Field originates surprisingly close to the surface
An international team of researchers, including engineers from Northwestern University, is getting closer to solving a 400-year-old mystery about the sun that even famous astronomer Galileo Galilei couldn’t figure out.
Since first observing the sun’s magnetic activity, astronomers have had a hard time figuring out where the process starts. Now, after running a series of complex calculations on a NASA supercomputer, the researchers found that the magnetic field is generated about 20,000 miles below the sun’s surface.
The finding goes against previous theories, which suggest the phenomenon starts much deeper — more than 130,000 miles below the sun’s surface.
The research will be published on May 22 in the journal Nature.
The new discovery not only helps us better understand how the sun works, but it could also help scientists more accurately predict powerful solar storms. Although this month’s strong solar storms gave us beautiful, extended views of the Northern Lights, similar storms can cause intense destruction — damaging satellites orbiting Earth, electricity grids, and radio communications.
“Understanding the origin of the sun’s magnetic field has been an open question since Galileo and is important for predicting future solar activity, like flares that could hit the Earth,” said study co-author Daniel Lecoanet. “This work proposes a new hypothesis for how the sun’s magnetic field is generated that better matches solar observations, and, we hope, could be used to make better predictions of solar activity.”
Lecoanet, an expert in astrophysical fluid dynamics, is an assistant professor of engineering sciences and applied mathematics at Northwestern’s McCormick School of Engineering and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics. Geoffrey Vasil, a mathematics professor at the University of Edinburgh in Scotland, led the study.
Over the years, astronomers have made significant progress in understanding the origins of the solar dynamo — the physical process that generates the magnetic field — but limitations have remained. Theories suggesting the dynamo has a deep origin, for example, predict solar features that astronomers have never observed, such as strong magnetic fields at high latitudes.
To solve this puzzle, the research team developed new, state-of-the-art numerical simulations to model the sun’s magnetic field. Unlike previous models, the new model accounts for torsional oscillations, a cyclical pattern of how gas and plasma flow within and around the sun. Because the sun is not solid like the Earth and moon, it doesn’t rotate as one body. Instead, its rotation varies with latitude. Like the 11-year solar magnetic cycle, torsional oscillations also experience an 11-year cycle.
“Because the wave has the same period as the magnetic cycle, it has been thought that these phenomena were linked,” Lecoanet said. “However, the traditional ‘deep theory’ of the solar magnetic field does not explain where these torsional oscillations come from. An intriguing clue is that the torsional oscillations are only near the surface of the sun. Our hypothesis is that the magnetic cycle and the torsional oscillations are different manifestations of the same physical process.”
When Kyle Augustson, a postdoctoral fellow in Lecoanet’s lab at Northwestern, ran the numerical simulations, the researchers found their new model provided a quantitative explanation for properties observed in the torsional oscillations. The model also explains how sunspots follow patterns of the sun’s magnetic activity — another detail missing from the deep origin theory.
With a better understanding of the sun’s dynamo, researchers hope to improve forecasts for solar storms. When solar flares and coronal mass ejections launch toward Earth, they can severely damage electrical and telecommunications infrastructure, including GPS navigation tools. This month’s recent solar storms, for example, knocked out navigational systems for farming equipment — right at peak planting season.
But the researchers look to an even more powerful solar storm that hit Canada in September 1859 as a cautionary tale. Dubbed the Carrington Event, the intense storm damaged the country’s fledgling telegraph system. With enough warning, engineers could take steps to prevent catastrophic damage in the future.
“While the recent solar storms were powerful, we’re worried about even more powerful storms like the Carrington Event,” Lecoanet said. “If a storm of similar intensity hit the United States today, it would cause an estimated $1 trillion to $2 trillion in damage. Although many aspects of solar dynamics remain shrouded in mystery, our work makes huge strides in cracking one of the oldest unsolved problems in theoretical physics and opens the way to better predictions of dangerous solar activity.”
The study, “The solar dynamo begins near the surface,” was supported by NASA (grant numbers 80NSSC20K1280, 80NSSC22K1738 and 80NSSC22M0162). Computations were conducted with support by the NASA High End Computing Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center.
