Our Sun is far from a smooth, white ball in the sky. Twisting and looping magnetic field lines rise from its surface like hair, causing dark, transient patches on the surface called sunspots wherever they emerge. On particularly turbulent days, the Sun spews intense bursts of radiation like the ones that recently graced Earth with spectacular, awe-inspiring aurorae. Those solar storms, the strongest in two decades, also caused breakdowns of navigational systems on farming equipment, which rely on accurate GPS signals from satellites to precisely distribute seeds in straight rows.
“While the recent solar storms were powerful, we’re worried about even more powerful storms like the Carrington Event,” Daniel Lecoanet of Northwestern University in Evanston, Illinois, said in a statement. “If a storm of similar intensity hit the United States today, it would cause an estimated $1 trillion to $2 trillion in damage.”
Accurately predicting dangerous solar storms relies on understanding how the Sun’s magnetic field operates. But its origins have remained murky despite decades of observation.
Now, a new simulation of the Sun’s magnetic field created by an international team of astronomers including Vasil and Lecoanet shows that activity in the Sun’s outermost layers produce the magnetic field at a shallow depth of 20,000 miles (32,180 km) below its surface. The work was published May 22 in Nature.
“We show that isolated perturbations near the Sun’s surface, far from the deeper layers, can grow over time to potentially produce the magnetic structures we see,” said study co-author Keaton Burns at MIT.
Additionally, unlike its predecessors, the new simulation was also able to reproduce the positions and times of sunspots that astronomers have tracked since the time of Galileo. Because it predicts sunspots and magnetic field patterns similar to those we have observed, scientists say the new simulation can help us better forecast powerful solar storms.
A shallow magnetic field
Previous theories suggested the Sun’s magnetic field originates deep within our star, about 130,000 miles (209,214 kilometers) below its surface. However, solar dynamo models, which are designed to reproduce the dynamics of the Sun’s magnetic field and were built on those theories, predict certain features never actually observed on the Sun, such as stronger magnetic fields near its poles rather than its equator.
“We know the dynamo acts like a giant clock with many complex interacting parts,” said Geoffrey Vasil, a researcher at the University of Edinburgh in Scotland. “But we don’t know many of the pieces or how they fit together.”
That previous work modeled movement of superheated plasma across many layers of the Sun. But the new research simulates plasma flows in just the top 5 to 10 percent of the Sun’s structure. The researchers say the new simulation is unique in that it accounts for the Sun’s remarkably skewed magnetic field, which features a cyclical pattern of plasma flows inside and around the Sun known as torsional oscillations.
“The traditional ‘deep theory’ of the solar magnetic field does not explain where these torsional oscillations come from,” said Lecoanet, adding that these waves occur only near the Sun’s surface. “Our hypothesis is that the magnetic cycle and the torsional oscillations are different manifestations of the same physical process.”
Rotating plasma
To develop their model, the team studied helioseismology data, which record vibrations on the Sun, to determine how ionized gas flows and is distributed just below the surface. “If you take a video of a drum and watch how it vibrates in slow motion, you can work out the drumhead’s shape and stiffness from the vibrational modes,” Burns said. “Similarly, we can use vibrations that we see on the solar surface to infer the average structure on the inside.”
That structure, Burns and his colleagues found, can be likened to an onion, with layers of swirling past each other. The researchers modeled perturbations in these plasma flows and found the motions in the Sun’s outermost layers are similar to the way matter rotates in an accretion disk around a black hole. And the rotation of these flows could generate instabilities that ultimately generate the Sun’s magnetic field, the researchers say.
“I think this result may be controversial,” added Burns. “Most of the community has been focused on finding dynamo action deep in the Sun. Now we’re showing there’s a different mechanism that seems to be a better match to observations.”
