The missing link in solar physics: Complex structures of solar magnetic fields could originate elsewhere than thought, near the surface of the Sun rather than deep within it, astrophysicists have discovered. Consequently, near-surface plasma flows act like a second magnetic dynamo that amplifies and modulates the solar magnetic field. The team says in the journal Nature that this mechanism could explain phenomena such as the sunspot cycle and equatorial magnetic fields better than previous models.

The recent solar storm impressively demonstrated just how active our star is. But where and how the Sun's magnetic fields and activity cycles arise is still unclear, as solar currents and fields are very complex. “We know that the solar dynamo works like a large clock with many complex, interacting parts,” explains lead author Geoffrey Vassell from the University of Edinburgh. “But we don't know all the pieces yet or how they fit together.”

Dynamo in the depths?

Previous models mostly assumed a deep origin for the solar magnetic dynamo. Accordingly, currents at the bottom of the solar convection zone at a depth of about 210,000 km must form the basis of the solar magnetic field. “But these global convection models often do not fit important solar observations, and require conditions that do not match solar reality,” Vasil and his colleagues explain. Even theoretically, this model cannot conclusively explain many phenomena.

For this reason, Vasil and his team set their sights on a different region of the Sun: the currents in the near-surface region, which make up between five and ten percent of the Sun. “We asked ourselves: Are there subtle perturbations or changes in the plasma flow that could be amplified to the point of generating the solar magnetic field?” says co-author Keaton Burns of the Massachusetts Institute of Technology (MIT). To do this, the researchers analyzed solar seismic data and created astrophysical simulations.

They arise in layers close to the surface

Indeed: Scientists have discovered a mechanism by which near-surface plasma flows can generate complex magnetic fields at a depth of about 32 thousand kilometers. The shear forces of plasma masses flowing past each other at different speeds modify and strengthen the Sun's simple dipole magnetic field and generate more complex, sometimes transverse solar magnetic field structures including its oscillations. This, in turn, shapes the appearance of sunspots.

This new model could thus establish the long-sought connection between the deep magnetic dynamo and its simple dipole field and the more complex structures and processes observed on the solar surface. “We have shown that isolated turbulence near the Sun's surface can grow over time and then form the magnetic structures we see,” Burns says.

Turbulence like around a black hole

The basis of this newly discovered surface dynamo is a mechanism that also occurs in plasma racing around black holes. In the so-called magnetic rotational instability (MRI), regions of plasma in the accretion disk around the black hole that flow at different speeds create inward drag, but at the same time cause turbulence that can strengthen magnetic fields. According to researchers, this is exactly what would happen in the upper layer of the Sun.

“Such a dynamo driven by rotational magnetic instability could also explain the formation of large minimums that sometimes occur,” Vasil and his team wrote. During phases such as the Maunder minimum about 400 years ago, the Sun is less active than usual and its magnetic field also goes through a weakening phase. According to the new model, these periods of weakness can occur because the MRI-driven amplification process at the surface sometimes “stutters” and stops.

Link notes and models

According to Vasil and his colleagues, their new model provides new insights into the complex physics of our star. “Understanding the origin of the solar magnetic field has been an open question since the time of Galileo Galilei,” says co-author Daniel Liquanette of Northwestern University in Illinois. “Our work now proposes a new hypothesis for solar magnetic field formation that is more consistent with solar observations.”

The researchers' model is still too simplistic and can only depict basic processes. However, they see this as a first step towards deciphering some still insufficiently elucidated processes on our home star.

Astrophysicist Ellen Zwibel of the University of Wisconsin, who was not involved in the study, sees something similar. “The findings by Vasil and colleagues are exciting,” she wrote in an accompanying commentary in Nature. “They could provide an interpretive framework for more detailed models, and are sure to inspire future studies.” This could also help predict more powerful solar storms in the future. Better than before (Nature, 2024, doi: 10.1038/s41586-024-07315-1)

Source: Massachusetts Institute of Technology, Northwestern University

May 23, 2024 – Nadja Podbrigar