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The sun has a strong magnetic field that creates sunspots on the star’s surface and unleashes solar storms like the one that bathed much of the planet in beautiful auroras this month.
But exactly how that magnetic field is created inside the Sun is a puzzle that has plagued astronomers for centuries, all the way back to the time of the Italian astronomer Galileo. who made the first observations of sunspots in the early 1600s, and noted how they varied over time.
The researchers behind the interdisciplinary study put forward a new theory in a report published Wednesday in the journal Nature. Unlike previous research that assumed that the Sun’s magnetic field originates from deep within the celestial body, they suspect that the source is much closer to the surface.
The model developed by the team could help scientists better understand the 11-year solar cycle and improve forecasting of space weather, which can disrupt GPS and communications satellites, as well as blind night sky watchers with the aurora borealis.
“This paper proposes a new hypothesis about how the Sun’s magnetic field is generated that better fits observations of the Sun and, hopefully, could be used to make better predictions of the Sun’s activity,” said Daniel Lecoanet, assistant professor of engineering and applied sciences. in mathematics at Northwestern University’s McCormick School of Engineering and a member of the Center for Interdisciplinary Research and Research in Astrophysics.
“We want to forecast whether the next solar cycle will be particularly strong, or perhaps weaker than usual. Previous models (assuming that the solar magnetic field is generated deep inside the Sun) have not been able to make accurate forecasts or (determine) whether the next solar cycle will be strong or weak,” he added.
Sunspots help scientists track the sun’s activity. They are the starting point for explosive flares and ejection events that release light, solar material and energy into space. The recent solar storm is evidence that the sun is approaching “solar maximum” – the point in its 11-year cycle when there are the most sunspots.
“Because we think the number of sunspots follows the strength of the magnetic field inside the Sun, we think the 11-year cycle of sunspots reflects a cycle in the strength of the Sun’s internal magnetic field,” Lecoanet said.
It is difficult to see the Sun’s magnetic field lines, which weave through the Sun’s atmosphere to form a complicated network of magnetic structures far more complex than Earth’s magnetic field. To better understand how the Sun’s magnetic field works, scientists are turning to mathematical models.
In a science first, the model developed by Lecoanet and his colleagues explained a phenomenon called torsional oscillations—magnetically driven flows of gas and plasma in and around the sun that contribute to the formation of sunspots.
In some areas, the rotation of this solar feature speeds up or slows down, while in others it remains stable. Like the 11-year solar magnetic cycle, torsional oscillations also have an 11-year cycle.
“Observations of the Sun have given us a good idea of how material moves inside the Sun. For our supercomputing calculations, we solved the equations to determine how the magnetic field changes inside the Sun due to the observed motions,” said Lecoanet.
“No one had done this calculation before because no one knew how to do the calculation efficiently,” he added.
The team’s calculations showed that magnetic fields can be generated about 20,000 miles (32,100 kilometers) below the surface of the sun – far closer to the surface than previously thought. Other models suggested it was much deeper – about 130,000 miles (209,200 kilometers).
“Our new hypothesis provides a natural explanation for torsional oscillations missing from previous models,” said Lecoanet.
An important advance was the development of new numerical algorithms for performing the calculations, Lecoanet said. The paper’s lead author Geoff Vasil, a professor at the University of Edinburgh in the United Kingdom, came up with the idea about 20 years ago, Lecoanet said, but it took more than 10 years to develop the algorithms and required a powerful NASA supercomputer to implementation of the simulation.
“We used about 15 million CPU hours for this investigation,” he said. “That means if I tried to do the calculations on my laptop, it would take me about 450 years.”
In a commentary published with the study, Ellen Zweibel, a professor of astronomy and physics at the University of Wisconsin-Madison, said the initial results are intriguing and will help inform future models and research. She was not involved in the study.
Zweibel said the team has added “a provocative ingredient to the theoretical mix that may prove key to unraveling this astrophysical enigma.”