UA Scientists Study Voyager 2's Plunge Through the Heliosphere
Scientists have discovered that our sun's 'atmosphere' is asymmetric, thanks to two champion spacecraft.

By Lori Stiles, University Communications
Dec. 10, 2007

For the second time in history, a spacecraft still communicating with Earth has reached the solar system's final frontier, a vast region at the edge of our solar system where the solar wind smashes into the thin gas between the stars.

NASA's Voyager 2 probe crossed the solar-wind "termination shock" on Aug. 30, which is 30 years and 10 days after the spacecraft launched from Cape Canaveral, Fla.

Voyager 2 has been flying out of the solar system in a different direction than Voyager 1 and was about 10 billion miles away from the place along the termination shock that Voyager 1 crossed on Dec. 17, 2004.

Voyager 2 was also almost a billion miles closer to the sun when it crossed the shock, confirming that the heliosphere is asymmetric rather than spherical. The heliosphere is the bubble carved in interstellar space by energetic, charged particles streaming out from the sun.

Scientists are set to talk about it today at the 2007 fall meeting of the American Geophysical Union in San Francisco.

University of Arizona Regents' Professor J. Randy Jokipii, Associate Professor Joe Giacalone and others in a group at the Lunar and Planetary Laboratory use advanced computer models to study energetic particles at astrophysical shock waves.

At today's meeting, Jokipii presented results on what energetic particles observed upstream of the solar-wind termination shock just prior to the Voyager spacecraft crossings suggest about the nature of the termination shock.

"Termination shocks are not unique to the heliosphere," Jokipii said. "You can see one in water running in your kitchen sink. The streaming water initially radiates outward faster than waves can propagate through it. As a result, the surrounding fluid cannot send an inward signal that its motion has been slowed. A shock front forms where the fast- and slow-moving parts of the fluid abruptly collide."

Jokipii and UA physicist K.C. Hsieh produced a movie that shows the termination shock created by water flowing from a tap.

The Voyager spacecraft have confirmed that the termination-shock boundary of our heliosphere, or that part of space dominated by the solar wind, like its water analog, is indeed irregular and turbulent.

"The Holy Grail for me will be when the spacecraft begin traveling in pure interstellar space," Jokipii said. "That's maybe 10 years away. Scientists now can start thinking about what they want to look for when the Voyagers break through the last barriers to true interstellar space."


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J. Randy Jokipii