🔭 astrophysics

Magnetic structure of coronal dark halos

By analyzing Solar Orbiter data of active region NOAA 12893, this study reveals that coronal dark halos are characterized by a radial decrease in magnetic flux density and distinct temperature-dependent emission patterns, suggesting that reduced heating from weak magnetic fields and specific loop structures contribute to their formation.

Original authors: J. D. Nölke, S. K. Solanki, J. Hirzberger, H. Peter, L. P. Chitta, K. H. Glassmeier, D. Calchetti, G. Valori

Published 2026-05-13

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: J. D. Nölke, S. K. Solanki, J. Hirzberger, H. Peter, L. P. Chitta, K. H. Glassmeier, D. Calchetti, G. Valori

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 surface as a bustling city. In the center of this city, there is a massive, energetic construction zone called an Active Region. This is where sunspots live, and it's a place of intense magnetic activity, like a giant power plant humming with electricity.

Surrounding this power plant, you might expect the neighborhood to be just as bright and lively. But instead, astronomers have noticed a strange phenomenon: a "dark halo" or a dim ring of fog surrounding the construction zone. In the ultraviolet light (which is like a special night-vision camera for the Sun), this ring looks significantly darker and dimmer than the quiet, peaceful neighborhoods further away.

This paper is a detective story about figuring out why this dark ring exists. The researchers used high-tech telescopes on the Solar Orbiter spacecraft and the SDO satellite to take a close-up look at the magnetic fields and the heat in this dark ring.

Here is what they found, explained simply:

1. The Temperature Twist

The most surprising discovery is that the dark ring behaves differently depending on how "hot" you look at it.

The "Cool" View (Around 1 million degrees): When looking at the Sun at temperatures around 1 million degrees (which is hot for us, but "cool" for the Sun's corona), the dark ring is indeed dark. It emits about 40% less light than the quiet Sun. It's like a street that is dimly lit compared to the bright city center.
The "Hot" View (Above 1.6 million degrees): But when they looked at even hotter temperatures, the story flipped! The dark ring actually became brighter than the quiet Sun. It was like finding that the street is actually flooded with high-intensity floodlights, but only if you use a special camera that sees super-hot light.

2. The Magnetic "Fence"

To understand why this happens, the team looked at the Sun's magnetic field, which acts like invisible scaffolding or fences that hold the solar plasma (super-hot gas) in place.

The Inner Ring: Close to the active region, the magnetic field is strong.
The Outer Ring: As you move further out into the dark halo, the magnetic field gets weaker. In fact, at the very edge of the dark ring, the magnetic field is actually weaker than in the quiet Sun neighborhoods.

3. The Loop Structure (The "Bridge" Analogy)

The researchers found that the shape of the magnetic "fences" (called loops) changes based on temperature:

At Cooler Temperatures: The loops are short and stubby, like small garden fences. Inside the dark halo, there are very few of these short fences. Without these fences to hold the gas, the gas doesn't get heated up enough to glow in the "cool" ultraviolet light. This explains why the ring looks dark at 1 million degrees.
At Hotter Temperatures: The loops are long and stretchy, like giant bridges reaching far out from the city center. These long bridges cross right over the dark halo. Because they connect to the powerful magnetic fields of the main city (the Active Region), they carry a lot of energy. This is why the halo glows brightly when looking at the super-hot temperatures.

4. Why is it Dark?

The paper suggests a few reasons for the darkness at the cooler temperatures:

Weak Power Supply: In the outer parts of the halo, the magnetic field is so weak that it simply can't generate enough heat to make the gas glow. It's like a neighborhood with weak electrical wiring that can't power the streetlights.
The "Pressing Down" Effect: Close to the active region, the strong magnetic field from the city center might be pushing the small, short loops down, keeping them too low to get hot enough to glow.

5. How is this different from a "Coronal Hole"?

The Sun also has other dark spots called Coronal Holes. These are like open doors where the magnetic field lines go straight out into space. The researchers found that dark halos are different:

Coronal Holes look dark at all temperatures because the gas escapes.
Dark Halos only look dark at cool temperatures. At hot temperatures, they actually glow brighter than the surroundings. This difference helps scientists tell them apart.

The Bottom Line

The "dark halo" isn't just a simple empty spot. It's a complex zone where the Sun's magnetic field changes its shape and strength.

Near the center, the magnetic field is strong but might be suppressing small loops.
Farther out, the magnetic field is too weak to heat the gas up.
However, giant, long magnetic bridges stretch across the whole area, carrying heat that makes the halo glow brightly if you look at it with the right "hot" eyes.

By understanding this, scientists are getting a better picture of how the Sun's magnetic field controls the heating of its atmosphere, solving a puzzle that has been around for decades.

