Solving a Plasma Physics Mystery: Magnetic Reconnection

Image courtesy of NASA

This graphic shows the magnetic field surrounding the Earth and how it reacted to energy and plasma from a solar flare caused by magnetic reconnection.

The sun doesn't normally deliver electrical shocks. But on two days in September 1859, a solar flare caused electricity to flow from the atmosphere down telegraph lines. In some cases, it flowed into the operators, delivering a nasty shock. In others, it lit the telegraph paper on fire. Telegraph systems in several countries failed outright.

If a similar flare and resulting geomagnetic storm happened today, "it would have an enormously disruptive effect on life on Earth," said Ellen Zweibel, a theoretical physicist at the University of Wisconsin-Madison. Massive telecommunications and electrical outages would be likely. "We would really love to know how to predict how strong a flare would be."

The key to forecasting such a flare is understanding a phenomenon called magnetic reconnection.

Magnetic reconnection is a process that occurs nearly anywhere there's plasma. The fourth state of matter, plasma, is gas made up of unbound ions and electrons. As plasma makes up the stars and 99 percent of the visible universe, magnetic reconnection is quite common. However, it is poorly understood. Scientists at universities, research institutes, the Department of Energy Office of Science's Princeton Plasma Physics Laboratory (PPPL), and NASA are coming close to mapping the process of magnetic reconnection. With the help of modeling, experimental, and observational data, they think their most recent theory may provide the definitive map to guide scientists through this fundamental phenomenon.

Small Collisions that Cause Big Problems

Anything that has a large amount of moving mass — from the Earth's core to galaxies — forms a magnetic field.

"You look every direction in the universe and it is magnetized," said Hantao Ji, a PPPL physicist.

Magnetic fields are made up of field lines. Electrons and ions flow along these invisible lines. When two sets of lines that have magnetic fields pointing in opposite directions get too close, they collide. As the field lines cross and form an X, they break and then reconnect to the other set of lines coming from the opposite direction. Forming U-shapes that push away from each other, they rearrange the magnetic field. By heating up and accelerating the particles in the plasma, that rearrangement transforms magnetic energy into particle energy. This tumultuous process is magnetic reconnection.

"Magnetic reconnection is one of the most important phenomenon throughout the whole universe," said Jim Burch, Vice President of the Southwest Research Institute and principal investigator of NASA's Magnetospheric Multiscale (MMS) mission.

Understanding magnetic reconnection could help us understand how magnetic fields arose early in the universe's history as well as protect us from its effects. When reconnection pierces the Earth's magnetic field, high-energy particles can flow from the sun into the Earth's atmosphere. Those particles can harm low-flying satellites and electrical grids. For PPPL scientists looking to recreate fusion — the energy source that powers the sun — magnetic reconnection reduces their control and can damage their machines.

Capturing Fast Reconnection in Theory

Any theory explaining magnetic reconnection has to explain how fast it occurs and how it transforms so much energy.

At first, scientists assumed they could explain magnetic reconnection using the standard theory that explains how fluids affected by magnetic fields behave. That didn't work. When scientists used this theory to calculate how quickly a solar flare develops, the answer was a million years. In reality, solar flares develop in only a few minutes.

Next up was the Sweet-Parker theory, proposed in 1957. It described thin, stretched-out sheets of electrical current forming in the plasma. Magnetic field lines sit on top of these sheets of electrical current. As the magnetic field lines break apart, the particles that normally flow along the field lines break away from them and stop being magnetized.

So far, so good. But reconnection according to the Sweet-Parker theory was still too slow. Using this theory, solar flares occurred hundreds of times slower than the real thing. This theory also didn't explain why the process released so much energy. A part of the map was missing.

On the plus side, this theory could explain reconnection in certain types of plasmas. It also laid the groundwork for research to come.

Filling in the Gaps on the Map

To expand their search, physicists turned to mathematical simulations, laboratory experiments, and observations in space. Theories provided the broad lines of the map, pointing the way for others to go.

"I try to make a mathematical model that uses a relatively small number of laws of physics to explain the essential phenomena," said Zweibel. "A good cartoonist, with just a few lines, can create an image."

Physicists that create computer models take those broad laws and enter them as limits for computers to follow. These simulations pencil in details to the broad lines.

Laboratory experiments and observations of space plasma make those penciled lines more definitive. Theories and simulations help them know what to look for.

While they had some data from fusion experiments, one of the first efforts to study magnetic reconnection was PPPL's Magnetic Reconnection Experiment (MRX). Physicist Masaaki Yamada and his colleagues designed the machine in the 1990s, repurposing pieces from a magnetic fusion experiment. Nearly 30 years later, MRX still creates conditions as close as it can to reconnection in space. After heating plasma to 20 times the temperature of the sun's visible surface, it triggers magnetic reconnection in a thousandth of a second. Using measurements of the plasma's density, temperature, and electrical fields, scientists create a 2D map of magnetic fields in the machine.

If the data match the theories and models, they reinforce those results. If not, it's back to the drawing board.

