The sun is an alien place where matter in a state rarely encountered on Earth roils and twines and sometimes erupts into space. Despite decades of telescope monitoring, scientists lack a fundamental understanding of how and why these outbursts happen. In the past couple of decades physicists have tried re-creating the situation on the sun’s surface in the controlled setting of the laboratory. One study recently wrapped up at the Princeton Plasma Physics Laboratory (PPPL) is turning up intriguing results.
About five kilometers north of the main Princeton campus in New Jersey the plasma lab is a complex of bland architecture housing equipment inside that looks like a cross between steampunk and Star Trektechnology. One of these pieces of hardware, the Magnetic Reconnection Experiment (MRX), is a cylindrical metal chamber 1.5 meters wide and about two meters long—large enough inside for a person to crawl around. MRX, built in 1995, is designed to study a process called reconnection, a collision of magnetic field lines that is central to solar eruptions. The device uses similar technology as the various nuclear fusion experiments dominating the PPPL’s work.
Physics graduate student Clayton Myers and his advisor Masaaki Yamada, MRX principal investigator, recently outfitted this chamber with a setup of wire coils and copper electrodes to mimic conditions during a so-called coronal mass ejection (CME). These events occur when magnetic energy builds up just above the solar surface and reaches a breaking point, causing the sun to belch plasma and magnetic fields into space. “The energy storage happens over a long timescale and the eruption happens very fast,” Myers says. “Our big question is the trigger. Exactly what determines when it snaps, when it can’t hold more magnetic energy?”
The researchers placed coils carrying electric current inside the experiment to simulate sunspots, magnetically active regions on the sun where current flows beneath the surface. On top of these coils, two copper electrodes serve as anchor points for a loop of plasma that mimics the arcs of plasma over the solar surface, or photosphere. The scientists injected gas into the chamber through tiny holes in the electrodes and then applied a voltage to heat the gas from room temperature into a 100,000-degree plasma. Once the sun simulacrum was set up, the experiment gradually pushed more and more magnetic energy into the plasma, and high-speed cameras watched to see if the energy was contained or reached a breaking point and erupted.
The whole process happened very quickly; the scientists sometimes conducted 300 runs, or “shots,” in a day. Myers built an array of seven long probes, each with 50 coils along its length, to measure the changing magnetic field at hundreds of locations throughout the chamber.
In the experiment eruptions happened less frequently than expected, and the researchers found that a component of the magnetic forces called the tension force, which serves to strap plasma down on the sun and hamper eruptions, could be more important than previously thought. “Clayton’s data is very exciting,” Yamada says. “We are finding a very interesting new phenomenon in solar flares.”
Myers says the discrepancy between his results and existing theory might involve failures of both theory and experiment. “There are certainly things that we’re not doing that the sun is doing, and identifying those things is very important.” One example is the structure of the experiment chamber itself. “What the sun doesn’t have is an outer wall,” he says. “That’s actually a big problem for us.”
Still, laboratory experiments such as the Princeton project can be surprisingly applicable to the situation on the sun, where the sizes and timescales are vastly different. “The reason is that if you look at the basic equations of the physics describing this, those equations don’t have an intrinsic scale; they work even if you make them a million times as big,” says Paul Bellan, a Caltech physicist who runs a competing solar laboratory experiment. And because remote observations only reveal so much about what is currently happening on the sun, laboratory astrophysics experiments offer unique opportunities. The MRX project “is a good example of a careful and systematic study of the physical processes presumed to be taking place on the sun that is only possible in the laboratory setting,” says astrophysicist Vyacheslav Lukin of the U.S. Naval Research Laboratory in Washington, D.C.
Beyond elucidating the complex physics of our nearest star, experiments such as these could offer practical benefits, because coronal mass ejections can wreak havoc on Earth. When the sun spews one in our planet’s direction, plasma slams into Earth’s magnetic field, releasing energy that can disrupt satellite communications and evenknock out power grids on the ground. A better understanding of what triggers CMEs could improve forecasting and provide precious advance notice when the sun’s wrath is headed our way.