Origin of the Sun’s magnetic field isn’t as deep as we thought

Earlier this month the Earth was hit by the  biggest solar storm in more than 20 years, resulting in striking auroras, which were spotted at unusually low and high latitudes.

Sunspot AR3664 is no longer facing the Earth, but as it takes the Sun approximately 25 Earth days to rotate once on its axis, it should come back into view within the next 2 weeks.

These solar phenomena are driven by the Sun’s magnetic field, which is generated internally in a process called the dynamo action. Now, a new study in the journal Nature is challenging long-held hypotheses about the depth at which this process occurs inside the Sun.

“The features we see when looking at the Sun, like the corona that many people saw during the recent solar eclipse, sunspots, and solar flares, are all associated with the sun’s magnetic field,” says study author Keaton Burns, a research scientist in the Department of Mathematics at Massachusetts Institute of Technology in the US.

“One of the basic ideas for how to start a dynamo is that you need a region where there’s a lot of plasma moving past other plasma, and that shearing motion converts kinetic energy into magnetic energy.

“People had thought that the sun’s magnetic field is created by the motions at the very bottom of the convection zone.”

An illustration of the sun's magnetic fields in yellow.
This illustration lays a depiction of the sun’s magnetic fields over an image captured by NASA’s Solar Dynamics Observatory. The complex overlay of lines can teach scientists about the ways the sun’s magnetism changes in response to the constant movement on and inside the sun. Credit: NASA/SDO/AIA/LMSAL

The convection zone comprises the top third of the Sun’s radius and stretches about 200,000 kilometres below its surface. 

The team developed new, state-of-the-art numerical simulations to model the sun’s magnetic field.

The found that when they simulated certain perturbations, or changes in the flow of plasma (ionised gas) within the top 5-10% of the Sun, these surface changes were enough to generate realistic magnetic field patterns similar to those observed on the Sun.

In contrast, their simulations of a dynamo in deeper layers produced less realistic solar activity.

Study co-author Daniel Lecoanet, an expert in astrophysical and geophysical fluid dynamics at Northwestern University in the US, adds that understanding the origin of the Sun’s magnetic field is “important for predicting future solar activity, like flares, that could hit the Earth”.

“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.”

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