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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 exactly how that magnetic field is generated inside the sun is a puzzle that has puzzled astronomers for centuries, dating back to the time of the Italian astronomer Galileo, who made the first observations of early sunspots in the 1600s, and noted how they changed over time. .
Researchers behind an interdisciplinary study put forth a new theory in a report published Wednesday in the journal Nature. In contrast to previous research that assumed the sun’s magnetic field comes from deep within the celestial body, they suspect 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 space weather forecasting, which could affect GPS and communications satellites as well as Night sky watchers dazzle with auroras.
“This work proposes a new hypothesis for how the solar magnetic field is generated that better matches solar observations, and, hopefully, could be used to make better predictions of solar activity,” a 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 Exploration and Research in Astrophysics.
“We want to predict whether the next solar cycle will be particularly strong, or perhaps weaker than usual. The previous models (assuming that the solar magnetic field is generated deep inside the Sun) were not able to make accurate predictions or (determine) whether the next solar cycle will be strong or weak,” he said.
Sunspots help scientists track the sun’s activity. They are the point of origin for the explosive flares and ejection events that release light, solar matter 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 highest number of sunspots.
“Because we think that the number of sunspots corresponds to the strength of the internal magnetic field of the Sun, we think that the 11-year sunspot cycle reflects a cycle in the strength of the internal magnetic field of the Sun,” said Lecoanet.
Modeling the sun’s magnetic field
It is difficult to see the solar magnetic field lines, which meander 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.
In a scientific first, the model developed by Lecoanet and his colleagues led to a phenomenon known as torsional oscillations – magnetically driven gas and plasma flows in and around the sun that contribute to sunspot formation.
In some areas, the rotation of this solar element speeds up or slows down, while in others it remains constant. Like the 11-year solar magnetic cycle, torsional oscillations also experience an 11-year cycle.
“Solar observations have given us a good idea of how matter moves around inside the Sun. For our supercomputing calculations, we solved equations to determine how the magnetic field within the Sun changes 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 said.
The group’s calculations showed that magnetic fields can be generated about 20,000 miles (32,100 kilometers) below the surface of the sun – 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 missing from previous models,” said Lecoanet.
an astrophysical enigma
One of the important ones was to develop new numerical algorithms to run the calculations, said Lecoanet. 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 address. the simulations.
“We’ve used about 15 million CPU hours for this investigation,” he said. “That means if I had tried to run 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 were interesting and would help inform future models and research. She was not involved in the study.
Zweibel said the team “added a tantalizing ingredient to the theoretical mix that may be key to solving this astrophysical enigma.”
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