Solar events such as sunspots and flares may be the result of a shallow magnetic field, according to surprising new findings that could help scientists predict space weather. The Sun’s surface is dazzled with sunspots and flares that are products of the Sun’s magnetic field, created by a mechanism known as a dynamo. Traditionally, astrophysicists believed that this magnetic field formed deep within the star. However, research from the Massachusetts Institute of Technology shows that these events may have actually occurred through a much smaller process.

Published in the magazine today (May 22) Nature Research by teams from the Massachusetts Institute of Technology, the University of Edinburgh and other institutions suggests that the Sun’s magnetic field may be responsible for instability in its outer layers.

By developing a detailed model of the Sun’s surface and simulating various perturbations in plasma flow in the upper 5 to 10 percent of the Sun, the researchers found that these surface changes can create magnetic field patterns very similar to those observed by astronomers. Conversely, simulations of the Sun’s deeper layers produced less accurate images of solar activity.

Shallow magnetic fields

The findings suggest that sunspots and flares may be the product of a shallow magnetic field rather than a field originating deeper within the sun, as scientists have largely assumed.

“The objects we see when we look at the sun, such as the corona that many people saw during the last solar eclipse, sunspots and solar flares, are related to the sun’s magnetic field,” says Keaton Burns, a researcher in the Faculty of Mathematics. MYTH. “We show that isolated perturbations near the Sun’s surface, away from deeper layers, can grow over time and potentially create the magnetic structures we see.”

If the Sun’s magnetic field truly originates from its outer layers, this could give scientists a chance to predict eruptions and geomagnetic storms that could damage satellites and telecommunications systems.

“We know that the dynamo works like a giant clock with many complex interacting parts,” says co-author Geoffrey Vasyl, a researcher at the University of Edinburgh. “But we don’t know most of the pieces or how they fit together. “This new insight into how the solar dynamo begins is important for understanding and predicting it.”

Co-authors of the study include Danielle Lecoinet and Kyle Augustson of Northwestern University, Jeffrey Oishi of Bates College, Benjamin Brown and Keith Julien of the University of Colorado at Boulder, and Nicholas Brummell of the University of California at Santa Cruz.

Dynamics of the convection zone

The Sun is a ball of white-hot plasma boiling on its surface. This boiling zone is called the “convection zone”, where layers and clouds of plasma rotate and flow. The convection zone covers the upper third of the Sun’s radius and extends to approximately 200,000 kilometers below the surface.

“One of the basic ideas of how to run a dynamo machine is that you need a region where lots of plasma passes next to other plasma, and the cutting action converts kinetic energy into magnetic energy,” Burns explains. “People thought that the Sun’s magnetic field was created by movements at the bottom of the convection zone.”

To determine exactly where the Sun’s magnetic field originates, other scientists used large 3D simulations to determine plasma flow in many layers of the Sun’s interior. “These simulations require millions of hours spent on national supercomputers, but what they produce is still not as turbulent as the real sun,” says Burns.

Instead of simulating the complex flow of plasma through the body of the Sun, Burns and his colleagues wondered whether examining the stability of the plasma flow near the surface would be sufficient to explain the origin of the dynamo process.

To investigate this idea, the team first used data from the field of “helioseismology,” in which scientists use oscillations observed at the Sun’s surface to determine the average structure and flow of plasma beneath the surface.

“If you videotape a drum and watch it vibrate in slow motion, you can tell the shape and stiffness of the drum head from the vibration patterns,” says Burns. “Similarly, we can use the vibrations we see at the sun’s surface to infer the average structure inside.”

sunny onion

For their new study, the researchers pieced together models of the Sun’s structure from helioseismic observations. “These midstreams resemble an onion, with different layers of plasma rotating over each other,” Burns explains. “Then we ask: Are there perturbations or small changes in the plasma flow that we can impose on this intermediate structure that can grow to cause the Sun’s magnetic field?”

To find such patterns, the team used Project Dedalus, a numerical framework developed by Burns that can simulate many types of fluid flows with high accuracy. The code has been applied to a wide range of problems, from modeling dynamics inside individual cells to ocean and atmospheric circulation.

“My team has been thinking about the solar magnetism problem for years, and now Dedalus’ capabilities have reached the point where we can solve it,” says Burns.

The team developed algorithms that they incorporated into Dedalus to find self-reinforcing changes in the Sun’s average surface fluxes. The algorithm discovered new patterns that could grow and lead to realistic solar activity. Specifically, the team found patterns that matched the positions and timelines of sunspots observed by astronomers since Galileo in 1612.

Sunspots are temporary structures on the Sun’s surface that are thought to be formed by the Sun’s magnetic field. These relatively cooler regions appear as dark spots relative to the rest of the sun’s white-hot surface. Astronomers have long noticed that sunspots occur in cycles, waxing and waning every 11 years, and generally receding around the equator rather than the poles.

In the team’s simulations, they found that certain changes in plasma flow in just the top 5 to 10 percent of the Sun’s surface layers were sufficient to create magnetic structures in the same regions. Conversely, changes in deeper layers create less realistic solar fields that are concentrated closer to the poles rather than closer to the equator.

The team was motivated to take a closer look at the near-surface flow patterns because the conditions there were similar to unstable plasma flows in completely different systems: accretion disks around black holes. Accretion disks are huge disks of gas and stardust that rotate towards the black hole under the influence of “magnetorotational instability”, which creates turbulence in the flow and causes the flow to fall inwards.

Burns and his colleagues suspected that a similar phenomenon was occurring in the sun, and that magneto-spin instability in the sun’s outermost layers could be the first step in creating the sun’s magnetic field.

Conflicting findings and ongoing research

“I think this conclusion may be controversial,” he said. “Most of the community was focused on finding a dynamo mobile under the sun. Now we show that there is another mechanism that fits the observations better.” Burns says the team is continuing to investigate whether the new surface area models can reproduce individual sunspots and the entire 11-year solar cycle.