Plasma loops created in the lab were recorded using high-speed cameras. Credit: Eve Stenson / Caltech |
In
orbit around Earth is a wide range of satellites that we rely on for
everything from television and radio feeds to GPS navigation. Although
these spacecraft soar high above storms on Earth, they are still
vulnerable to weather—only it’s weather from the sun. Large solar
flares—or plasma that erupts from the sun’s surface—can cause widespread
damage, both in space and on Earth, which is why researchers at the
California Institute of Technology (Caltech) are working to learn more
about the possible precursors to solar flares called plasma loops. Now,
they have recreated these loops in the lab.
“We’re
studying how these solar loops work, which contributes to the knowledge
of space weather,” says Paul Bellan, professor of applied physics at
Caltech, who compares the research to studying hurricanes. For example,
you can’t predict a hurricane unless you know more about the events that
precede it, like high-pressure and low-pressure fronts. The same is
true for solar flares. “It takes some time for the plasma to get to
Earth from the sun, so it’s possible that with more research, we could
have up to a two-day warning period for massive solar flares.”
The
laboratory plasma loop studies were conducted by graduate student Eve
Stenson together with Bellan and are reported in the August 13 issue of
the journal Physical Review Letters.
They
found that two magnetic forces control the behavior of arching loops of
plasma, which is hot, ionized gas. “One force expands the arch radius
and so lengthens the loop while the other continuously injects plasma
from both ends into the loop,” Bellan explains. “This latter force
injects just the right amount of plasma to keep the density in the loop
constant as it lengthens.”
The
duo says that in simpler terms, this process is like squeezing
toothpaste into a tube from both ends, except that the toothpaste has
little magnets in it, so there are magnetic forces acting internally.
Stenson and Bellan studied plasma loops that they generated with a
pulse-powered, magnetized plasma gun. Inside a vacuum chamber,
electromagnets create an arched magnetic field. Then, hydrogen and
nitrogen gas is released at the two footpoints of the arch. Finally, a
high-voltage electrical current is applied at the footpoints to ionize
the gas and turn it into plasma, which travels at a minimum speed of
about six miles per second.
“All
three steps—the magnetic field, and the gas, and the high
voltage—happen in just a flash of light inside the chamber,” says
Stenson. “We use high-speed cameras with optical filters to capture the
behavior of the plasmas.”
By
color-coding the inflowing plasma, the optical filters vividly
demonstrated the flow from the two ends of the loop. According to
Bellan, no one has ever used this technique before. On camera, red
plasma flows into the loop from one footpoint while blue plasma
simultaneously flows into the loop from the other end.
“For each experiment, you’ll only see the light from the hydrogen side or
the nitrogen side in the images,” explains Stenson. “But these
experiments are very reproducible, so we can put separate images on top
of each other to see both plasmas in one picture.”
Next,
Bellan’s lab will test how two loops interact with each other. “We want
to see if they can merge and form one big loop,” says Bellan. “Some
people believe that this is how larger plasma loops on the sun are
formed.”
Funding for the research outlined in the Physical Review Letters
paper, “Magnetically Driven Flows in Arched Plasma Structures,” came
from the National Science Foundation, the U.S. Department of Energy, and
the Air Force Office of Scientific Research.
