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Astrophysicists solve the mystery of sunspot creation—and much more

A team of solar scientists has made a breakthrough in understanding the engine that drives much of the sun’s volatile behavior
A team of solar scientists has made a breakthrough in understanding the engine that drives much of the sun’s volatile behavior. (CREDIT: Creative Commons)

A team of solar scientists has made a breakthrough in understanding the engine that drives much of the sun’s volatile behavior, leading to the formation of sunspots and causing the sun’s activity to fluctuate in 11-year cycles. This discovery sheds light on one of the oldest unsolved problems in physics: the solar dynamo.

Benjamin Brown, a solar physicist at CU Boulder, calls the solar dynamo "one of the oldest unsolved problems of physics." In recent research, Brown and his colleagues used mathematical equations to simulate the sun's behavior. They propose that the dynamo begins in the star’s outer layers, contrary to the long-held belief that it originates deep within the sun.


This quest to understand the solar dynamo dates back to Galileo Galilei, who first observed sunspots in 1612. “Galileo first observed the sunspots 400 years ago, and he learned quite a bit about them, including how they move over the sun’s surface,” said Brown. “But he couldn’t figure out where they came from. We’ve struggled with the question ever since.”

The team, led by Geoffrey Vasil of the University of Edinburgh, published their findings in the journal Nature. Among the co-authors was Keith Julien, an applied mathematician from CU Boulder who passed away recently.


The solar dynamo, a complex interplay of physics and chemistry, generates the sun’s magnetic fields. Earth has its own dynamo that powers its magnetic field, which is why compasses point north. Brown emphasizes the importance of understanding the sun’s dynamo as it influences solar storms that can disrupt power grids and create auroras.

This study is the culmination of decades of work. “Geoff Vasil and I have been thinking about these ideas ever since we were both grad students at CU Boulder 20 years ago,” Brown said.


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Scientists generally agree that the solar dynamo begins in the sun’s convection zone, the outer third of its interior. In this zone, hot, charged particles, known as plasma, rise to the surface. Unlike Earth’s mostly uniform magnetic field, the sun’s plasma creates a chaotic, twisted magnetic field that resembles a bowl of noodles.

Despite this chaos, the solar dynamo exhibits surprisingly predictable behavior. Every 11 years, the sun transitions from a period of low activity, with fewer sunspots, to a period of high activity, before resetting. “You could practically set a calendar to the solar dynamo,” Brown noted. “How is it so wild yet also so orderly?”


Investigating the Dynamo’s Origins

In the 1990s, scientists suggested that the dynamo originated about 130,000 miles below the sun’s surface, a theory known as the “dynamo in the deep.” However, this theory struggles to explain the solar dynamo's predictability. Vasil, Brown, and their colleagues turned to a phenomenon called “magnetorotational instability,” which occurs when magnetic fields interact with rotating plasmas that move faster as you go deeper.

“It’s kind of like dance partners slinging each other around in a spin while holding arms,” Brown explained.

While this phenomenon is well-studied in the disks of hot gases around black holes, its role in the sun is less clear. The researchers conducted computer simulations to explore how this instability could impact the sun’s activity. They found that it could indeed drive the solar dynamo, explaining the 11-year cycles. This process occurs in the outer 10% of the sun, about 20,000 miles from the surface, suggesting that the dynamo is relatively shallow.


Future Research and Legacy

Although there is still work to be done to fully disprove the “dynamo in the deep” theory, Brown is optimistic that their study will spark new research in the field. Vasil emphasized the importance of their late colleague, Keith Julien, in their work.

“My advisors and mentors were Nic Brummell, Juri Toomre, and Keith Julien,” Vasil recalled. “I had a ‘huh, that’s funny’ moment about the sun’s near-surface instability in 2004 while reading an astrophysics textbook. Keith was the first person I ran to tell. He supported ideas and gave encouragement to a generation of young researchers. It’s astonishing he won’t be here anymore. But he was thrilled this work was published in Nature. His ideas and personality will live on in the many people he inspired.”


This new understanding of the solar dynamo not only honors the legacy of past scientists but also opens doors for future discoveries, ensuring that the study of our sun continues to evolve.

For more science and technology news stories check out our New Discoveries section at The Brighter Side of News.


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