🔭 astrophysics

Full-Disk Spectroscopy of the Solar Corona Across a Solar Cycle with Hinode/EIS

By analyzing 18 full-disk spectroscopic scans from Hinode/EIS spanning solar cycles 24 and 25, this study reveals that disk-integrated coronal intensity variability is driven primarily by the changing fraction of the solar disk occupied by active regions, while the intrinsic plasma properties (Doppler and non-thermal velocities) of both active regions and the quiet Sun remain largely insensitive to the solar cycle.

Original authors: James McKevitt, Ignacio Ugarte-Urra, Peter R. Young

Published 2026-06-04

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CC BY 4.0

Original authors: James McKevitt, Ignacio Ugarte-Urra, Peter R. Young

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 giant, glowing stage. For over a decade, astronomers have been trying to understand how the "actors" on this stage change their performance as the show goes through its 11-year cycle. Sometimes the stage is quiet and calm (solar minimum), and sometimes it's chaotic and crowded with dramatic scenes (solar maximum).

The big question this paper asks is: When we look at the Sun from far away (like we do with other stars), does the whole show get louder because the actors are shouting louder, or simply because there are more actors on stage?

Here is the story of what James McKevitt and his team discovered, using a special "mosaic" of observations from the Hinode satellite.

1. The Camera and the Puzzle

The Sun is huge, but the camera on the Hinode satellite (called EIS) is like a high-powered magnifying glass with a very narrow view. It can see tiny details, but it can only see a small patch of the Sun at a time.

To get a picture of the entire Sun, the team had to take 18 different "snapshots" over 11 years (from 2013 to 2024) and stitch them together like a giant jigsaw puzzle. This gave them a full-disk view of the Sun's atmosphere (the corona) at different points in its life cycle. They focused on a specific type of super-hot gas (plasma) that glows in a specific color of light.

2. The Two Main Findings

Finding A: The "Volume" Goes Up and Down with the Crowd

When the team looked at the total amount of light coming from the entire Sun (as if they were a distant star-watcher), they found a perfect match with the solar cycle.

The Analogy: Imagine a stadium. When the game is exciting (solar maximum), the crowd is loud. When the game is boring (solar minimum), the crowd is quiet.
The Result: The Sun gets much brighter in the corona when the solar cycle is at its peak. This confirms what we see with other stars: more activity means more light.

Finding B: The Actors Don't Change Their Scripts

Here is the surprising part. The team looked closely at the specific "actors" (the active regions where the magnetic storms happen) and the "background extras" (the quiet Sun). They measured two things:

Doppler Velocity: How fast the gas is moving toward or away from us.
Non-thermal Velocity: How much the gas is jittering or swirling (like turbulence).

The Analogy: Imagine a group of actors. You might expect that during the "exciting" part of the show, they would start shouting louder or moving faster. But the team found that they didn't.
The Result: Whether it was the quiet part of the cycle or the chaotic part, the gas in the active regions moved at the same speeds and jittered with the same amount of energy. The "script" for how the gas behaves didn't change.

3. The Big Conclusion: It's About the "Fill Factor"

So, why does the Sun get brighter?

The paper concludes that the Sun gets brighter not because the individual storms become more violent or the gas gets hotter, but simply because there are more storms on the stage.

The Metaphor: Think of the Sun as a room full of lightbulbs.
- Old Theory: Maybe the lightbulbs get brighter during the peak of the cycle?
- New Finding: No, the lightbulbs stay the same brightness. But during the peak cycle, the room is filled with more lightbulbs. During the quiet cycle, most of the bulbs are turned off.

The team calls this the "filling factor." The variability we see from Earth (or from other stars) is driven by how much of the Sun's surface is covered by these active regions, not by changes in the physics of the regions themselves.

4. A Small Mystery

The team did notice one small quirk. When they looked at the brightness of the active regions per unit of area (how bright a single storm is, regardless of how many there are), it was slightly brighter during the peak of the cycle.

They aren't 100% sure why yet. It might be that the storms near the peak of the cycle are just naturally more intense because the Sun's magnetic field is stronger overall, or perhaps some "fuzzy" light from the edges of the storms is being counted. But this is a minor detail compared to the main discovery.

Summary

In simple terms: The Sun's 11-year cycle is a story of quantity, not quality. The gas in the Sun's atmosphere behaves the same way whether the cycle is high or low. The only thing that changes is how much of the Sun is covered by these active regions. When the Sun is "busy," it's just because there are more busy spots, not because the spots themselves are doing anything different.

This helps astronomers understand other stars: if we see a star getting brighter and dimmer, it's likely because the number of "sunspots" on that star is changing, not because the physics of the star's surface is fundamentally shifting.

