After more than 30 years in space, NASA's Voyager 2
continues to make new discoveries and is
upending our understanding of the processes at work at the
very edges of the sun's influence. The
spacecraft has found that exotic particles from outside the
solar system dominate the dynamics of
this distant region, and that it is far more complex than
had been predicted.
On Aug. 31, 2007, when the spacecraft reached the
termination shock — where the solar wind
slows to subsonic speeds as it collides with interstellar
gas — it found temperatures lower than what
theories had predicted, and high-velocity particles that no
models had foreseen.
"This was totally unexpected," said Stamatios "Tom"
Krimigis of Johns Hopkins'
Applied Physics Laboratory and principal investigator
for Voyager 2's Low-Energy Charged Particles instrument,
which
detected the very highest energy particles. "The
environment is totally unlike what the models
predicted," he said. Researchers from APL and their
colleagues are using the new data to modify
existing models and create new ones to describe this more
complicated reality.
Voyager 2 is traveling toward the outer limits of the
heliosphere, a bubble in space created by
the solar wind flowing from the sun in all directions.
Material from the sun and the interstellar gas
outside the heliosphere begin to affect one another at the
termination shock, which is more than 83
times farther than the distance between Earth and the
sun.
The sudden slowdown of the solar wind at this
interface, from approximately 217 miles per
second to about 81 miles per second, releases a tremendous
amount of energy. Theories predicted
that the energy went into heating the now relatively
low-speed ions and electrons that make up the
solar wind.
"When Voyager 2 measured the temperature of the plasma
at the termination shock, there was
a mystery," said APL's Rob Decker, lead author on a paper
about the finding in the July 3 issue of
Nature. "The temperature was too low to account for the
energy loss. We had to ask, Where was that
energy going?"
It now appears that most of that energy is going into
accelerating particles from outside our
solar system that have made their way into the
heliosphere.
While the solar wind races away from the sun at many
hundreds of kilometers per second, a
relatively light breeze of neutral hydrogen atoms floats
into the heliosphere at about 16 miles per
second. The neutral particles are unaffected by the
heliosphere's magnetic and electric fields, but
sometimes they collide with solar wind particles and lose
an electron, becoming ions.
As freshly charged particles, they are now swept along
by the solar wind's magnetic and electric
fields, back toward the termination shock. As the rest of
the solar wind slows down, the kinetic
energy it loses does provide some heating of the solar
wind, but most of the energy goes into
accelerating the "pickup" ions, so called because they were
picked up by the solar wind.
Beyond the termination shock is the heliosheath, where
the slowed solar wind is diverted away
from the approaching interstellar medium and forms our
bullet-shaped heliosphere. It now appears
that the high-velocity, or nonthermal, pickup ions play a
large role in determining the behavior of this
region. The heliosheath plasma interacts with and possibly
mixes with the interstellar medium across
the heliopause, which is the outer boundary of the
heliosphere and the next goal for the Voyager
spacecraft.
"Once they are beyond the termination shock, the
pickup ions affect how that medium behaves,"
Decker said. "They are carrying a lot of energy and
therefore play a large role in the dynamics of the
flowing plasma, modifying the heliosheath's width and its
magnetic field structure, for example. But
that's something we're still trying to understand."
The researchers also report in the Nature
article on the differences between what Voyager 1
and Voyager 2 have observed at the termination shock. The
two spacecraft crossed the interface
approximately 10 billion miles apart, Voyager 1 above the
plane of the ecliptic — the plane on which most
planets orbit the sun — and Voyager 2 below it.
It was expected that the two would find themselves in
similar environments once across the
termination shock; however, Voyager 2 is finding much more
variability than did Voyager 1.
"Now it's time to rewrite the models," Krimigis
said.
The Voyagers were built by NASA's Jet Propulsion
Laboratory in Pasadena, Calif., which
continues to operate both spacecraft.
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