dates:

16 July 2024: Continuous observation from 12:30 to 22:30 UT with the slit positioned at 1.05 $R_\odot$ on the west limb.
05 August 2024: Continuous observation from 01:15 to 22:15 UT with the slit positioned at 1.12 $R_\odot$ on the west limb.

Data processing involved dark subtraction, flat-field correction, curvature correction, and background subtraction. Emission profiles were extracted and fitted with Gaussian functions to derive peak intensities, line widths, and Doppler velocities. To isolate the turbulent signal, the authors removed known periodicities (13 and 33 minutes) attributed to spacecraft pointing issues via Fourier filtering. Power Spectral Density (PSD) analysis was then performed on the time series of the line widths and Doppler velocities to determine the spectral slopes before and after the CME onsets.

Key Results

Line Width Broadening: In both events, the emission line widths were already broader than thermal values (calculated for a formation temperature of ~1.8 MK) during the pre-CME phase. Following the CME onset, the mean line widths increased further by approximately 15% (16 July) and 7% (05 August).
Doppler Velocity Changes: Significant increases in Doppler velocities were observed post-CME. For the 16 July event, the velocity shifted from $-0.63 \pm 1.08$ km/s to $12.94 \pm 4.75$ km/s. For the 05 August event, it shifted from $-0.65 \pm 0.67$ km/s to $3.49 \pm 2.56$ km/s.
Power Spectral Density (PSD) Analysis: The PSD distributions of the line widths exhibited a power-law behavior.
- 16 July Event: The PSD slope was $-1.60 \pm 0.29$ before the CME and $-1.47 \pm 0.14$ after.
- 05 August Event: The PSD slope was $-1.42 \pm 0.07$ before the CME and $-1.33 \pm 0.09$ after.
- These slopes are consistent with the Kolmogorov turbulence slope of $-5/3$ ( $\approx -1.67$ ) and fall within the range of spectral indices reported for interplanetary CMEs and solar wind fluctuations.
Doppler Velocity PSD: For the 16 July event, the PSD slopes for Doppler velocities were $-1.98 \pm 0.31$ (pre-CME) and $-1.89 \pm 0.16$ (post-CME), aligning with reported ranges for solar wind velocity fluctuations ( $-5/3$ to $-2$).

Significance and Claims
The authors claim that the observed line widths, which exceed thermal values both before and after CMEs, are primarily attributable to turbulence. The consistency of the PSD slopes with the Kolmogorov slope suggests that the underlying spatial spectrum of turbulent fluctuations follows a Kolmogorov-type cascade.

The paper posits that the additional broadening observed after the CME onset is likely caused by enhanced turbulence. This enhancement is attributed to the coronal dimming associated with the CME and the subsequent reconfiguration of the coronal magnetic field. The study supports the view that magnetic reconnection and turbulence are closely linked, with the reconfiguration of magnetic field lines driving increased non-thermal motions.

The authors modestly note that while the slopes align with Kolmogorov turbulence, the variation in measured slopes ($-1.33$ to $-1.98$) could also imply contributions from Iroshnikov-Kraichnan turbulence or shock turbulence. Furthermore, the shallower slopes observed in the 05 August event compared to the 16 July event are acknowledged as potentially being influenced by data noise. The paper concludes that these findings provide valuable insights into the inner solar corona's turbulent dynamics during CME events, utilizing the unique capabilities of the Aditya-L1/VELC instrument.

1. The "Sit and Stare" Strategy

2. The "Blurry" Light

3. The Explosion Makes it Worse

4. The "Fingerprint" of Chaos

5. The Big Picture

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