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Space + FlightSpace & Flight

This ‘Big Red Ball’ can simulate the Sun’s bizarre magnetic field

Physicists built a machine that might help explain how solar wind forms—all without leaving Earth’s atmosphere.

ByKatherine J. WuNOVA NextNOVA Next
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The Big Red Ball, a plasma confinement vessel, can mimic certain aspects of the Sun's magnetic field. Image Credit: Courtesy of Cary Forest, University of Wisconsin-Madison

At this very moment, NASA’s Parker Solar Probe is in the midst of its third lap around the Sun. Since its launch last August, the spacecraft has been furiously collecting data for Earthlings eager to unravel the mysteries of the Solar System’s life-sustaining star.

By the end of 2024, the Probe will have flown within 4 million miles of the Sun’s sweltering surface. But the device closest to capturing the essence of our Solar System’s cosmic centerpiece might actually be nestled in the bowels of an astrophysics lab in Madison, Wisconsin.

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That device is the Big Red Ball—an aptly-named machine built to generate and contain simulations of solar plasmas. In truth, the 10-foot-wide aluminum vessel, the product of a team led by University of Wisconsin-Madison physicist Cary Forest, looks more like the lovechild of a Mars rover and a crimson papier-mâché ball than a middle-aged star.

But as Forest and his colleagues report today in the journal Nature Physics, the reactions generated within the Big Red Ball can mimic some of the features of the Sun’s spiral-shaped magnetic field. In doing so, their invention recreates a phenomenon that’s never before been observed on Earth.

To be clear, the Big Red Ball is not the Sun. By design, the device recapitulates only a sliver of the intricate dynamics that characterize the surface of the Sun, which dwarfs it in both complexity and scale. All the same, the team’s findings have the potential to shed serious (sun)light on the origin and evolution of phenomena like solar wind—and, in doing so, provide stellar aficionados with a source of solar data that’s much closer to home.

“This paper is incredibly exciting,” says Elizabeth Jensen, a solar physicist at the Planetary Science Institute who was not involved in the study. “Working on these problems in the lab is something we’ve needed for a very long time now. That they’ve been able to push this through at a moment in time when we have a spacecraft out there, taking measurements, makes the overall study of the Sun that much richer.”


An illustration of NASA's Parker Solar Probe approaching the Sun. Image Credit: NASA/Johns Hopkins APL/Steve Gribben

For something so essential to life, the Sun remains remarkably poorly understood. One of the biggest enigmas in solar physics is the source of solar wind, the stream of hot, charged particles of gas, or plasma, that jets off the Sun’s surface at extremely high speeds.

These energetic emissions are bursting with so much ammo that they can interfere with satellites orbiting Earth, or ripple the skies with dazzling auroras. Even Uranus and Pluto can be rattled by solar wind’s wayfaring particles, which have been detected by spacecraft meandering in that cold, distant neck of the cosmic woods.

Before any of that can happen, though, plasma needs to muster the energy required to break free of the Sun’s hefty magnetic field, which confines would-be solar wind like the bars of a jail cell. But the particles manage the feat—and doing so, they don’t just liberate themselves from their magnetic prison. They haul along bits of their former cage like cosmic cargo, ferrying thread-like shreds of the Sun’s magnetic field deep into interplanetary space.

All the while, these hunks of magnetic field remain tied to their source, stretching out like strings of cheese oozing off a pizza slice. Then, as they permeate the Solar System, these tenuous connections get twisted by the slow rotation of the Sun, collectively forming a vast magnetic structure called the Parker spiral. Viewed from afar, it’s thought to resemble the billowing skirts of a pirouetting stellar dancer.

Scientists still don’t have a good understanding of how solar wind acquires the enormous cache of energy it needs to launch itself and the Sun’s magnetic field into space. Figuring that out, of course, requires intel from the Sun itself, and there’s no replacement for the data that’s being collected by Parker Solar Probe, as well as future missions like PUNCH, which will use satellites to track the wind’s trajectory.

