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The Sun has a powerful magnetic field that creates sunspots on the star’s surface and triggers solar storms like the one that bathed much of the planet in beautiful auroras this month.
But exactly how this magnetic field is generated inside the Sun is a mystery that has puzzled astronomers for centuries, dating back to the time of the Italian astronomer Galileo. who made the first observations of sunspots in the early 16th century and noticed how they changed over time.
Researchers behind an interdisciplinary study have put forward a new theory in a report published on Wednesday in the journal Nature. Unlike previous research, which assumed that the sun’s magnetic field originates deep inside 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 prediction of space weather, which can disrupt GPS and communications satellites and blind night sky watchers with auroras.
“This work proposes a new hypothesis about how the Sun’s magnetic field is generated that better fits solar observations and, we hope, could be used for better predictions of solar activity,” said Daniel Lecoanet, assistant professor of engineering and applied mathematics at the McCormick School of Engineering at Northwestern University 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 normal. The previous models (assuming that the solar magnetic field is generated deep inside 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 the sun’s activity. They are the origin of explosive flares and ejection events that release light, solar material and energy into space. The recent solar storm is evidence that the Sun is nearing its “solar maximum” – the point in its 11-year cycle where there are the highest number of sunspots.
“Because we believe that the number of sunspots is related to the strength of the magnetic field within the Sun, we think that the 11-year sunspot cycle reflects a cycle in the strength of the Sun’s internal magnetic field,” Lecoanet said.
NASA/GSFC/Solar Dynamics Observa
This view of the Sun’s magnetic field was created by NASA’s Solar Dynamics Observatory.
It is difficult to detect the Sun’s magnetic field lines, which wind through the Sun’s atmosphere and form an intricate network of magnetic structures far more complex than Earth’s magnetic field. To better understand how the sun’s magnetic field works, scientists use mathematical models.
The model developed by Lecoanet and his colleagues was the first scientific model to take into account a phenomenon called torsional oscillation – magnetically driven gas and plasma flows within and around the Sun that contribute to sunspot formation.
In some areas the rotation of this solar formation speeds up or slows down, while in others it remains constant. 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 matter moves inside the sun. For our supercomputing calculations, we solved equations to determine how the magnetic field within the Sun changes based on the observed movements,” said Lecoanet.
“Nobody had done this calculation before because nobody knew how to do the calculation efficiently,” he added.
The group’s calculations showed 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 had suggested it was much deeper – about 130,000 miles (209,200 kilometers).
“Our new hypothesis provides a natural explanation for the torsional vibrations that is missing in previous models,” said Lecoanet.
An important breakthrough was the development of new numerical algorithms to carry out 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 the algorithms took over a decade to develop and required a powerful NASA supercomputer to carry them out the simulations were necessary.
“We used around 15 million CPU hours for 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 alongside the study, Ellen Zweibel, a professor of astronomy and physics at the University of Wisconsin-Madison, said the initial results were intriguing and would help develop future models and research. She was not involved in the study.
Zweibel said the team “added a provocative ingredient to the theoretical mix that may prove to be the key to solving this astrophysical puzzle.”