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Vector Magnetic Field associated with an Active Region Filament Observed by SUNRISE III/SCIP in the Ca II 8542 Å Line

This paper reports the first unambiguous detection of linear polarization in the Ca II 8542 Å line associated with a solar filament using SUNRISE III/SCIP, revealing a vector magnetic field structure of approximately -80 G along the line of sight and 300–500 G in the transverse direction that is nearly parallel to the filament axis.

Original authors: Takuma Matsumoto, Yukio Katsukawa, Masahito Kubo, Yusuke Kawabata, Takayoshi Oba, Ryohtaroh T. Ishikawa, Yoshihiro Naito, Hirohisa Hara, Toshifumi Shimizu, Fumihiro Uraguchi, Toshihiro Tsuduki, Kazuya

Published 2026-06-16

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CC BY 4.0

Original authors: Takuma Matsumoto, Yukio Katsukawa, Masahito Kubo, Yusuke Kawabata, Takayoshi Oba, Ryohtaroh T. Ishikawa, Yoshihiro Naito, Hirohisa Hara, Toshifumi Shimizu, Fumihiro Uraguchi, Toshihiro Tsuduki, Kazuya Shinoda, Tomonori Tamura, Yoshinori Suematsu, Carlos Quintero Noda, Sami K. Solanki, Andreas Lagg, Achim Gandorfer, Jose Carlos Del Toro Iniesta, Pietro Bernasconi, Thomas Berkefeld, Alex Feller, Tino L. Riethmüller, Alberto Álvarez-Herrero, H. N. Smitha, David Orozco Suárez, Bianca Grauf, Michael Carpenter, Alexander Bell, Valentín Martínez Pillet, Francisco Javier Bailén, Julian Blanco Rodríguez, Juan Sebastián Castellanos Durán, Edvarda Harnes, Johannes Hölken, Francisco A. Iglesias, Azaymi L. Siu Tapia, Hanna Strecker, Dušan Vukadinović, Pablo Santamarina Guerrero, Nour E. Raouafi

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, bubbling pot of hot plasma. Floating inside this pot are dark, cool ribbons of gas called filaments. Think of them like invisible, magnetic "fishing lines" holding up a heavy net of cool gas high above the Sun's surface. For a long time, scientists have struggled to see the invisible magnetic "fishing lines" that hold these ribbons together because the signals are so faint and the instruments are often too shaky to catch them.

This paper is like a high-definition, steady-handed detective story where the team finally gets a clear look at the invisible magnetic forces holding up one of these solar ribbons.

The High-Tech Camera

The scientists used a special instrument called SCIP, which was strapped to a giant balloon called Sunrise III. Imagine this balloon as a floating observatory that rises above the Earth's atmosphere. By going up high, the balloon avoids the "twinkling" and blurring caused by our air (like trying to take a photo of a star through a wavy heat haze). This allowed the camera to take incredibly sharp, steady pictures of the Sun's surface.

The "Magnetic Fingerprint"

To see the magnetic fields, the team didn't just take a regular photo. They used a technique called spectropolarimetry. Think of light as a wave. When light bounces off a magnetic field, it gets "twisted" in a specific way, like a corkscrew.

Circular polarization is like a corkscrew spinning left or right. This tells us about the magnetic field pointing toward or away from us.
Linear polarization is like a wave vibrating side-to-side or up-and-down. This is the "holy grail" the team was looking for, as it reveals the magnetic field running across our view (sideways).

In the past, scientists had only seen the "corkscrew" (circular) signals in these filaments. The "side-to-side" (linear) signals were too weak to see, or the instruments were too shaky to distinguish them from noise.

The Big Discovery

On July 15, 2024, the team pointed their camera at a quiet, stable solar filament near the center of the Sun's disk. They looked at a specific color of light (infrared) that comes from a layer of the Sun's atmosphere called the lower chromosphere.

Here is what they found:

The First Clear Signal: They successfully detected the "side-to-side" linear polarization signals for the first time in a solar filament using this specific type of light. It's like finally hearing a whisper in a noisy room because you finally found a microphone that works perfectly.
The Magnetic Map: By analyzing these signals, they mapped the magnetic field. They found that the magnetic field inside the filament is strong (about 300 to 500 units of magnetic strength) and runs almost perfectly parallel to the ribbon itself.
- Analogy: Imagine a river flowing along a valley. The magnetic field is the riverbed, and the cool gas of the filament is the water flowing right on top of it. The water follows the path of the riverbed perfectly.
A New Layer: Previous studies used a different type of light (from Helium) to look at the top of the filament. This study used Calcium light to look at the bottom of the filament. They found that the magnetic "riverbed" at the bottom is actually quite strong and organized, which helps explain how the filament stays suspended.

Why This Matters

Before this, scientists were like people trying to understand a bridge by only looking at the very top of the arch. They knew the bridge was there, but they couldn't see the foundation. This paper gives us a clear view of the foundation (the lower atmosphere) of these solar structures.

The team concludes that the magnetic field is the "skeleton" holding the filament up. Because they could finally see the linear polarization clearly, they can now start to understand exactly how these magnetic skeletons are built and why they sometimes collapse, which can lead to massive solar storms.

In short: By using a balloon-borne camera to avoid Earth's atmospheric blur, the team finally took a clear "magnetic fingerprint" of a solar filament, proving that strong, organized magnetic fields run along the bottom of these structures, holding them in place.

