Massachusetts Institute of Technology | David Chandler | 2012 Feb 26
Computer simulation identifies source of aurora-causing high-speed electrons in space
A mysterious phenomenon detected by space probes has finally been explained, thanks to a massive computer simulation that was able to precisely align with details of spacecraft observations. The finding could not only solve an astrophysical puzzle, but might also lead to a better ability to predict high-energy electron streams in space that could damage satellites.Click to play embedded YouTube video.
Jan Egedal, an associate professor of physics at MIT and a researcher at the Plasma Science and Fusion Center, working with MIT graduate student Ari Le and with William Daughton of the Los Alamos National Laboratory (LANL), report on this solution to the space conundrum in a paper published Feb. 26 in the journal Nature Physics.
Egedal had initially proposed a theory to explain this large-scale acceleration of electrons in Earth’s magnetotail — a vast and intense magnetic field swept outward from Earth by the solar wind — but until the new data was obtained from the computer simulation, “it used to be people said this was a crazy idea,” Egedal says. Thanks to the new data, “I don’t get that anymore,” he says.
The simulation shows that an active region in Earth’s magnetotail, where “reconnection” events take place in the magnetic field, is roughly 1,000 times larger than had been thought. This means a volume of space energized by these magnetic events is sufficient to explain the large numbers of high-speed electrons detected by a number of spacecraft missions, including the Cluster mission.
Solving the problem required a staggering amount of computer power from one of the world’s most advanced supercomputers, at the National Institute for Computational Science at Oak Ridge National Laboratory in Tennessee. The computer, called Kraken, has 112,000 processors working in parallel and consumes as much electricity as a small town. The study used 25,000 of these processors for 11 days to follow the motions of 180 billion simulated particles in space over the course of a magnetic reconnection event, Egedal says. The processing time accumulated gradually, squeezed in during idle time between other tasks. The simulation was performed using a plasma-physics code developed at LANL that rigorously analyzes the evolution of magnetic reconnection.
Egedal explains that as the solar wind stretches Earth’s magnetic-field lines, the field stores energy like a rubber band being stretched. When the parallel field lines suddenly reconnect, they release that energy all at once — like releasing the rubber band. That release of energy is what propels electrons with great energy (tens of thousands of volts) back toward Earth, where they impact the upper atmosphere. This impact is thought, directly or indirectly, to generate the glowing upper-atmosphere plasma called the aurora, producing spectacular displays in the night sky.
What had puzzled physicists is the number of energetic electrons generated in such events. According to theory, it should be impossible to sustain an electric field along the direction of the magnetic field lines, because the plasma (electrically charged gas) in the magnetotail should be a near-perfect conductor. But such a field is just what’s needed to accelerate the electrons. And, according to the new simulation, the volume of space where such fields can build up can, in fact, be at least 1,000 times larger than the theorists had thought possible — and thus large enough to explain the observed electrons.
“People have been thinking this region is tiny,” Egedal says. But now, “by analyzing the spacecraft data and doing the simulation, we’ve shown it can be very large, and can accelerate many electrons.” As a result, “for the first time, we can reproduce the features” observed by the Cluster spacecraft.
That could be important because, among other things, “these hot electrons can destroy spacecraft,” Egedal says, which is why both the military and NASA “would like to understand this better.”
Although this analysis was specific to the phenomena in Earth’s magnetotail, Egedal says similar phenomena may be taking place in much bigger regions of magnetized plasma in space — such as in mass ejections that erupt from the sun’s corona, which occupy regions 10,000 times larger, or even regions surrounding pulsars or other high-energy objects in deep space, which are much larger still. In the future, he hopes to carry out simulations that would apply to the sun’s coronal mass ejections. “We think we can scale up the simulation” by a hundredfold, he says.
Michael Brown, a professor of physics at Swarthmore College who was not involved in this research, says Egedal “is emerging as a real leader in experimental [and] observational aspects of magnetic reconnection,” and his co-author Daughton “is the recognized leader in state-of-the-art plasma simulations.” The new result “is very significant, and I think is surprising to the rest of the community. … I think this picture will gain more and more acceptance, and we have to go beyond” the presently accepted picture of plasmas, he says.
Northern Lights Mystery May Be Solved
Space.com | Science & Astronomy | 2012 Feb 27
Scientists may have solved a longstanding mystery about the origin of the energetic particles that cause Earth's dramatic aurora displays.
The electrons responsible for the auroras — also known as the northern and southern lights — are likely accelerated to incredible speeds in an active region of Earth's magnetosphere, according to a new study. This region is 1,000 times larger than scientists had thought possible, providing enough volume to generate lots of the fast-moving electrons, the study reports.
"People have been thinking this region is tiny," lead author Jan Egedal, of the Massachusetts Institute of Technology, said in a statement. But now, he added, "we’ve shown it can be very large, and can accelerate many electrons."
Egedal and his colleagues analyzed data gathered by various spacecraft, including the European Space Agency's four Cluster probes. They also performed simulations using a supercomputer called Kraken at the United States Department of Energy's Oak Ridge National Laboratory in Tennessee.
Kraken has 112,000 processors working in parallel. The team used 25,000 of these processors for 11 days, following the motions of 180 billion simulated particles in space to map out how aurora-generating electrons move.
The researchers determined that these electrons are likely being rocketed to their tremendous speeds in the magnetotail, a portion of Earth's protective magnetosphere that has been pushed far into space by the solar wind.
As the solar wind — the million-mph stream of charged particles coming from the sun — stretches Earth's magnetic-field lines, the field stores energy like a rubber band being stretched, Egedal said. When the normally parallel field lines reconnect, that energy is released like a rubber band being snapped, and electrons are propelled back toward our planet at fantastic speeds.
When these fast-moving electrons hit molecules in Earth's upper atmosphere, the impact generates the phenomenon that we know as the northern and southern lights. [Photos: Dazzling Northern Lights of February 2012]
Some physicists had viewed this origin story for the aurora-causing electrons as improbable, because they didn't think the active magnetotail region was big enough to generate the huge numbers of electrons that slam into Earth's atmosphere.
Egedal and his team found, however, that the region is likely plenty big — roughly 1,000 times larger, in fact, than theorists had thought possible.
"It used to be people said this was a crazy idea," Egedal said. "I don’t get that anymore."
In addition to creating a beautiful glow at Earth's higher latitudes, these super-energetic electrons can damage or destroy spacecraft. So a better understanding of their behavior may help operators better protect their satellites, researchers said.
Large-scale electron acceleration by parallel electric fields during magnetic reconnection - Jan Egedal, William Daughton, Ari Le
- Nature Physics (online 26 Feb 2012) DOI: 10.1038/nphys2249