"You cannot always trust the simulation all of the time," said Fatima Ebrahimi, a PPPL modeler. "If it doesn't match [the experiment] very well, that means you need to go back to your simulation and probably revise your model."

But Did It Work?

This back-and-forth led to a new theory: two-fluid reconnection. Unlike previous theories, it modeled electrons and ions like two fluids that move separately from one other.

When they ran the numbers, scientists were consistently getting reconnection rates that matched observations. It looked like the theory they had been searching for.

Initial findings from the MRX looked promising.

"We compared the data and had agreement between the MRX data and the numerical simulation," said Yamada. "It was really amazing."

But scientists wanted more details about reconnection in larger plasmas. A comprehensive theory has to describe how magnetic reconnection occurs in different types of plasmas, from small ones in laboratories all the way up to the massive Crab Nebula in space. Scientists also categorize the different types of plasma depending on how much particles interact inside them. In the plasma inside the sun and early stages of solar systems, particles interact a lot. In the space plasma near Earth and in fusion research facilities, particles interact only a little.

Unfortunately, plasma physicists couldn't collect these details on the ground. But NASA's satellites could.

Scientists from PPPL worked with NASA to design the Magnetospheric Multiscale (MMS) mission. The first mission specifically designed to study magnetic reconnection, the MMS has four identical spacecraft. Launched in 2015, these craft are currently circling the globe. As they dart between the Earth's magnetic field and the sun's, they collect data to create 3D maps of magnetic fields. In the first year, they crossed the boundary 9,000 times and recorded 25 reconnection events.

Theory, simulations, and data from the MRX provided a map to direct the MMS. The MMS searches for certain magnetic fields that the two-fluid theory predicts. The NASA scientists used the predictions to design parameters for the MMS's computers. Because the MMS collects so much data that it can only send down about 5 percent of it with full detail, it needs to be choosy.

Since it first captured data on magnetic reconnection in 2016, the MMS has confirmed much of what two-fluid theory predicts.

"It's just how science should work. [Theorists] made predictions and we came along and verified some," said Burch. "And we found new things and now [theorists] are trying to figure them out."

But two-fluid theory still had a hole. The theory explains reconnection in small and medium plasmas but doesn't explain how reconnection happens in very large plasmas that have few interactions between their particles. As these plasmas make up much of the universe, it was a big gap.

Enter the Plasmoid

In 2007, Nuno Loureiro, then a postdoctoral researcher at PPPL and a participant of the DOE Center for Multiscale Plasma Dynamics at the University of Maryland, developed what might be the final piece of the map. It was a new theory that became known as the plasmoid instability. It connects the Sweet-Parker and two-fluid models into a single theory. Later work at PPPL has expanded on his initial idea.

Like Sweet-Parker, the plasmoid instability model starts with a stretched-out, thin sheet of electrical current with an accompanying magnetic field. Like the two-fluid model, it assumes the electrons and ions that flow along the magnetic field lines break away at different times.

What makes this model different is that it is based on the fact that the sheets of electrical current are extremely unstable. As the sheets stretch, they break and form new ones, each thinner than the original. As these sheets separate, chains of magnetic bubbles (plasmoids) form between them. While previous theories had described these bubbles, no one had provided a good explanation of how and why they form.

The theory proposes that as more bubbles form and sheets break up, the magnetic lines crash into each other and break. The lines disconnect from the ions first, then the electrons. The breaking feeds magnetic energy into the particles, heating them up and accelerating them. As time goes on, the whole process becomes faster and faster. It creates a runaway effect — fast reconnection.

Unlike Sweet-Parker, this model provides the "oomph" to give fast reconnection its speed. Unlike the two-fluid theory, it illustrates why and how the process starts after that initial sheet of strong electric current forms.

Modeling and experimental data have held the theory up — so far.

Simulations such as Ebrahimi's have backed it up. By modeling reconnection on both a single sheet and multiple sheets, she discovered that bubbles sometimes grow in multiple sheets in 3D due to large-scale magnetic field generation when they do not in a single sheet in 2D. Her simulations also predicted the plasmoid instability in a large fusion device during plasma startup.

In 2016, the best supporting evidence so far emerged. At the Terrestrial Reconnection Experiment at the University of Wisconsin (TREX), researchers supported by the Office of Science directly observed bubbles in a similar type of plasma to one that's common in space. The next year, observations at the MRX reinforced these results. Both observations lined up with simulations and theory.

Unfortunately, both machines' technical limits mean that this isn't the end of the story. The plasmas' small size and restrictions on how well they can conduct electricity keep them from fully mimicking the processes that occur inside the sun or beginnings of solar systems.

But the solution may be here soon. The MMS will continue to provide insight into plasma around the Earth. On the ground, future experimental devices will be able to investigate the type of plasmas that make up so much of the universe. Only then can scientists know if the plasmoid instability model holds the key or not.

"We need to do research to find out if it is true," said Ji. "This is the grand challenge we are facing."

The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit https://science.energy.gov.

Shannon Brescher Shea is a senior writer/editor in the Office of Science, [email protected].