Technical Summary: Full-Disk Spectroscopy of the Solar Corona Across a Solar Cycle with Hinode/EIS

Problem Statement
The structure and dynamics of the solar corona are governed by the Sun's 11-year magnetic cycle. While the evolution of the global magnetic field from a dipolar minimum to a complex multipolar maximum is well understood, how this variability manifests in disk-integrated, "Sun-as-a-star" observables remains poorly constrained. Stellar activity studies rely on integrated spectra, yet the specific contributions of active regions versus the quiet Sun to these integrated signals are difficult to disentangle without spatial resolution. Previous work using broadband imaging and Soft X-ray Differential Emission Measure (DEM) analysis suggested that solar-cycle variability in stellar coronae is driven primarily by the changing fraction of the disk occupied by active regions (the filling factor), rather than intrinsic changes in the plasma properties of those regions. However, this hypothesis has not been rigorously tested using high-resolution spectroscopic data capable of diagnosing plasma kinematics (Doppler velocities) and non-thermal broadening across the full solar disk.

Methodology
To address this, the authors compiled and analyzed 18 full-disk spectroscopic mosaic scans obtained by the Hinode/EUV Imaging Spectrometer (Hinode/EIS) between 2013 and 2024. This dataset spans the declining phase of solar cycle 24, the minimum, and the rise of solar cycle 25.

Observations: The study utilized narrow-slit (2 arcsec) mosaic scans with a 4 arcsec step size (sparse rastering) to complete full-disk coverage within a 48-hour window.
Data Processing: The authors employed the EISMaps pipeline to assemble the mosaics. Spectra were corrected for instrument effects and radiometrically calibrated using Del Zanna et al. (2025).
Spectral Analysis: The analysis focused on the Fe XII 195.119 Å line (log T [K] ∼6.2), which provided sufficient signal-to-noise for deriving non-thermal velocities. Gaussian profiles were fitted using the MPFIT algorithm within the EISPAC code. Two-component fits were used for Fe XII to account for blending with the 195.179 Å line.
Feature Classification: Active regions were identified using Space-weather HMI Active Region Patches (SHARP), while coronal holes were located via multi-wavelength imaging and a neural network method. The "quiet Sun" was defined as disk pixels not classified as active regions or coronal holes. Pixels near the limb ( $\theta > 85^\circ$ ) were excluded to minimize Doppler cosine correction artifacts.
Metrics: The study derived spatially resolved maps and disk-integrated values for intensity, Doppler velocity, and non-thermal velocity ( $v_{nt}$ ).

Key Contributions
This work provides the first comprehensive, long-term spectroscopic survey of the full solar disk covering a significant portion of a solar cycle. Unlike previous studies relying on broadband imaging or X-ray DEMs, this dataset offers direct diagnostics of plasma motions and wave activity. The authors release processed full-disk maps of intensity, Doppler velocity, and non-thermal velocity for log T [K] ∼4.7–7.1 plasma, alongside a specific analysis of the log T [K] ∼6.2 component.

Results

Disk-Integrated Intensity: The total disk-integrated coronal intensity shows a strong positive correlation with the solar cycle ( $r=0.91$ ), consistent with stellar observations. This variability tracks the sunspot number closely.
Plasma Distributions (Velocity and Non-thermal Broadening):
- Full Disk & Quiet Sun: The distributions of Doppler and non-thermal velocities show no clear modulation across the solar cycle. The velocity centroid remains stable at $\sim 0$ km/s, and the spread of velocities is consistent throughout the cycle.
- Active Regions: A strong dependence of Doppler velocity on intensity is observed; high-intensity regions cluster near 0 km/s or show slight redshifts, while lower-intensity regions skew toward upflows. Crucially, the relationship between upflowing plasma and elevated non-thermal broadening persists throughout the cycle with no clear variation.
Active Region Intensity per Unit Solid Angle: While total active region intensity tracks the cycle, the intensity normalized by the active region's solid angle shows only a moderate correlation with the solar cycle ( $r=0.53$ ). This suggests that while the area of active regions drives the global signal, there may be a secondary effect where active regions near solar maximum possess higher intrinsic intensity per unit area, or that diffuse emission from the outskirts of active regions (not captured by SHARP masks) influences the normalization.

Significance and Claims
The paper concludes that the variability of disk-integrated coronal intensity across the solar cycle is driven primarily by the changing fraction of the disk occupied by active regions (the filling factor), rather than by changes in the intrinsic spectroscopic properties (kinematics and non-thermal broadening) of the plasma within those regions.

The persistence of the correlation between upflows and non-thermal broadening in active regions throughout the cycle implies that the underlying kinematic mechanisms (e.g., impulsive heating, unresolved wave activity, or reconnection) are insensitive to the global magnetic field configuration. This supports the hypothesis used in modeling X-ray activity cycles for solar analogues, where per-region plasma physics is treated as relatively fixed, and cycle variability is attributed to filling factors.

The authors note a modest caveat: the moderate correlation between normalized active region intensity and the solar cycle suggests that either the intrinsic intensity of active regions increases with cycle phase (potentially due to higher unsigned magnetic flux) or that the masking technique does not fully exclude diffuse emission. Distinguishing between these possibilities would require direct comparison with HMI magnetic flux data, which is beyond the scope of the current analysis. The study reinforces the utility of "Sun-as-a-star" spectroscopy for interpreting stellar activity cycles but highlights the need for future high-throughput spectrometers (like SOLAR-C/EUVST) to refine measurements of quiet Sun non-thermal velocities.