Technical Summary: Magnetic Structure of Coronal Dark Halos

Problem and Context
Active regions (ARs) on the Sun are often surrounded by extended areas of significantly reduced extreme-ultraviolet (EUV) emission at low coronal temperatures (around or below 1 MK) compared to the quiet Sun (QS). These structures, termed "dark halos" (also known as dark canopies, emerging dimmings, or dark moats), have been observed by instruments such as SDO/AIA and Solar Orbiter/EUI. While several models exist to explain their nature—ranging from EUV-absorbing neutral material to magnetic configurations that suppress loop heights—there is no consensus on the underlying physical mechanisms. Specifically, the relationship between the photospheric magnetic field within these halos and the resulting coronal emission across different temperatures remains unclear.

Methodology
The authors utilized co-temporal observations from the Solar Orbiter mission, specifically combining data from the high-resolution Polarimetric and Helioseismic Imager (SO/PHI-HRT) and the Extreme Ultraviolet Imager (EUI-HRIEUV). These observations were taken on November 5, 2021, during a cross-calibration campaign where Solar Orbiter was nearly aligned with the Sun-Earth line. The study focused on Active Region NOAA 12893.

To analyze the magnetic and emission properties, the authors:

Defined Boundaries: Established the dark halo boundaries using a 75% intensity threshold relative to a reference QS area, based on smoothed HRIEUV images.
Spatial Analysis: Determined the geometric center of the AR and created concentric rings extending from the inner to outer boundaries of the dark halos.
Multi-Temperature Comparison: Analyzed coronal emission at various temperatures using HRIEUV (~1 MK) and SDO/AIA channels (171 Å, 193 Å, and 211 Å, corresponding to ~0.8 MK, 1.6 MK, and 2.0 MK, respectively).
Magnetic Field Characterization: Calculated the unsigned magnetic flux density ( $\langle|B_{LOS}|\rangle$ ) and magnetic flux imbalance within the dark halo patches and compared them to the QS and the brighter areas between halo patches.
Structural Imaging: Applied multi-scale Gaussian normalization to enhance contrast and identify loop structures in the corona.

Key Results

Magnetic Flux Density: The unsigned magnetic flux density within the dark halos is not uniform. It exhibits a strong radial dependence, dropping by approximately 38% from the inner boundary (6.1 G) to the outer boundary (3.8 G). While the average flux density across the entire halo is similar to the QS (5.0 G vs. 5.3 G), the outermost parts of the halo fall significantly below QS levels.
Emission at Low Temperatures ( $\le$ 1 MK): In the HRIEUV and AIA 171 Å channels, the dark halo emission is consistently $\sim$ 40% below QS levels. Crucially, this reduced emission shows no significant dependence on the distance from the AR center; it remains uniformly low despite the magnetic field strength varying from above to below QS levels across the halo.
Emission at High Temperatures ( $\ge$ 1.6 MK): In contrast to the low-temperature behavior, emission in the 193 Å and 211 Å channels exceeds QS levels near the AR and declines radially outward toward QS values.
Loop Structures: Enhanced imaging reveals distinct loop populations based on temperature. At lower temperatures ( $\sim$ 0.8 MK), the corona is dominated by short loops, which are fewer in number within the dark halo. At higher temperatures ( $\ge$ 1.6 MK), bright, long loops are observed extending radially from the AR periphery, crossing over, or terminating within the dark halo. These hot loops appear rooted in stronger magnetic patches within the halo.
Distinction from Coronal Holes (CHs): The study confirms that dark halos can be distinguished from equatorial CHs. While CHs show strong emission contrasts at temperatures above 1 MK, dark halos do not; they only appear dark at lower temperatures. One observed halo patch (DH3) overlapped with a CH, showing emission dips at high temperatures corresponding to the CH location, but otherwise behaving like a standard dark halo.

Significance and Conclusions
The paper concludes that the reduced emission in dark halos cannot be explained by a single mechanism acting uniformly across the structure. The findings suggest a dual nature:

Outer Regions: The drop in unsigned magnetic flux density below QS levels in the outer halo suggests that reduced coronal heating due to weak underlying magnetic flux may be partially responsible for the reduced emission around 1 MK, similar to the mechanism proposed for coronal voids.
Inner Regions: Closer to the AR, where magnetic flux is stronger than in the QS, the emission remains low. The authors propose that other mechanisms, such as suppressed horizontal flows (reducing Poynting flux) or specific magnetic configurations that restrict loop heights, may be responsible for the reduced heating in these inner regions.

The study emphasizes that the different loop structures detected for hotter and cooler coronal temperatures play a crucial role in understanding dark halos. The authors argue that future studies should not rely solely on mean values averaged over entire halos but must account for the radial dependence of both magnetic fields and emission. Furthermore, the distinct behavior of dark halos at high temperatures compared to CHs provides a clear observational criterion for distinguishing between these EUV-dark structures.