They say beauty is only skin deep, and apparently so is the Sun’s magnetic field, according to a recent study.
The Sun’s magnetic field is shallow, suggests new research, which used computer simulation to model the flow of plasma in the outer layers of the Sun. Astrophysicists had suspected that the Sun’s powerful magnetic field, which has a huge impact on communications, navigation, and other electronics on and around Earth, was produced by motion much deeper in the star (similar to the way Earth’s magnetic field is generated by the motion of our planet’s metallic core). Armed with a better understanding of where the Sun’s magnetic field comes from, physicists may soon stand a better chance of predicting how much activity each 11-year solar cycle holds in store.
University of Edinburgh mathematician Geoffrey Vasil and his colleagues published their work in the journal Nature.
Shallow, No Diving
Vasil and his colleagues used a computer simulation to model how plasma flows in the outer layers of the Sun, called the convective zone: a 124,000-mile-deep sea of roiling plasma, heated from below by the Sun’s thermonuclear core. Specifically, they were interested in what happened in the top 20,000 miles or so of the plasma.
Massachusetts Institute of Technology mathematician Keaton Burns, a coauthor on the study, designed the program, called the Dedalus Project, to be very good at simulating how fluids flow, from the movements of liquid inside a cell to the circulation of Earth’s oceans and atmosphere. And the Sun, although we tend to picture it as a big ball of fire, is actually a big ball of super-hot gas.
Physicists know that the magnetic field is driven by an enormous dynamo: physical mass in motion, whose mechanical energy becomes electrical energy.
“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,” explains Burns in a recent statement.
Most astrophysicists are convinced that process happens more than 125,000 miles deep inside the Sun, at the very bottom of the convection layer. That explanation makes sense in a lot of ways, but simulations based on it don’t actually produce results that look much like the churning, roiling plumes of plasma in the Sun’s outer layers — or the pattern of sunspots (dark, relatively cool patches of the Sun’s surface, where magnetic field lines meet and tangle) that astronomers actually see on the Sun’s surface. The real Sun is much more turbulent than most simulations suggest. And in simulations of a magnetic field coming from deep within the Sun, sunspots tend to gather near the Sun’s poles; on the real Sun, most sunspots form near the equator.
Knowing more about how the Sun’s magnetic field works could help scientists make predictions about the kinds of solar activity that produced stunning auroras — and briefly stalled spring planting on some farms by disrupting navigation systems on tractors and other equipment — earlier in May.
Just beneath the surface of the Sun
Vasil and his colleagues’ simulations showed that the Sun’s outermost reaches are made of layers of plasma, which normally rotate past each other (picture an onion; now picture an onion 865,000 miles wide and made of super-hot gas with a giant thermonuclear reactor running at its center). But tiny changes in how one layer flowed past the next — like a little plume or a slight eddy — could create an instability that fed on itself and rapidly grew into major turbulence.
“It doesn’t take much to trigger an instability! If a system is unstable, it is like a pencil balanced on its point — any slight breeze is enough to knock it over,” co-author Daniel Lecoanet, an assistant professor of Engineering Sciences and Applied Mathematics at Northwestern University, tells Inverse. “The outer part of the Sun is convective, which is kind of like it is boiling. These roiling motions are definitely enough to seed the instability.”
Most of the instabilities that eventually formed got their start thanks to powerful winds in the interior of the Sun, which move at different speeds depending on how deep into the Sun you go. As you sink deeper into the Sun, the winds get stronger — until about 21,000 miles below the outer edge, when they suddenly slow down.
“The difference in wind speed at the surface vs. deeper into the Sun is a source of energy that can drive instability,” says Lecoanet. That energy is eventually converted into the Sun’s magnetic field.
And in Vasil and his colleagues’ simulations, those instabilities in the outer layers of the simulated Sun eventually produced a pattern of magnetic field lines and sunspots that looked remarkably like the real thing.
“Our work provides strong evidence that the solar cycle starts near the surface of the Sun in the equatorial region,” says Lecoanet.
Predicting Solar Storm Seasons
Eventually, Vasil and his colleagues hope their work will help astrophysicists forecast how active the next solar cycle will be, similar to the way that meteorologists predict how active the coming hurricane season will be.
“This work is not trying to make predictions about individual solar storms,” says Lecoanet.
At the moment, NOAA’s Space Weather Prediction Center can predict the severity of a solar storm a few days in advance, around the time a huge belch of plasma called a coronal mass ejection leaves the Sun’s surface. But we don’t have a good way to predict what the next few years hold; some solar cycles are more active than others, and knowing what’s coming could help us prepare.
“We want to forecast if the next solar cycle will be particularly strong, or maybe weaker than normal,” says Lecoanet. “The previous models (assuming the solar magnetic field is generated deep within the Sun) have not been able to make accurate forecasts of if the next solar cycle will be strong or weak. We are hoping to make predictions of if a cycle will have a lot of solar storms or not a lot.”
