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Signatures of photospheric convection throughout the solar atmosphere: the EVE Sun-as-a-star mHz continuum

Using high signal-to-noise "Sun-as-a-Star" observations from the SDO/EVE instrument, this study reveals a broadband Doppler power continuum driven by photospheric convection throughout the solar atmosphere, demonstrating that while non-thermal velocities are consistent with previous microturbulence estimates, there is no evidence for Kolmogorov turbulence and that coronal lines exhibit significantly less granulation-scale Doppler amplitude than chromospheric lines.

Original authors: Hugh Hudson, Anne-Marie Broomhall, Lyndsay Fletcher, David Graham, Sargam M. Mulay, Chris Osborne, Kirsty Williamson, Graham Woan

Published 2026-07-07

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CC BY 4.0

Original authors: Hugh Hudson, Anne-Marie Broomhall, Lyndsay Fletcher, David Graham, Sargam M. Mulay, Chris Osborne, Kirsty Williamson, Graham Woan

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 not as a static, burning ball of gas, but as a giant, churning pot of soup. Deep down, in the visible surface layer (the photosphere), this soup is boiling with convection currents—hot bubbles rising and cool ones sinking. These bubbles, called "granules," are constantly moving, creating a rhythmic, churning motion.

This paper is like listening to the "sound" of that boiling pot, but instead of hearing it with ears, the scientists are listening with a special space telescope called EVE (Extreme ultraviolet Variability Experiment) aboard the Solar Dynamics Observatory. They are looking at the Sun as a single, whole star ("Sun-as-a-star") rather than zooming in on specific spots.

Here is a breakdown of what they found, using simple analogies:

1. The "Boiling Pot" Echoes Upward

Usually, when we think of the Sun's boiling surface, we think of it staying down there. But this study shows that the churning motion of the surface bubbles sends ripples all the way up through the Sun's atmosphere, reaching high into the hot, thin gas of the corona (the Sun's outer atmosphere).

Think of it like a drum. If you hit the drumhead (the surface), the vibration travels up the drumstick and into the air above it. The scientists measured how fast the gas in the upper atmosphere was wobbling back and forth (Doppler shifts) to see if it was still feeling the "beat" of the surface bubbles.

2. The "Harvey" Pattern vs. The "Turbulence" Myth

The scientists were looking for a specific type of chaotic motion called Kolmogorov turbulence. In physics, this is like the swirling, chaotic eddies you see in a fast-flowing river or smoke rising from a candle. If the Sun's atmosphere was full of this kind of turbulence, the energy of the motion would follow a very specific, flat mathematical rule (like a flat road).

What they found instead:
The data did not show this flat, turbulent road. Instead, it showed a pattern that looks like a rolling hill that gets steeper and steeper as you go faster.

The Analogy: Imagine a music equalizer. If the Sun were full of Kolmogorov turbulence, the bars on the equalizer would be flat across the high notes. Instead, the bars drop off sharply, like a steep slide.
The Result: The motion they see is best described by "Harvey-like" components. This is a mathematical way of saying the motion is driven by the specific size of the surface bubbles (granules). The energy drops off quickly at high speeds, meaning the "chaos" isn't the kind of swirling turbulence we might have expected.

3. The "Fading Echo" at Higher Altitudes

The researchers looked at different layers of the Sun's atmosphere, from the lower "transition region" up to the high "corona."

The Finding: The "boiling" motion (the granulation) is very loud and clear in the lower layers. But as you go higher up into the corona, the echo of that surface boiling gets much quieter.
The Analogy: It's like standing near a waterfall (the surface). You feel the mist and hear the roar. If you climb a ladder 100 feet up, you still feel a little bit of the spray, but the roar is much fainter. The study found that the high-altitude gas is moving much less violently than the gas closer to the surface.

4. No "P-Mode" Singing

The Sun has a famous "song" called p-modes (sound waves that bounce around inside the Sun). These are like the resonant notes of a bell.

The Finding: While these notes are very loud in the surface data, the scientists could not hear them clearly in the high-altitude EUV data.
The Analogy: It's like trying to hear a specific bell tone from inside a thick, sound-proof wall. The wall (the lower atmosphere) blocks the sound from getting through to the outside air (the corona) in a way that the telescope could detect.

5. How Fast is the Gas Moving?

Even though the "turbulence" wasn't the chaotic kind they looked for, the gas is still moving fast.

The Result: The total speed of this wobbling gas is about 15 kilometers per second (roughly 33,000 mph).
The Context: This matches what other scientists have guessed before by looking at how "fuzzy" the Sun's light lines are. It confirms that the Sun's upper atmosphere is a very active, fast-moving place, even if it's not "turbulent" in the way we usually imagine.

Summary

In short, this paper used a powerful space telescope to listen to the "heartbeat" of the Sun's entire atmosphere. They discovered that the churning motion of the Sun's surface bubbles sends ripples all the way to the top, but these ripples fade out as they go higher. They also proved that the high-speed motion in the upper atmosphere is not the result of chaotic, swirling turbulence (Kolmogorov), but rather a structured, fading echo of the surface boiling. This gives us a clearer picture of how energy travels from the Sun's surface into its hot outer atmosphere.

