Supercomputer simulations give a better picture of the sun’s magnetic field

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SSD flow and solution visualization. Flow velocity (left) and magnetic field strength (right) from a high-resolution active SSD run with Re=18,200 and PrM=0.01 on the simulation box surface. Credit: Nature astronomy (2023). DOI: 10.1038/s41550-023-01975-1

The sun’s strong and dynamic magnetic field can catapult huge jets of plasma known as coronal mass ejections (CMEs) into the solar system. Sometimes these hit Earth, where they can knock out power grids and damage satellites.

Scientists don’t fully understand how magnetic fields are generated and amplified within the sun, but a study recently published in Nature astronomy answers one of the fundamental questions about this complex process. By elucidating the dynamics underpinning solar weather, these findings could help predict major solar events a few days ahead, providing us with vital extra time to prepare.

The sun’s magnetism comes from a process known as a solar dynamo. It consists of two main parts, the large-scale dynamo and the small-scale dynamo, neither of which scientists have yet been able to fully model. In fact, scientists aren’t even sure that a small-scale dynamo could exist under the conditions found in the sun. Addressing this uncertainty is important, because a small-scale dynamo would have a large effect on solar dynamics.

In the new study, scientists from Aalto University and the Max Planck Institute for Solar System Research (MPS) tackled the small-scale dynamo issue by running massive computer simulations on petascale supercomputers in Finland and Germany. The joint computing power allowed the team to directly simulate whether the sun could have a small-scale dynamo.

“Using one of the largest possible computational simulations currently available, we have achieved the most realistic setup to date in which to model this dynamo,” says Maarit Korpi-Lagg, astroinformatics group leader and associate professor in the Department of Computer Science at the Aalto University. “We have shown not only that the small-scale dynamo exists, but also that it becomes more feasible as our model looks more like the sun.”

Some previous studies have suggested that the small-scale dynamo may fail under the conditions found in stars such as the sun, which have very low magnetic Prandtl number (PrM), a measure used in fluid and plasma physics to compare the rapidity with which the variations of the magnetic field and the velocities equalize. Korpi-Lagg’s research team modeled turbulent conditions with unprecedented PrM values ​​and found that, contrary to what was thought, a small-scale dynamo can occur at such low values.

“This is an important step toward understanding magnetic field generation in the sun and other stars,” says Jrn Warnecke, senior postdoctoral researcher at MPS. “This result will bring us closer to solving the puzzle of CME formation, which is important for devising protection for Earth against dangerous space weather.”

The research team is currently expanding its study to even lower magnetic Prandtl numbers using GPU-accelerated code on the new pre-exascale pan-European supercomputer LUMI. Next, they plan to study the interaction of the small-scale dynamo with the large-scale dynamo responsible for the 11-year solar cycle.

More information:
Jrn Warnecke et al, Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers, Nature astronomy (2023). DOI: 10.1038/s41550-023-01975-1

About the magazine:
Nature astronomy

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