The solar wind, originating in the million-degree-Celsius solar corona, blows through the solar system and interacts with the protective magnetosphere of the Earth. Huge blobs of plasma, known as coronal mass ejections, occasionally erupt from the solar surface and can interfere with satellite communications when they collide with the Earth’s magnetosphere. (Image courtesy of NASA) |
The sight of an aurora evokes feelings of
mystery and awe in the weekend star gazer and scientist alike. The stargazer
may ponder the vastness of our universe or how such vivid color can be created
in space, but for the scientist, the questions lie in the composition of the
aurora—and how little we actually know about it.
The ionized gases known as astrophysical
plasma give us auroras, comet tails, solar wind, and much of the spiraling
matter falling into a black hole. Created when a gas becomes so hot that
electrons are pulled away from atoms, such plasma is a fourth state of matter,
in addition to solids, liquids, and gases, and makes up much of the known
matter in space. Scientists began investigating the mysteries of space plasma
decades ago but are still at a loss to explain many aspects of plasma’s
behavior, especially the random, chaotic movements known as turbulence.
“Turbulence has always been a major
unsolved problem in physics, and in the realm of space physics it’s a
particularly important one,” said University of Iowa
astrophysicist Gregory Howes. A team led by Howes is pursuing this mystery on
Oak Ridge National Laboratory’s Jaguar supercomputer. The team was allotted
time through the Innovative and Novel Computational Impact on Theory and Experiment,
or INCITE, program.
The project initially used the GS2 code,
which computes the behavior of ions and electrons in a doughnut-shaped tokamak
fusion reactor. To make the application more useful for space plasmas, Howes’s
team stripped the code of its tokamak geometry, creating a faster simulation
code with shorter development time. The resulting code—called AstroGK—was
completed in 2007 and is routinely run using 51,000 of Jaguar’s 224,000
processing cores.
Without comprehending the effect of turbulence,
scientists cannot understand the unusual properties of plasma, from its role in
heating up the supermassive black hole in the center of our galaxy to the
creation of the solar wind that blows away from the sun, creating a giant
bubble surrounding the solar system.
Although some characteristics of the solar
wind are well understood, many others are still a mystery, such as the way it
is heated by turbulence. “Plasmas are notoriously complicated
entities,” Howes said. “The range of phenomena is amazing.”
Laws of attraction
The turbulence of space plasmas is fundamentally unlike the more familiar
turbulence of wind and water. On Earth the particles in air and water
constantly collide with one another and therefore move at essentially the same
speed. This uniformity allows them to be described as a fluid, depending on
only density, velocity, and temperature.
Astrophysical plasmas do not behave this
way. The particles in them rarely collide and can therefore travel at different
speeds. In addition, not only do these particles move along with the plasma as
a whole, but they also bounce around erratically and independently within the
plasma, adding another three-dimensional behavior that must be accounted for.
This combination of characteristics requires a kinetic, rather than a fluid,
description of the plasma.
“Essentially the physics of the plasmas
is six dimensional, and a seventh [dimension] is time. This is very
computationally challenging, and this leads into why we need to use the
resources through INCITE,” Howes said.
Turbulence in astrophysical plasmas can have
different causes depending on where the plasma is and how a magnetic field is
influencing it. In the atmosphere above the sun’s surface, the solar corona,
turbulence plays an unknown role in heating the plasma from 6,000 to about 1
million degrees Celsius (roughly 10,000 to 1.8 million degrees Fahrenheit).
Intense heat and gravitational pull from the sun create turbulence in solar
wind. On the other hand, the gravitational pull from a black hole creates
turbulence by forcing the arms of gaseous matter—known as accretion disks—to
slide against one another as they move toward the center.
Howes’s team focuses on understanding the
heating of the solar wind plasma as the turbulence is dissipated at scales from
kilometers to hundreds of kilometers. Petascale supercomputers such as Jaguar,
which is capable of more than 2 quadrillion calculations per second (2
petaflops), have finally allowed scientists to simulate the intricate web of
motions within space plasmas and unravel the physical mechanisms involved in
the turbulence and plasma heating.
Even with petascale systems, however,
getting meaningful data out of six-dimensional simulations is difficult. Much
past research in the field has relied on magnetohydrodynamics, which treats the
plasma as a fluid and calculates results based on the three spatial dimensions
of the plasma. The independent motions and interactions of the plasma particles
are unaccounted for in this case, leaving an incomplete description of the
substance.
To more accurately describe the kinetic
properties of solar wind, the team uses a mathematical approach called
gyrokinetic theory, which eliminates unnecessary physics from kinetic theory to
more efficiently simulate solar wind turbulence. This approximation averages
over the spiral motion of ions and electrons in the interplanetary magnetic
field, reducing the kinetic descriptions to five instead of six dimensions.
Some of the solar wind physics may lie outside the gyrokinetic description,
though. “It is an approximation, and one must keep that in mind doing this
research,” Howes said.
Travelling outward
This team has secured time on Jaguar for three years. In 2010 it focused on
establishing the connection between turbulence in the solar wind, the resulting
generation of heat, and the eventual fate of that heat. Knowing how the
turbulence behaves would contribute to our ability to predict coronal mass
ejections—massive bursts of solar wind ejected from the sun that are capable of
disrupting satellites and GPS location systems. Anticipating “space
weather” would allow for less disruption of communication systems on
Earth.
Research into space exploration has long
been concerned with the effect of high-energy particles on the human body.
Finding the origins of these particles and being able to calculate their
acceleration and movements would also allow us to better predict how such
particles travel through the solar system. Such insight could give astronomers
on the ground time to warn astronauts fixing a space station or satellite that
potentially fatal particles are heading toward them and that they should move
to somewhere safe.
For Howes’s team, though, these
questions will come later. Its current, fundamental aim is to understand a
material that, despite its abundance in space, is still as mysterious to us as
the dark side of the moon. This research may not be able to paint the whole
picture of plasmas yet, but it is already giving researchers the basic
information to explain some of the happenings in our vast and mysterious solar
system.