Technical Summary: Signatures of Photospheric Convection Throughout the Solar Atmosphere: The EVE Sun-as-a-Star mHz Continuum

Problem and Context
Convectively driven motions in the solar photosphere generate broadband Doppler variability that propagates through the chromosphere, transition region, and corona. While the power spectrum of photospheric convection is well-characterized by "Sun-as-a-star" observations of Total Solar Irradiance (TSI) and visible light (e.g., via the Harvey 1985 model), the vertical transmission of these motions into the upper solar atmosphere remains less understood in the Extreme Ultraviolet (EUV) regime. Previous studies have tracked convective continua into the corona via imaging, but a global, spectroscopic view of Doppler variability across the mHz frequency range (specifically above the p-mode band) has been lacking. This study addresses the need to characterize the broadband power-spectral continuum in the EUV to understand how photospheric energy and momentum are transported upward.

Methodology
The authors analyzed data from the Extreme Ultraviolet Variability Experiment (EVE) aboard the Solar Dynamics Observatory (SDO), specifically utilizing the Multiple EUV Grating Spectrograph B (MEGS-B) which covers the 35–104 nm wavelength range.

Data Selection: The study focused on daily 3-hour contiguous data chunks from January 2011, selected for high data quality.
Spectral Analysis: For 26 specific emission lines (spanning ions of H, He, O, Mg, Ne, Si, C, N, and Fe), the authors fitted Gaussian profiles to 10-second spectra to extract centroid wavelengths. These centroids were converted into Doppler velocity residuals relative to the median of each 3-hour chunk.
Power Spectral Density (PSD): The authors computed incoherent-sum power spectra from these velocity residuals. They averaged spectra from 231 valid 3-hour chunks to achieve high signal-to-noise ratios, extending the analysis up to the Nyquist frequency of 50 mHz.
Comparative Baseline: The EVE results were benchmarked against "ground truth" Helioseismic and Magnetic Imager (HMI) line-of-sight velocity data to contextualize the photospheric driver.
Modeling Approach: The spectra were analyzed for Harvey-like components (Lorentzian-like functions describing convective scales) and tested against the $f^{-5/3}$ power law expected from Kolmogorov turbulence.

Key Results

Broadband Continuum Detection: The study presents the first clear detection of Sun-as-a-star EUV Doppler variability above $\sim$ 10 mHz. The power spectra reveal a broad continuum with two Harvey-like components. One component extends to the Nyquist frequency (50 mHz) with a steep power-law tail.
Granulation Signature: A distinct broad maximum is observed at approximately 5 mHz across all lines, corresponding to granulation-scale convection.
Temperature Dependence: There is a systematic decrease in Doppler amplitude at higher frequencies (specifically the 5 mHz Harvey component) as the line formation temperature increases. Lines formed in the corona exhibit substantially less Doppler amplitude in the granulation band compared to chromospheric/transition region lines.
Absence of Kolmogorov Turbulence: The observed continuum does not exhibit the flat component predicted by Kolmogorov turbulence ( $\langle v^2 \rangle \propto f^{-5/3}$ ). By fitting a Kolmogorov law to the high-frequency "noise" band (45–50 mHz), the authors derive an upper limit for the ratio of turbulent variance to total variance of $W_{turb}/W_{tot} \leq 10^{-5}$ .
Non-Thermal Velocities: The total inferred non-thermal RMS velocities (integrated above 0.1 mHz) are approximately 15 km/s. This is consistent with previous estimates of coronal "microturbulence" derived from non-thermal line widths.
p-mode Detection: No distinct p-mode peaks are resolved in the EUV power spectra at the current frequency resolution, consistent with the expectation that acoustic waves are evanescent above the photosphere. The continuum below 5 mHz does not resemble the p-mode envelope seen in HMI data.
FIP Bias: A comparison of Mg VII (low First Ionization Potential) and Ne VIII (high FIP) lines shows significant differences in power spectra. However, the authors attribute the Ne line behavior to its high-temperature contribution function (coronal dominance) rather than a detectable FIP bias in the power spectrum, as dynamical effects appear to mask any fractionation signatures.

Significance and Claims
The paper claims to provide the first comprehensive, global view of EUV Doppler variability driven by photospheric convection across the mHz band. By demonstrating that the high-frequency EUV power spectrum follows a steep power law rather than a Kolmogorov turbulence spectrum, the study places stringent upper limits on the amplitude of coronal turbulence. The authors assert that these observations validate the potential of full-disk EUV spectroscopy to probe turbulent energy transport throughout the solar atmosphere.

The study remains modest regarding quantitative interpretation, noting that without a global Sun-as-a-star modeling framework that accounts for optical depth and contribution functions, the precise physical mechanisms driving the observed line-to-line differences (particularly in the Lyman series and FIP pairs) cannot be fully resolved. The work serves as a foundational assessment of EVE data, establishing a high-precision baseline for future studies of solar-cycle properties, rotational modulation, and detailed turbulence modeling.