Where does the solar magnetic field originate?


What drives the solar dynamo and thus the magnetic fields and sunspot activity on the sun? © NASA/Goddard, SDO

The recent solar storm impressively demonstrated how active our star can be. But where and how exactly its magnetic fields and activity cycles arise is still unclear – it is usually assumed that it originates at the bottom of the solar convection zone deep inside the sun. But now a research team has found evidence that the solar magnetic dynamo could also be located much closer to the sun’s surface. According to their data, plasma flows in the top five to ten percent of the sun provide the magnetic forces and turbulence that could explain the formation of the complex magnetic fields and also the sunspot cycle.

The activity of our sun follows a regular cycle: sunspots and solar flares reach a maximum approximately every eleven years. At the same time, the plasma currents inside the sun change and the magnetic field reverses. But how this regular rhythm and the phenomena that accompany it come about are only partially understood. The main reason for this lack of clarity is the complexity of solar currents and magnetic fields. The solar magnetic field is similar to the Earth’s dipole field, but its field lines are distorted: some run in a polar direction, others are deflected laterally and run almost parallel to the solar equator. The sunspots are due to local magnetic field disturbances that are more characterized by this torsoidal field, while the poloidal magnetic field lags behind the changes in the sunspot cycle by almost a quarter of a year.

Solar dynamo wanted

But where the dynamo for the various solar magnetic field phenomena is located and how exactly it works is unclear. “To get such a dynamo going, the common assumption is that you need a region in which a lot of plasma flows past other plasma,” says co-author Keaton Burns from the Massachusetts Institute of Technology (MIT). “This shearing motion converts kinetic energy into magnetic energy.” But the Sun’s interior is filled with a variety of plasma currents, from the rising and descending plasma bubbles of the convection current, to deep overturning currents reaching from the pole to the equator, to the plasma currents at various depths triggered by the Sun’s differential rotation. Which of these currents forms the motor of the solar magnetic dynamo is controversial. “We know that the solar dynamo works like a large clock with many complex, interacting parts,” explains lead author Geoffrey Vasil from the University of Edinburgh. “But we don’t know all the pieces yet or how they fit together.”

However, previous models usually locate the motor of the solar magnetic dynamo at the bottom of the convection zone, at a depth of around 210,000 kilometers. “But these global convection models often do not fit important solar observations and require conditions that do not correspond to solar reality,” explain Vasil and his colleagues. Even in theory, this model cannot conclusively explain many phenomena. That’s why Vasil and his team took a closer look at another area of ​​the sun: the currents in the near-surface zone, which makes up five to ten percent of the sun. Information about the differential currents in this upper area is provided by subtle vibrations of the sun’s surface, which are recorded in helioseismology using solar observatories. “We asked ourselves: Are there disturbances or tiny changes in the plasma flow that can amplify to the point where they create the solar magnetic field?” says Burns. The team investigated this using astrophysical analysis algorithms and simulations.

Increasing turbulence near the surface

In fact, the evaluations showed that the physical processes near the sun’s surface are sufficient to cause the magnetic phenomena observed on the sun. Specifically, Vasil and his team attribute this to a mechanism that also occurs in the racing plasma around black holes, the so-called magneto-rotational instability. In this case, plasma areas flowing at different speeds create an inward suction, but at the same time cause turbulence that can strengthen magnetic fields. According to the researchers, this is exactly what could happen in the upper layer of the sun: the gravitational forces of the plasma masses flowing past each other at different speeds influence and strengthen the sun’s bipolar magnetic field and generate the toroidal magnetic fields and their oscillations. This in turn shapes the appearance of sunspots. “We show that the isolated turbulence near the Sun’s surface can grow over time and then form the magnetic structures we see,” says Burns.

According to the scientists, the near-surface processes they have identified could explain better than previous models how the complex solar magnetic fields and the phenomena associated with them come about. “Understanding the origin of the solar magnetic field has been an open question since the time of Galileo Galilei,” says co-author Daniel Lecoanet of Northwestern University in Illinois. “Our work now proposes a new hypothesis for the origin of the solar magnetic field that is more consistent with observations of the sun.” The team’s model is still very simplified and can only depict the basic processes. Nevertheless, they see it as a first step towards deciphering some of the processes on our home star that are still inadequately explained.

Source: Geoffrey Vasil (University of Edinburgh, UK) et al., Nature, doi: 10.1038/s41586-024-07315-1

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