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Source of the sun’s magnetic field may hide right under its surface

The sun’s magnetic field may not be as deep as we thought. For decades, scientists thought the sun’s dynamo – the area that generates its powerful magnetic field – was located far within the star. Now, evidence suggests the dynamo lurks just under the sun’s surface.

The strength of the sun’s magnetic field fluctuates in a distinct 11-year cycle. During the strongest part of the cycle, sunspots and powerful winds emerge near the solar equator, along with the plumes of material that cause the aurora borealis on Earth. Ideas for how the magnetic field is generated have had a difficult time explaining how all of those phenomena are connected.

Essentially, the sun behaves like a giant clock, with the many eddies and flows of plasma within it acting as the gears that make it tick, says Geoffrey Vasil at the University of Edinburgh in the UK. “Nobody really knows how those things fit together or even what they all are, and you can’t explain the whole clock if you don’t know how it starts.”

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Vasil and his colleagues suggest that the sun’s magnetic field might stem from instability in the rotation of plasma inside the star, which is common in other astrophysical objects like the discs of hot matter orbiting some black holes. Such instability may occur in the outermost 5 to 10 per cent of the sun.

The researchers modelled how this instability would churn the plasma that makes up the outer layers of the sun. It may give rise to sunspots and create the powerful winds that whip around the sun during its period of maximum activity, they found, along with other magnetic phenomena. Simulations with a dynamo close to the surface matched observed magnetic patterns on the sun much more closely than those with a deep dynamo.

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“There are all of these clues, and we’ve been piecing these things together for nearly 20 years,” says Vasil. “It’s very satisfying to have lots of things fit into place and make a lot of sense.”

If the sun’s dynamo is generated near its surface, that could make it much easier to study the solar magnetic field and predict its behaviour. “If the magnetic fields are sitting there, then there is the most hope for actually being able to study them,” says Vasil.

This could allow us to better forecast the solar activity that spawn stunning aurorae – and mess with electrical grids on Earth.

Journal reference:

Nature DOI: 10.1038/s41586-024-07315-1

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