New research has found that the secrets of the sun’s magnetic field, which has remained shrouded in mystery for four centuries, could lie close to the surface.
The Sun’s magnetic field is responsible for generating dark spots called sunspots, erupting solar flares, and even explosive ejections of matter called coronal mass ejections (CMEs). But 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 puzzled even Galileo Galilei.
The discovery means that sunspots and solar flares are likely the product of a shallow magnetic field rather than one that originates deeper in the sun, something previously 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 energy infrastructure, while also drawing a strange link between the Sun’s outer layers and powering of black holes.
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Using a NASA supercomputer, the team behind this study performed a series of complex calculations that showed that the Sun’s magnetic field is generated about 40,000 miles below its surface, the photosphere. This may seem incredibly deep, but the Sun has a radius of about 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 we look at the Sun, such as the corona that many people saw during the recent solar eclipse, sunspots and solar flares, are all related to the Sun’s magnetic field,” said team member Keaton Burns, a research scientist at the Sun. University of California. Massachusetts Institute of Technology (MIT), in a statement. “We show that isolated disturbances near the Sun’s surface, far from the deeper layers, can grow over time and potentially produce the magnetic structures we see.”
Sunspots are cool and dark spots on the Sun’s surface that scientists think form when magnetic field lines go haywire. Solar scientists have found that sunspots increase in number during the maximum solar period of the sun’s eleven-year solar cycle. Observations have shown that sunspots often occur closer to the Sun’s equator than to our star’s poles.
The solar dynamo is not that deep
The sun generates its magnetic field through a physical process that scientists call the solar dynamo. Previous models of this dynamo have suggested that it starts in a swirling and turbulent region of the Sun called the convection zone. Here, hot plasma rises away from the Sun’s core, where most of the energy is generated, and transports heat and energy to the Sun’s surface, the photosphere.
After the energy is deposited, the plasma cools and falls back through the convective zone, which has a depth of about 200,000 km and accounts for about 30% of the Sun’s volume, past the next “batch” of ascending heated plasma . .
“One of the basic ideas for starting a dynamo is that you need an area where there is a lot of plasma moving past other plasma and that sliding motion converts kinetic energy into magnetic energy,” Burns explains. “People thought that the sun’s magnetic field is created by the motions at the very bottom of the convective zone.”
Other teams of researchers have previously created three-dimensional simulations of the Sun to model the plasma flow through the different layers and thus determine where the magnetic field comes from. The team claims that these simulations have failed to pinpoint the true starting point of the solar dynamo because they have failed to paint an accurate picture of how chaotic and turbulent the Sun actually is.
Burns and his team took a different approach. Instead of modeling plasma flow through all layers in the Sun’s interior, they focused on the stability of plasma at the solar surface. They wanted to determine whether changes in this surface area would be enough to start the solar dynamo.
How the sun’s magnetic fields move with the current
To start, Burns and colleagues used a process called “helioseismology,” which measures captured sound waves as they ripple through the Sun and cause oscillations called “starquakes” at the Sun’s surface to determine the Sun’s interior. 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 it vibrate in slow motion, you can determine the shape and stiffness of the drumhead from its vibration modes,” Burns said. “Similarly, we can use vibrations we see on the solar surface to infer the average structure on the inside. These average currents look like an onion, with different layers of plasma spinning 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 small changes or ‘perturbations’ could be introduced into the regular structure that would make this could cause. grow and cause the solar dynamo.
Their algorithm discovered new patterns in the plasma flow that can grow and create a picture of real solar activity. These patterns matched the locations of sunspots observed by astronomers since 1612 and Galileo’s observations.
Sunspots are cool and dark spots on the Sun’s surface that scientists think form when magnetic field lines go haywire. Solar scientists have found that sunspots increase in number during the maximum solar period of the sun’s eleven-year solar cycle. Observations have shown that sunspots often occur closer to the Sun’s equator than to our star’s poles.
The Dedalus Project simulations showed that changes in plasma currents in the upper 5% to 10% of the Sun were sufficient to generate magnetic structures that could explain the observed sunspot activity. When they modeled deeper regions of our star as the source of magnetic fields, this led to sunspots gathering at the Sun’s poles instead of the equator, which is the opposite of what astronomers actually see.
When Burns and colleagues took a closer look at how plasma flowed along the Sun’s surface, they also discovered a surprising similarity to the immediate environment of black holes.
A strange connection between the sun and feeding black holes
When stars move too close to black holes, they can be destroyed by enormous gravitational forces that flatten them horizontally and compress them vertically, causing them to ‘splash’ in an event called a tidal disruption event (TDE).
Furthermore, in situations where a star in a binary star system orbits a black hole and is either too close or its outer layers have “swollen out,” the black hole’s gravitational influence can strip away stellar material.
In both cases of stellar cannibalism and in less extreme situations where black holes are located in regions of gas and dust, this superheated plasma has angular momentum (or spin), which means it cannot simply fall into the black hole.
Instead, this plasma forms an oblate cloud around the black hole that gradually feeds it and is subject to enormous frictional forces due to the gravity of the black hole that heats it, causing it to glow. This ‘dish’ of plasma is called an accretion disk. Accretion disks are turbulent and feed black holes due to the so-called “magnetorotational instability” in their plasma flow. This turbulence is caused by magnetization material closer to the edge of an accretion disk moves more slowly than material closer to the center.
Burns and his team think a similar phenomenon occurs in the Sun’s magnetic field, and it is this magnetorotational instability in the Sun’s outer layers that is the first step in generating the Sun’s magnetic field.
“I think this result could be controversial,” Burns added. “Most of the community has focused on finding dynamo action deep within the Sun. Now we show that another mechanism exists that seems to fit better with observations.”
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The team will now continue their research by studying magnetic field patterns on the surface and trying to determine whether they can create individual sunspots in their simulations and determine how these relate to the Sun’s overall 11-year cycle.
“We know that the dynamo behaves like a giant clock with many complex, interacting parts,” says Geoffrey Vasil, team member and researcher at the University of Edinburgh. “But we don’t know much about 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 Wednesday (May 22) in the journal Nature.