New research has revealed that the secrets of the sun's magnetic field, which has remained shrouded in mystery for four centuries, could lie close to its surface.
The sun's magnetic field is responsible for generating dark patches called sunspots, erupting solar flares, and even explosive ejections of matter called coronal mass ejections (CMEs). Yet, ever since astronomers began investigating the sun's magnetic fields, the point from which they originate has remained undetermined. Now, an international team of researchers may be closer to solving this 400-year-old mystery that confounded even Galileo Galilei.
The discovery means that sunspots and flares are likely to be the product of a shallow magnetic field rather than a field that originates deeper within the sun, something that had previously been theorized. The team's findings could help solar scientists better predict solar flares and geomagnetic storms that pose a risk to Earth's satellites, communications systems, and power infrastructure while also providing a strange link between the outer layers of the sun and feeding black holes.
Using a NASA supercomputer, the team behind this research performed a series of complex calculations that showed the sun's magnetic field is generated around 40,000 miles (64,000 kilometers) below its surface, the photosphere. This may seem incredibly deep, but the sun has a radius of around 433,000 miles (697,000 km), meaning the magnetic fields are generated in the outer 10% of the sun's superheated plasma.
"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," team member Keaton Burns, a research scientist at the Massachusetts Institute of Technology (MIT), said in a statement. "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."
Sunspots are cool and dark patches on the surface of the sun that scientists think are created when magnetic field lines tangle. Solar scientists have found sunspots increase in number during the solar maximum period of the sun's 11-year solar cycle. Observations have shown that sunspots tend to be found closer to the equator of the sun rather than at the poles of our star.
The solar dynamo isn't that deep
The sun generates its magnetic field via a physical process that scientists call the solar dynamo. Previous models of this dynamo have suggested it is kick-started in a roiling and turbulent region of the sun called the convection zone. Here, hot plasma rises away from the core of the sun, where the majority of its energy is generated, carrying heat and energy to the surface of the sun, the photosphere.
After the energy is deposited, the plasma cools and falls back through the convection zone, which has a depth of around 124,000 miles (200,000 km) and accounts for around 30% of the sun's volume, past the next "batch" of rising heated plasma.
"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," Burns explained. "People had thought that the sun's magnetic field is created by the motions at the very bottom of the convection zone."
Other teams of researchers have previously created three-dimensional simulations of the sun to model the flow of plasma throughout its various layers and to thus determine where its magnetic field originates. The team argues that these simulations failed to pinpoint the true starting point of the solar dynamo because they failed to capture the true picture of just how chaotic and turbulent the sun actually is.
Burns and his team took a different approach. Instead of modeling the flow of plasma throughout all the layers in the interior of the sun, they focused on the stability of plasma at the solar surface. They wanted to determine if changes in this surface region would be enough to start the solar dynamo.
How the sun's magnetic fields go with the flow
To begin, Burns and colleagues used a process called "helioseismology" that measures trapped soundwaves as they ripple through the sun and cause oscillations called "starquakes" at the solar surface to determine the interior of the sun. This allowed them to determine the structure and flow of plasma just beneath the solar 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. These average flows look sort like an onion, with different layers of plasma rotating past each other."
The team turned to the Dedalus Project, a framework developed by Burns that can simulate fluid flows with high precision, to look at this flow of solar plasma and then see if any tiny changes or "perturbations" could be introduced to the regular structure that could grow and cause the solar dynamo.
Their algorithm discovered new patterns in the flow of plasma that can grow and create a picture of real solar activity. These patterns matched the locations of sunspots that astronomers have been seeing since 1612 and the observations of Galileo.
Sunspots are cool and dark patches on the surface of the sun that scientists think are created when magnetic field lines tangle. Solar scientists have found sunspots increase in number during the solar maximum period of the sun's 11-year solar cycle. Observations have shown that sunspots tend to be found closer to the equator of the sun rather than at the poles of our star.
The Dedalus Project simulations revealed that changes in plasma flows in the top 5% to 10% of the sun were sufficient enough to generate magnetic structures that can account for observed sunspot activity. When they modeled deeper regions of our star as the source of magnetic fields, this led to sunspots congregating at the poles of the sun rather than its equator, which is the opposite of what is actually seen by astronomers.
Taking a closer look at how plasma flowed at the surface of the sun, Burns and colleagues also found a surprising similarity to the immediate environments of black holes.
A strange connection between the sun and feeding black holes
When stars venture too close to black holes, they can be destroyed by massive gravitational forces that squash them horizontally and squeeze them vertically, "spaghettifying" them in an occurrence called a tidal disruption event (TDE).
Furthermore, in situations when a star orbits a black hole in a binary system and is either too close, or its outer layers have "puffed out," the gravitational influence of the black hole can strip away stellar material.
In both the cases of stellar cannibalism and in less extreme situations when black holes are in regions of gas and dust, this superheated plasma has angular momentum (or spin), and that means it can't just fall into the black hole.
Instead, this plasma forms a flattened cloud around the black hole that gradually feeds it and is subject to immense frictional forces due to the gravity of the black hole that heats it, causing it to glow. This 'platter' of plasma is called an accretion disk. Accretion disks are turbulent and feed black holes because of co-called "magnetorotational instability" in their flow of plasma. This turbulence is created when magnetized material closer to the edge of an accretion disk moves more slowly than material closer to its center.
Burns and team think that a similar phenomenon is happening in the sun's magnetic field, and it is this magnetorotational instability in the sun's outermost layers that is the first step in generating the sun's magnetic field.
"I think this result may be controversial," Burns added. "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."
The team will now continue their investigation by studying surface magnetic field patterns and attempting to determine if they can create individual sunspots in their simulations and to determine how they link to the overall 11-year cycle of the sun.
"We know the dynamo acts like a giant clock with many complex interacting parts," Geoffrey Vasil, team member and researcher at the University of Edinburgh, said. "But we don't know many of the pieces or how they fit together. This new idea of how the solar dynamo starts is essential to understanding and predicting it."
The team's research was published on Wednesday (May 22) in the journal Nature.
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