But even a top-notch spacecraft has its limits. Extraterrestrial missions are difficult, hazardous, and expensive. And, compared to its target, the car-sized Parker Solar Probe is but a pinprick in space, limiting the scope of its data collection at any single point in time, explains study author Ethan Peterson, a graduate student working under Forest’s supervision.

These restrictions, however, all but melt away in the right plasma lab—and that’s where the Big Red Ball comes in. To mimic a magnetized star, Forest and his team inserted a magnet into the center of the device’s main chamber and shrouded it with a cloud of hot, energy-rich plasma. They then drove a small current into the machine that forced the plasma to spin.


An illustration of the Sun's skirt-like magnetic field permeating the Solar System. Image Credit: Werner Heil, Wikimedia Commons

At first, the churning ball of plasma wasn’t much to look at. Constrained by the force of their inner magnet, the particles mostly moved within a relatively confined space. But when the twirling reached a speed of about 22,000 miles per hour, something extraordinary happened: The plasma broke free.

This critical threshold represents the point at which the plasma acquired enough energy to overcome its magnetic restraints, Peterson explains. The newly liberated particles then spun out with some of the inner magnetic field in tow. And before long, the whole mess had been whipped into its own undulating Parker spiral, right here on Earth.

The simulation also appeared to showcase a simplified version of how so-called slow solar wind—which moves at “sluggish” speeds of around 1 million miles per hour—manages to escape the Sun’s equator, where the star’s grasp on plasma can be especially firm.


A view into the Big Red Ball, which can simulate plasma dynamics like those observed on the Sun. Image Credit: Jeff Miller, University of Wisconsin-Madison

As hot, charged particles spun within the Big Red Ball, parts of the magnetic field began to fragment, effectively fraying some of the seams holding the roiling plasma in place. The magnetic infrastructure was quick to repair itself—but not before a few blobs of plasma spewed from the cracks. Similar explosions have been observed on a grander scale on the Sun itself, Peterson says, hinting that this process could be one of the many ways in which solar wind finds its way into the interplanetary realm.

Several of these details have been known for a long time, thanks to observations of the star of the show itself, says Kelly Korreck, a solar physicist at the Harvard-Smithsonian Center for Astrophysics and Parker Solar Probe team member who was not involved in the study. But this is the first time they’ve been simulated—and that’s a big deal.

“Some of what’s in this paper makes me go, ‘Wow, they’ve been able to do something that looks so similar to what we think we’ll see at the Sun,’” she says. “We now have a way on Earth to start to try to understand how stars make their winds.”


A shot of the Big Red Ball in Sterling Hall at the University of Wisconsin-Madison. Image Credit: Jeff Miller, University of Wisconsin-Madison

The Big Red Ball is no substitute for the Sun itself, though, and there are limitations to its ability to ape the star’s magnetism, Forest notes. For one, the plasma in the device can’t generate a strong gravitational pull like the Sun does. And what happens inside a 10-foot-wide machine won’t scale up perfectly to a gargantuan, temperamental star.

But in a way, that makes it all the more exciting that Parker Solar Probe and other missions will be feeding us intel for years to come, says Heather Elliott, a solar physicist at the Southwest Research Institute in San Antonio and PUNCH team member who was not involved in the study. One of the Probe’s main objectives is to dip close enough to the Sun’s surface to witness the birth of solar wind in its natural habitat. What’s been observed in the Big Red Ball may yet be corroborated and built upon by future observations—which, in turn, could inspire labs on Earth to tweak and advance their simulations.

The Sun is a constantly evolving force, Korreck says. Peering into the belly of the beast—and actually understanding what’s inside—will take years of experiments, models, and observations here, there, and everywhere in between. In the meantime, we need all the help we can get, she says. “That’s what makes [the Big Red Ball] such a great tool.”

In the grand scheme of things, not much has changed: Our inscrutable Sun still taunts us from 100 million miles away. But here on Earth, it would seem we’re now a little less in the dark.

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