'Solar dynamo' and the secret origin of the Sun's magnetic field

The sun, a brilliant display of sunspots and flares, has captivated astronomers for centuries. These mesmerizing features are driven by the solar magnetic field, generated by a process called “dynamo action.”

Having long been thought to originate deep within the star’s interior, a new study challenges this long-held assumption, suggesting that the sun’s magnetic activity may be shaped by a much shallower process.

Shallower origins of solar dynamo

In a paper published in Nature, the research team, led by Keaton Burns, a research scientist in MIT’s Department of Mathematics, presents a precise model of the sun’s surface.

By simulating perturbations in the flow of plasma within the top 5–10% of the sun, they discovered that these surface changes were sufficient to generate realistic magnetic field patterns, closely resembling those observed by astronomers.

Geoffrey Vasil, a researcher at the University of Edinburgh and co-author of the study, emphasizes the importance of this new idea, stating.

“We know the dynamo acts like a giant clock with many complex interacting parts. But we don’t know many of the pieces or how they fit together. This new idea of how the solar dynamo starts is essential to understanding and predicting it,” Vasil explained.

Forecasting space weather

If the sun’s magnetic field does indeed arise from its outermost layers, it could have significant implications for space weather forecasting.

By better understanding the origins of sunspots and flares, scientists may be able to more accurately predict geomagnetic storms that have the potential to damage satellites and telecommunications systems.

“The features we see when looking at the sun, like the corona that many people saw during the recent solar eclipse, sunspots, and solar flares, are all associated with the sun’s magnetic field,” explains Keaton Burns.

“We show that isolated perturbations near the sun’s surface, far from the deeper layers, can grow over time to potentially produce the magnetic structures we see.”

Simulating sunspots and the solar dynamo

Rather than attempting to simulate the complex flow of plasma throughout the entire body of the sun, which requires millions of hours on national supercomputing facilities, Burns and his colleagues focused on studying the stability of plasma flow near the surface.

By leveraging data from helioseismology, a field that uses observed vibrations on the sun’s surface to determine the average structure and flow of plasma beneath the surface, the team was able to develop a more targeted approach.

Burns draws an analogy to understanding the shape and stiffness of a drumhead by observing its vibrational modes in slow motion. “Similarly, we can use vibrations that we see on the solar surface to infer the average structure on the inside,” he explains.

Dedalus Project: Powerful fluid dynamics tool

To search for patterns that could explain the sun’s magnetic field, the team utilized the Dedalus Project, a numerical framework developed by Burns that can simulate various types of fluid flows with high precision.

The code has been applied to a wide range of problems, from modeling the dynamics inside individual cells to ocean and atmospheric circulations.

“My collaborators have been thinking about the solar magnetism problem for years, and the capabilities of Dedalus have now reached the point where we could address it,” Burns notes.

Sunspots and the solar cycle

The team’s simulations revealed patterns that match the locations and timescales of sunspots observed by astronomers since Galileo in 1612.

Sunspots, which are relatively cooler regions that appear as dark spots on the sun’s surface, are thought to be shaped by the sun’s magnetic field and follow a cyclical pattern, growing and receding every 11 years.

The researchers found that certain changes in the flow of plasma within the top 5–10% of the sun’s surface layers were enough to generate magnetic structures in the same regions where sunspots are typically observed.

In contrast, changes in deeper layers produced less realistic solar dynamo fields that were concentrated near the poles rather than the equator.

Inspired by black hole accretion disks

The team’s motivation to examine flow patterns near the sun’s surface stemmed from the similarity between these conditions and the unstable plasma flows observed in accretion disks around black holes.

Accretion disks are massive disks of gas and stellar dust that rotate towards a black hole, driven by the “magnetorotational instability,” which generates turbulence in the flow and causes it to fall inward.

Burns and his colleagues suspected that a similar phenomenon might be at play in the sun and that the magnetorotational instability in the sun’s outermost layers could be the first step in generating the sun’s magnetic field.

While the team’s findings may be met with some controversy, as most of the scientific community has been focused on finding solar dynamo action deep within the sun, Burns believes their results provide a better match to observations.

The team continues to study whether these new surface field patterns can generate individual sunspots and the full 11-year solar cycle.

Challenging the deep solar dynamo theory

In summary, this important study challenges the long-held belief that the sun’s magnetic field originates from a solar dynamo deep within its interior.

By simulating perturbations in the flow of plasma near the sun’s surface and comparing the results with observational data, the team has shown that the sun’s magnetic field and its associated features, such as sunspots and flares, may arise from much shallower surface depths than previously thought.

This finding offers a new perspective on the fundamental processes driving solar activity, and holds promise for improving space weather forecasting and protecting vital satellite and telecommunications systems.

As the scientific community continues to debate and build upon this research, we move closer to unraveling the enigmatic nature of our nearest star and its far-reaching influence on our technology-dependent world.

The full study was published in the journal Nature.

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