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The sun has a powerful 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 how exactly this magnetic field is generated inside the sun is a mystery that has plagued astronomers for centuries, going back to the time of the Italian astronomer Galileo. who made the first observations of sunspots in the early 1600s and noticed how they varied with time.
Researchers behind an interdisciplinary study put forth a new theory in a report published Wednesday in the journal Nature. Unlike previous research that suggested the sun’s magnetic field originates deep within the celestial body, they suspect the source is much closer to the surface.
The model the team developed could help scientists better understand the 11-year solar cycle and improve the prediction of space weather, which can disrupt GPS and communications satellites and dazzle night sky watchers with auroras.
“This work offers a new hypothesis about how the sun’s magnetic field is generated that better matches solar observations and can hopefully be used to make better predictions about solar activity,” said Daniel Lecoinet, assistant professor in engineering and applied mathematics at Northwestern University’s McCormick School of Engineering and a member of the Center for Interdisciplinary Studies and Research in Astrophysics.
“We want to predict whether the next solar cycle will be particularly strong or perhaps weaker than normal. Previous models (assuming that the solar magnetic field is generated deep within the Sun) have not been able to make accurate predictions or (determine) whether the next solar cycle will be strong or weak,” he added.
Sunspots help scientists track solar activity. They are the starting point for explosive eruptions 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 it has the largest number of sunspots.
“Because we think that the number of sunspots follows the strength of the magnetic field in the Sun, we think that the 11-year sunspot cycle reflects a cycle in the strength of the Sun’s internal magnetic field,” Lecoinet said.
NASA/GSFC/Solar Dynamics Observa
This view of the Sun’s magnetic field was generated by NASA’s Solar Dynamics Observatory.
It is difficult to see the sun’s magnetic field lines, which run through the solar atmosphere to form a complex web of magnetic structures far more complex than Earth’s magnetic field. To better understand how the sun’s magnetic field works, scientists turn to mathematical models.
For the first time in science, the model that Lecoanet and his colleagues developed takes into account a phenomenon called torsional oscillation – 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 go through an 11-year cycle.
“Solar observations have given us a good idea of how material moves inside the Sun. For our supercomputer calculations, we solved equations to determine how the magnetic field in the Sun changes due to the observed motions,” said Lecoinet.
“No one had done this calculation before because no one knew how to do the calculation efficiently,” he added.
The team’s calculations show that magnetic fields can be generated about 20,000 miles (32,100 kilometers) below the sun’s surface — much 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 the torsional oscillations that were missing from previous models,” said Lecoinet.
An important breakthrough was the development of new numerical algorithms to perform the calculations, Lecoinet said. The paper’s lead author, Jeff Vassil, a professor at the University of Edinburgh in the United Kingdom, came up with the idea about 20 years ago, Lecoinet said, but it took more than 10 years to develop the algorithms and required a powerful NASA supercomputer to run the simulations.
“We used about 15 million CPU hours on this investigation,” he said. “That means if I had tried to do the calculations on my laptop, it would have taken 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 did not participate in the research.
Zweibel said the team has added “a provocative ingredient to the theoretical mix that may prove key to unraveling this astrophysical enigma.”