Technical Summary: Vector Magnetic Field Associated with an Active Region Filament Observed by SUNRISE III/SCIP in the Ca II 8542 Å Line

Problem Statement
Solar filaments are dense, cool structures suspended in the hot corona, with their stability and eruption governed by underlying magnetic field configurations. Despite their importance in triggering coronal mass ejections, direct measurements of the magnetic field vector within filaments remain challenging. Previous investigations have primarily relied on spectral lines such as He I D3, Hα, and Hβ, with a recent shift toward the He I 10830 Å triplet. However, the He I 10830 Å line forms in the upper chromosphere and is sensitive to both Hanle and Zeeman effects, complicating interpretation. Conversely, the Ca II 8542 Å line forms in the lower chromosphere, where the Zeeman effect dominates for magnetic fields stronger than ~100 G, simplifying analysis. Despite this advantage, polarimetric observations of filaments in the Ca II 8542 Å line are scarce. Previous studies could only establish upper limits on transverse field strengths due to insufficient sensitivity or weak magnetic signals, and no clear detection of the transverse component (linear polarization) in filaments using infrared Ca II lines had been reported prior to this work.

Methodology
This study utilizes high-spatial-resolution spectropolarimetric observations obtained on July 15, 2024, using the Sunrise Chromospheric Infrared spectro-Polarimeter (SCIP) onboard the Sunrise III balloon-borne observatory. The target was a prominent active region filament located near the solar disk center, adjacent to NOAA Active Region 13744. SCIP performed full-Stokes spectropolarimetry in the 850 nm band, capturing the chromospheric Ca II 8542 Å line (effective Landé factor $g_{eff} = 1.1$ ) and the photospheric Fe I 8468 Å line.

The data underwent standard calibration, including dark-current subtraction, flat-fielding, polarimetric correction, and cross-talk removal. The estimated noise level in the calibrated Stokes Q, U, and V signals was approximately $10^{-3}$ in units of continuum intensity. To verify calibration, the linear polarization orientation was cross-checked with simultaneous Hinode Spectro-Polarimeter observations and sunspot data. To enhance the signal-to-noise ratio (S/N), spatial binning of $4 \times 4$ pixels was applied to the linear polarization maps, reducing noise to $4 \times 10^{-4} I_{cont}$ .

Magnetic field diagnostics were performed using the Weak Field Approximation (WFA). This method assumes the Zeeman effect dominates the observed polarization and that Hanle contributions are negligible. The WFA was applied to derive the line-of-sight (LOS) magnetic field ( $B_{LOS}$ ) and the transverse magnetic field ( $B_{\perp}$ ) from the Stokes profiles. The LOS field for the Fe I line was derived using the center-of-gravity method, while WFA was used for the Ca II line variables.

Key Results

Detection of Linear Polarization: The study reports the first unambiguous detection of significant linear polarization signals associated with a solar filament in the Ca II 8542 Å line. The detected signals within the filament region exceeded the $2\sigma$ noise level and exhibited a characteristic Zeeman double-lobe spectral shape in Stokes Q and U, distinguishing them from scattering-induced polarization.
Magnetic Field Strength: Using the WFA, the derived magnetic field strength along the line of sight ( $B_{LOS}$ ) is approximately $-80$ G. The transverse magnetic field ( $B_{\perp}$ ) is estimated to be between 300 and 500 G. These values represent integrated magnetic properties along the line of sight within the formation height of the Ca II 8542 Å line, likely including contributions from both filament threads and the underlying chromosphere.
Field Orientation: The orientation of the magnetic field vector is found to be nearly parallel to the filament axis in the northeastern portion of the filament (which was fully captured within the field of view). The linear polarization azimuths align with the thread-like structures observed in the line-core intensity map.
Comparison with Photosphere: The photospheric transverse magnetic field below the filament is typically around 180 G, significantly lower than the 300–500 G derived for the chromospheric region above the filament. This suggests that the photospheric field alone cannot explain the inferred chromospheric field strength.
Spectral Signatures: The Stokes profiles show a characteristic double-lobe structure in linear polarization, consistent with the transverse Zeeman effect, while no significant central single-lobe component (indicative of scattering dominance) was detected.

Significance and Claims
The authors claim that this work constitutes the first clear detection of linear polarization associated with a solar filament using the Ca II 8542 Å line. This detection opens a new diagnostic window for probing the vector magnetic structure of solar filaments in the lower chromosphere, complementing existing He I-based diagnostics that probe the upper chromosphere.

The successful detection is attributed to the high polarimetric sensitivity and spectral fidelity of SCIP, combined with the stable, seeing-free environment of the balloon platform. The ability to perform effective spatial binning without losing critical spatial information allowed for the extraction of subtle signals that previous ground-based instruments (such as CRISP, SOLIS, or SPINOR) may have missed due to atmospheric jitter or insufficient sensitivity.

The results provide new observational constraints on filament magnetic structures, suggesting that active region filaments in the lower chromosphere may possess stronger and more coherent magnetic fields than previously inferred from He I observations or upper limits. While the authors note that the simplified assumptions of the WFA warrant cautious interpretation of absolute field strengths, the alignment of the magnetic field with the filament axis supports models of highly sheared arcades or weakly twisted flux ropes. The paper concludes that these findings demonstrate the powerful capability of high-resolution infrared spectropolarimetry for routine and quantitative measurements of magnetic fields in filaments.

The High-Tech Camera

The "Magnetic Fingerprint"

The Big Discovery

Why This Matters

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