Origin of the Sun’s magnetic field discovered under the star’s surface

For many years, scientists have searched for the source of the Sun’s magnetism. It’s possible they’ve been looking in the wrong place.

A new study suggests that the solar dynamo, the process which generates the Sun’s magnetic field, lies about 32,000 kilometres beneath its surface. Though this distance is almost three times the diameter of our planet, it’s relatively shallow in solar terms.

By better understanding where the solar dynamo is, researchers hope to create more accurate simulations of what’s happening under the star’s surface. In turn, this could help to forecast not just when the Northern Lights might appear, but when solar storms could put satellites, radio networks and power grids at risk.

Dr Geoff Vasil is the study’s lead author from the University of Edinburgh's School of Mathematics. He says that their work helps to solve questions about the Sun which have been unanswered for hundreds, if not thousands, of years.

“Alongside the Moon, the Sun is one of only two astrophysical objects in the universe that has direct daily consequences for us on Earth,” Geoff says. “We Earthlings have been Sun worshippers for our entire history, and something fundamental within us drives us to understand our primary life-giver.”

“Solar storms are particularly important to understand. While they’re most recognisable for the beautiful interactions they make with the atmosphere, they also affect anything large and metal. They can put power grids, satellites and astronauts at serious risk, so forecasting solar events is vital to protecting our planet.”

The findings of the study were published in the journal Nature.

Revealing the Sun’s magnetic field

Despite being the most vital part of our solar system, the Sun has hidden depths which we’re only just beginning to come to grips with.

Some of the longest lasting questions are about sunspots and where they come from. Some of the earliest records of these short-lived solar blotches come from China more than 2,000 years ago, while scientists like Galileo observed them with telescopes in the 1600s.

We now know that sunspots are a symptom of the Sun’s magnetism. For reasons that aren’t well understood turbulent interactions between streams of plasma generate a powerful magnetic field.

“Think of plasma as a large bucket of small magnets,” Geoff explains. “If you pour them out of the bucket, they flow like a fluid, but their magnetic field adds a bit of ‘rigidity’ to them.”

“When you push similarly charged magnets together, they push away from each other. In the Sun, however, the intense heat and pressure means this isn’t an option. The energy from pushing has to go somewhere, so it makes the magnetic field a bit stronger.”

While advances in our understanding of thermodynamics, electricity and quantum mechanics have helped bring us closer to an explanation of the Sun’s magnetism, it largely remains a mystery. Geoff has been determined to answer the question of the solar dynamo for over 20 years.

“For many years, it had been thought that the dynamo was in the deeper regions of the Sun, hundreds of thousands of kilometres beneath its surface,” Geoff says. “When I investigated a part of the deeper Sun called the tachocline, however, nothing about the models worked as planned.”

“It was during this time that I started thinking of alternatives because of this and hit on the near-surface region.”

To investigate this theory, the researchers turned to probe observations of how the Sun’s surface vibrates. Known as helioseismology, this can reveal the average flow and structure of plasma beneath the surface.

The resulting data was used to build models of the Sun’s surface, resulting in patterns that appeared to match with real-world observations of sunspots. Further simulations suggested that changes in the flow of plasma within the outer 10% of the Sun, rather than the deeper regions, were enough to generate realistic magnetic fields.

Forecasting solar storms

The team’s research couldn’t be timelier. The Sun is currently approaching its solar maximum, which is the point in the 11-year-cycle of its magnetic field when its activity is at its strongest. This produces powerful solar storms.

The most powerful solar storm recorded history took place in 1859, in what is known as the Carrington Event. Though at the time the damage was largely limited to the telegraph system, advances in technology mean we are much more vulnerable today. Since 2000, solar storms have been linked to radio and GPS disruption, as well as temporary blackouts.

In the worst-case scenario, it’s estimated that another Carrington-level storm could cause as much as $2.6 trillion dollars of damage in North America alone.

In the long term, the team hopes that their research could help to give advanced warning of when such powerful storms might arrive. This will allow steps to be taken to protect important infrastructure.

“For fields originating below the surface, we will have much better opportunities to predict where large sunspot groups will emerge,” Geoff says. “We should also know much more about those groups and their dynamics after they form. With enough data, we should be able to make a statistical database to indicate the regions that pose the danger.”

For this to become a reality, more work will be needed to understand other regions of the Sun, to piece together a unified model of how it works.

“We'll never fully be able to predict storms without knowing what's happening below the surface,” Geoff adds. “I sincerely believe that when we get it, the final answer will be simple.”