Groundbreaking New Maps of the Sun’s Coronal Magnetic Fields

If you enjoyed this summer’s display of aurora borealis, thank the Sun’s corona. The corona is the Sun’s outer layer and is the source of most space weather, including aurorae. The aurora borealis are benign light shows, but not all space weather produces such harmless displays; some of it is dangerous and destructive.

In an effort to understand space weather and the solar corona, the National Science Foundation aimed the world’s most powerful solar telescope, the Daniel K. Inouye Solar Telescope, at the corona to map its magnetic fields.

Space weather affects Earth’s magnetosphere, ionosphere, thermosphere, and exosphere. It includes solar flares, coronal mass ejections (CME), and the solar wind.

Solar flares are powerful bursts of electromagnetic energy that can damage satellites and disrupt radio communications and are frequently associated with sunspots. CMEs are ejections of plasma from the corona that collide with the magnetosphere, causing geomagnetic storms and aurorae and, when powerful enough, disrupting power grids. The solar wind is a constant stream of charged particles that streams from the solar corona and causes aurorae. Since the solar wind never stops, it can also change the orbit of satellites.

The solar corona is made of plasma, and though it’s quite dim, it’s very hot.

This image shows the Sun’s layers in false colour for clarity. Solar prominences are precursors to CMEs, though not all prominences escape the corona to become CMEs. Image Credit: By Kelvinsong – Own work, CC BY-SA 3.0, https://ift.tt/hqswmlI

Scientists know the large role the solar corona plays in space weather, but they don’t understand how the Sun’s magnetic fields drive it. However, the Daniel K. Inouye Solar Telescope (DKIST) has successfully mapped the corona’s magnetic field for the first time. Understanding the magnetic field is critical for understanding and predicting space weather.

The results are in a new paper titled “Mapping the Sun’s coronal magnetic field using the Zeeman effect.” It’s published in the journal Science Advances, and the lead author is Thomas Schad, an associate astronomer at the National Solar Observatory, the organization that operates the DKIST.

“This breakthrough promises to significantly enhance our understanding of the solar atmosphere and its influence on our solar system.”

Thomas Schad, NSO

Thomas Schad is the lead author of the new paper but has been working with the DKIST for several years. In a 2023 paper, Schad and his co-authors explained that “The possibility of measuring coronal magnetic fields from the Zeeman-effect-induced circular polarization has been a generational goal for understanding the Sun’s outer atmosphere.”

The National Science Foundation’s (NSF) Daniel K. Inouye Solar Telescope is a four-meter solar telescope on the island of Maui, Hawai’i. It’s the largest solar telescope in the world. Image Credit: National Solar Observatory.

To do this, DKIST relies on one of its primary instruments, the Cryogenic Near-Infrared Spectropolarimeter (cryo-NIRSP). The Cryo-NIRSP is uniquely suited for polarimetric observations of the solar corona. In 2023, Schad and his co-authors explained that “One of the main Cryo-NIRSP goals is to routinely and sensitively measure coronal intensities, velocities, densities, and magnetic fields with unprecedented temporal, spatial, and polarimetric resolution.”

The Zeeman effect allows the DKIST to measure the fields by observing spectral line splitting. Spectral lines are like ‘fingerprints,’ and they result from either the absorption or emission of light by specific atoms or molecules. In the presence of a static magnetic field, spectral lines are split. The splitting gives researchers insight into the Sun’s magnetic properties.

Astronomers have attempted to study the Zeeman effect and spectral line splitting in the past, but the observations lacked detail and regularity. The DKIST has changed that.

The problem with observing the Sun’s corona is its faintness compared to the rest of the Sun. The corona is about one million times fainter than the solar disk, and the corona was only observable during a solar eclipse. The DKIST uses coronagraphy to create artificial eclipses, bringing the corona into view. That lets the telescope see the extremely faint polarized signals, which are a staggering one billion times fainter than the disk.

“The Inouye’s achievement in mapping the Sun’s coronal magnetic fields is a testament to the innovative design and capabilities of this trailblazing unique observatory,” said Schad. “This breakthrough promises to significantly enhance our understanding of the solar atmosphere and its influence on our solar system.”

This figure illustrates some of the research’s results. The top panel is a composite image from the Solar Dynamics Observatory and its Atmospheric Image Assembly, and the bottom panel is from DKIST. The black dotted lines show solar radii. Together, the images show that polarization amplitude increases inside the dense coronal structures above the surface of the corona. ?B stands for Bohr magneton, a way of expressing the strength of a magnetic field in units. DN/s stands for Data Numbers per second, a way of measuring changes in solar activity over time. Image Credit: Schad et al. 2024.

Coronal Mass Ejections are the most dangerous type of space weather. Earth’s magnetosphere has a protective effect, but CMEs can slam into it and overwhelm it, creating a geomagnetic storm. The most powerful geomagnetic storm we know of is the Carrington Event of 1859. At that time, the USA’s telegraph was new, and the storm disabled parts of it. It also started fires and injured some people.

In our modern satellite age, a storm that powerful could be devastating. If we can predict them, we can harden our satellites and power grids and minimize the effects. By understanding how the Sun’s coronal magnetic fields work, scientists hope to be able to anticipate when a powerful CME is coming our way.

“Just as detailed maps of the Earth’s surface and atmosphere have enabled more accurate weather prediction, this thrillingly complete map of the magnetic fields in the sun’s corona will help us better predict solar storms and space weather,” said Dr. Carrie Black, NSF program director for the NSO. “The invisible yet phenomenally powerful forces captured in this map will propel solar physics through the next century and beyond.”

The overplotted lines in this figure from the research show the direction of linear polarization in the Sun’s corona. The scale on the right shows the percentage of polarized amplitudes of the magnetic lines. Image Credit: Schad et al. 2024.

“Reconstructing the 3D distribution of coronal plasma and its embedded magnetic stresses remains essential for understanding coronal energetics,” the authors explain in their research. “These first reported maps of the coronal Zeeman effect, made possible by DKIST, unveil the wealth of information that polarimetric diagnostics provide for the solar corona, particularly for its key driver: the magnetic field.”

These results go beyond just the Sun and local space weather. This detailed knowledge will build our understanding of stars in general.

“Mapping the strength of the magnetic field in the corona is a fundamental scientific breakthrough, not just for solar research, but for astronomy in general,” said NSO Director Christoph Keller. “This is the beginning of a new era where we will understand how the magnetic fields of stars affect planets, here in our own solar system and in the thousands of exoplanetary systems that we now know about.”

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