This visualization from a kinetic magnetic reconnection model shows magnetic flux ropes (blue) along a selection of magnetic field lines (yellow). A movie of such a simulation helps scientists explore the three-dimensional structure of the process, including the flux ropes interacting. (These findings come from work published this year by Los Alamos National Laboratory’s Yi-Hsin et al. in Physical Review Letters, 110.265004.)
Many textbooks include diagrams of simple bar magnets with roughly semicircular field lines emanating from one end and returning to the other. Imagine those lines breaking and rejoining in a process called magnetic reconnection.
This phenomenon arises often in plasmas – ionized gases that include protons and electrons. In fact, magnetic reconnection is thought to occur in plasmas ranging from laboratory experiments to planetary, solar and astrophysical environments.
But magnetic reconnection is particularly troubling when it leads to explosive energy releases during solar magnetic storms, spewing out billion of tons of hot gas traveling at several million miles per hour toward Earth. Magnetic reconnection also lets this hot gas penetrate Earth’s protective magnetic field, leading to what’s popularly called space weather.
One of the worst of solar storms, known as the Carrington event, happened in 1859. An amateur English astronomer, Richard Carrington, tracked its magnetic field, which roared through space and ripped through Earth’s own magnetosphere – the magnetic field surrounding our planet. The storm destroyed telegraph systems around the world, even causing some to shoot sparks.
Today, a magnetic storm of similar magnitude could cause $2 trillion in damage, says Homayoun Karimabadi, a research scientist in the department of electrical and computer engineering at the University of California, San Diego. “It could take out transformers over a vast area, and it could take several months to repair.”
Scientists estimate there’s at least a 10 percent chance of a solar storm of the 1859 magnitude hitting Earth in the next 10 years. “In July 2012,” Karimabadi says, “we barely missed – by just a few days – an event the size of the Carrington event. Had it happened a few days later, the storm would have been pointing right at us.”
Researchers seek ways to forecast space weather, in hopes that countries could avoid such magnetically driven catastrophes, perhaps just by knowing ahead of time to turn off electronic devices. These forecasts require a better understanding of the processes behind magnetic reconnection.
It starts with plasmas. “Most of the visible matter in the universe is in the form of plasma,” Karimabadi says. “Most of this ionized gas has a magnetic field embedded in it.” The Earth’s magnetic field usually deflects most of the magnetized plasma coming from the sun, but the shielding is less effective during extreme solar storms, creating havoc on technological systems, including communication satellites and power grids.
Bill Daughton, a physicist at Los Alamos National Laboratory, explains. First, magnetic field lines in a plasma move with it, so if part of a large-scale plasma moves, the magnetic field lines can distort. “Think of the lines like rubber bands. They can become stressed in regions and, figuratively, snap and then move back together.” These magnetic fields store energy, and the snapping can take place quickly, releasing that energy explosively, something visible in a solar flare bursting off the sun.
At the smallest scale, a plasma consists of particles moving in a magnetic field (see sidebar, “Sizing up the scales.” [1]) “At that scale, there are too many particles to track in a real system,” Daughton says, “so people look for statistical descriptions.”
Three-dimensional kinetic simulations of magnetic reconnection reveal turbulence generation, which was not evident in two-dimensional simulations. The turbulence could play a fundamental role in forecasting space weather.
The simplest statistical approach describes a plasma with fluid equations, “like the flow around an airplane, but with additional forces from the electric and magnetic fields,” Daughton explains. A relatively large system can be studied with this approach, but it depends on making approximations, some of which might be invalid.
For example, a fluid model of plasmas works well for most of a system, except where the magnetic field lines get very close together. In these so-called thin regions, the model breaks down. “We are debating what physics causes this,” Daughton says.
Instead of treating the plasma only as a fluid, researchers looked at particle characteristics in a kinetic model. “A fully kinetic model makes fewer assumptions than a fluid one,” Daughton says. It’s essentially a statistical description of the plasma working from first-principles (fundamental physical laws).
Since magnetic reconnection tends to occur in thin areas, scientists want to “couple the dynamics of the large scale to those thin areas,” says Vadim Roytershteyn, a plasma physicist at SciberQuest in Del Mar, Calif. “To obtain the full description, one must simultaneously resolve the large-scale dynamics and the tiny layers.”
Daughton, Karimabadi and Roytershteyn took this new thinking – plus the three-dimensional kinetic plasma simulation code called VPIC – to the Cray XK6 at Oak Ridge National Laboratory. The Department of Energy’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program provided 37 million processor hours for the work.
There were intriguing results. Karimabadi says the team found that starting with a 2-D system that is purely laminar – smooth, rather than turbulent – it stays laminar even after magnetic reconnection. When the team ran the simulation in 3-D, the system self-generated turbulence. “Turbulence can increase the mixing efficiency,” he says, “and space weather is all about mixing of the solar-wind plasma with that inside the Earth’s magnetosphere.” This mixing can amplify the effects of space weather during a massive storm.
Overall, the research suggests magnetic reconnection in a plasma behaves more like a fluid on larger scales and more kinetic on smaller scales.
“The Holy Grail is a model that would allow bigger computational cells and fluid-like behavior where possible and to automatically treat the system kinetically where needed,” Daughton says. “That is very challenging, but work on it is beginning.”