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Voyager probes find surprise at solar system's outer edge
Posted: June 9, 2011

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NASA's aging Voyager spacecraft, more than three decades outbound from Earth and approaching the outermost limits of the solar system, may be seeing signs of what scientists believe are huge magnetic bubbles churning at the interface between the sun's influence and interstellar space. The unexpected bubbles, shaped like sausages more than 100 million miles across, likely affect how high-energy cosmic rays pass into the inner solar system and may shed light on how stars interact with their galactic environments.

Old and new views of the heliosheath. Red and blue spirals are the gracefully curving magnetic field lines of orthodox models. New data from Voyager add a magnetic froth (inset) to the mix. Credit: NASA
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"It's exciting. We're learning new things almost every day," Voyager project scientist Ed Stone said in a telephone interview.

The outward flow of charged particles making up the solar wind, along with the sun's magnetic field, define a gigantic region in space known as the heliosphere. It is shaped somewhat like a teardrop because of the sun's motion, due to the rotation of the Milky Way, combined with the effects of the sun's passage through a cloud of interstellar debris produced by ancient supernova. The tail of the teardrop stretches away in the opposite direction of travel.

The outward flow of the solar wind and the entwined magnetic field act as a shield of sorts for the inner solar system, affecting the passage of high-energy cosmic rays from deep space.

NASA's twin Voyager 1 and 2 spacecraft were launched in 1977. Voyager 1 is roughly 11 billion miles from Earth while Voyager 2 is about nine billion miles out. Data from the spacecraft are transmitted to Earth with 20-watt transmitters.

"So it's 20 watts coming from 11 billion miles away," Stone said. "It's really a tiny amount of power. We have to use either the largest antennas, the 70-meter antennas, or we have to combine the signals from two 34 meter antennas in order to recover the data."

The Voyagers are moving through a region known as the heliosheath and have reached a point where the outward velocity of the solar wind has dropped to near zero. Just in front of the heliosheath, which defines the outermost limit of the sun's influence, is the heliopause, the boundary between the heliosphere and the interstellar wind the sun is moving through. Just in front of the heliopause is a so-called bow shock where the interstellar wind crashes into the heliosphere.

"This heliospheric boundary is very important in shielding the heliosphere from galactic cosmic rays that come from outside our home," said Merav Opher, an astronomer at Boston University. "What shields us is the heliopause, this last region that separates us from the rest of the galaxy."

Scientists believed the structure of the magnetic field at the interface between the heliosphere and interstellar space was relatively smooth. But based on data from the two Voyager spacecraft, it appears that huge magnetic bubbles may form in the heliosheath in the sun's direction of travel.

"A basic principle of magnetic fields in space is that they are tied to the ionized gas, because that gas is an excellent conductor of electricity, and where the gas goes, the field is stretched along with it and wherever the field goes, the gas has to go with it, too," said Eugene Parker, professor emeritus of physics at the University of Chicago. "So they're always together, stretched out through the solar system in this case."

Field lines extend from the sun's north and south poles and are carried away by the solar wind. Because of the sun's rotation, the field lines become "twisted and wrinkled, a bit like a ballerina's skirt," Opher said in a statement. "Far, far away from the sun, where the Voyagers are, the folds of the skirt bunch up."

Based on the Voyager data, scientists suspect magnetic field lines rapidly reconnect in the heliopause and form huge, disconnected bubbles that act like a permeable membrane to incoming cosmic rays. At least those coming from the sun's direction of travel.

"There had been recently some suggestions that the reconnections between the magnetic field in the heliosheath might be important," Stone told CBS News. "We hadn't really included that in any of the models because it's something that happens on a very small scale, it's hard to model.

"There are now models that suggest these bubbles will develop and that will change the nature of that region of the layer we're in, the heliosheath, and that might affect how cosmic rays from outside, can actually get inside the heliosphere. They have to sort of manage to get across all these bubbles."

Stone is hopeful the Voyagers will cross the heliopause and move into interstellar space in the next few years.

"We must be getting very close because the outward motion of the (solar) wind in the heliosheath is basically zero, it's not moving outward any more, which means we're close," he said. "But how close? We don't know. It could happen any day, it could happen in the next several years. My own feeling is that it would be surprising if we don't cross the heliopause in the next four years or so."