An argon plasma jet forms a rapidly growing corkscrew, known as a kink instability. This instability causes an even faster-developing behavior called a Rayleigh-Taylor instability, in which ripples grow and tear the jet apart. This phenomenon, the Caltech researchers say, has never been seen before and could be important in understanding solar flares and in developing nuclear fusion as a future energy source. Credit: A. L. Moser and P. M. Bellan, Caltech |
January saw the biggest solar storm since 2005, generating some
of the most dazzling northern lights in recent memory.
The
source of that storm—and others like it—was the sun’s magnetic field,
described by invisible field lines that protrude from and loop back into
the burning ball of gas. Sometimes these field lines break—snapping
like a rubber band pulled too tight—and join with other nearby lines,
releasing energy that can then launch bursts of plasma known as solar
flares. Huge chunks of plasma from the sun’s surface can zip toward
Earth and damage orbiting satellites or bump them off their paths.
These
chunks of plasma, called coronal mass ejections, can also snap Earth’s
magnetic field lines, causing charged particles to speed toward Earth’s
magnetic poles; this, in turn, sets off the shimmering light shows we
know as the northern and southern lights.
Even
though the process of field lines breaking and merging with other
lines—called magnetic reconnection—has such significant effects, a
detailed picture of what precisely is going on has long eluded
scientists, says Paul Bellan, professor of applied physics in the
Division of Engineering and Applied Science at the California Institute
of Technology (Caltech).
Now,
using high-speed cameras to look at jets of plasma in the lab, Bellan
and graduate student Auna Moser have discovered a surprising phenomenon
that provides clues to just how magnetic reconnection occurs. They
describe their results in a paper published in the February 16 issue of
the journal Nature.
“Trying
to understand nature by using engineering techniques is indeed a
hallmark of the Division of Engineering and Applied Science at Caltech,”
says Ares Rosakis, the Theodore von Kármán Professor of Aeronautics and
professor of mechanical engineering and the chair of engineering and
applied science.
In
the experiments, Moser fired jets of hydrogen, nitrogen, and argon
plasmas at speeds of about 10 to 50 km per second across a distance of
more than 20 cm in a vacuum. Plasma is a gas so hot that atoms are
stripped of their electrons. As a throughway for speeding electrons, the
jets act like electrical wires. The experiment requires 200 million W
of power to produce jets that are a scorching 20,000 K and carry a
current of 100,000 A. To study the jets, Moser used cameras that can
take a snapshot in less than a microsecond, or one millionth of a
second.
As
in all electrical currents, the flowing electrons in the plasma jet
generate a magnetic field, which then exerts a force on the plasma.
These electromagnetic interactions between the magnetic field and the
plasma can cause the jet to writhe and form a rapidly expanding
corkscrew. This behavior, called a kink instability, has been studied
for nearly 60 years, Bellan says.
But when Moser looked closely at this behavior in her experimental plasma jets, she saw something entirely unexpected.
She
found that—more often than not—the corkscrew shape that developed in
her jets grew exponentially and extremely fast. The jets in the
experiment formed 20-cm-long coils in just 20 to 25 microseconds. She
also noticed tiny ripples that began appearing on the inner edge of the
coil just before the jet broke—the moment when there was a magnetic
reconnection.
In
the beginning, Moser and Bellan say, they did not know what they were
seeing—they just knew it was strange. “I thought it was a measurement
error,” Bellan says. “But it was way too reproducible. We were seeing it
day in and day out. At first, I thought we would never figure it out.”
But
after months of additional experiments, they determined that the kink
instability actually spawns a completely different kind of phenomenon,
called a Rayleigh-Taylor instability. A Rayleigh-Taylor instability
happens when a heavy fluid that sits on top of a light fluid tries to
trade places with the light fluid. Ripples form and grow at the
interface between the two, allowing the fluids to swap places.
What
Moser and Bellan realized is that the kink instability creates
conditions that give rise to a Rayleigh-Taylor instability. As the
coiled plasma expands—due to the kink instability—it accelerates
outward. Just like a passenger being pushed back into the seat of an
accelerating car, the accelerated plasma is pushed down on the vacuum
behind it. The plasma tries to swap places with the trailing vacuum by
forming ripples that then expand—just like when gravity forces a heavy
fluid to try to change places with a light fluid underneath. The
Rayleigh-Taylor instability—as revealed by the ripples on the trailing
side of the accelerating plasma—grows in about a microsecond.
“People have not observed anything like this before,” Bellan says.
Although
the Rayleigh-Taylor instability has been studied for more than 100
years, no one had considered the possibility that it could be caused by a
kink instability, Bellan says. The two types of instabilities are so
different that to see them so closely coupled was a shock. “Nobody ever
thought there was a connection,” he says.
What
is notable is that the two instabilities occur at very different
scales, the researchers say. While the coil created by the kink
instability spans about 20 centimeters, the Rayleigh-Taylor instability
is much smaller, making ripples just two centimeters long. Still, those
smaller ripples rapidly erode the jet, forcing the electrons to flow
faster and faster through a narrowing channel. “You’re basically choking
it off,” Bellan explains. Soon, the jet breaks, causing a magnetic
reconnection.
Magnetic
reconnection on the sun often involves phenomena that span scales from a
million meters to just a few meters. At the larger scales, the physics
is relatively simple and straightforward. But at the smaller scales, the
physics becomes more subtle and complex—and it is in this regime that
magnetic reconnection takes place. Magnetic reconnection is also a key
issue in developing thermonuclear fusion as a future energy source using
plasmas in the laboratory. One of the key advances in this study, the
researchers say, is being able to relate phenomena at large scales, such
as the kink instability, to those at small scales, such as the
Rayleigh-Taylor instability.
The
researchers note that, although kink and Rayleigh-Taylor instabilities
may not drive magnetic reconnection in all cases, this mechanism is a
plausible explanation for at least some scenarios in nature and the lab.
The title of Moser and Bellan’s Nature
paper is “Magnetic reconnection from a multiscale instability cascade.”
This research was funded by the U.S. Department of Energy, the National
Science Foundation, and the Air Force Office of Scientific Research.
