An artist’s rendition of Earth’s magnetosphere. A magnetic tail, or magnetotail, is formed by pressure from the solar wind on a planet’s magnetosphere. Image: NASA |
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.
Jan
Egedal, an associate professor of physics at Massachusetts Institute of Technology
(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 in 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.
