Sun’s shock wave goes missing

Spacecraft observations redraw astronomers’ ideas about the local stellar environment

The sun isn’t quite the speed demon scientists once suspected. It chugs around the galactic center at a relatively pokey 83,500 kilometers per hour — or roughly 11,000 kilometers per hour slower than expected, says a report appearing online May 10 in Science.

While that might not sound like a big deal, the sun’s slower pace clashes with theories describing the solar system’s local environment — a protective, sun-blown bubble called the heliosphere. The sun’s speed helps shape the size and boundary of this elastic bubble, along with the interstellar dust and gas clouds it moves through.

In particular, scientists thought a shock wave — called the bow shock — preceded the bubble’s journey through space. ”We’ve spent the last quarter-century assuming there was a bow shock,” says study coauthor David McComas, a space scientist at the Southwest Research Institute in San Antonio.

Though the ramifications of a missing bow shock are still unknown, scientists can now study more precisely how the heliosphere responds to the sun’s travels through different interstellar voids and clouds.

“We’re talking about this structure around our own star,” says Seth Redfield, an astronomer at Wesleyan University in Middletown, Conn. “And yet we still, up until now, have not had a very clear idea of what was going on out there.”

McComas and his colleagues clocked the solar system’s speed by using helium atoms captured by NASA’s orbiting Interstellar Boundary Explorer. Since helium atoms have no electrical charge, they can travel through magnetic fields and easily cross the turbulent boundary, called the heliopause, between the heliosphere and the local gas cloud. Thousands of helium atoms revealed the speed and direction of the traveling sun, McComas says.

The scientists also measured the magnetic field in the surrounding cloud and found it was stronger than expected — meaning even faster speeds would be needed to generate a shock wave.

At least for now. When the sun plows into a different cloud, that might change.

“The protective shield of the heliosphere has to have changed over the lifetime of the sun,” Redfield says. In 2008, he and his colleagues used a different method to chart the sun’s galactic journey. After studying the movement and starlight from 157 nearby stars, the team came up with a solar velocity very close to what McComas found.  “When they matched, I was extremely excited,” says Redfield, who is now studying similar bubbles, or astrospheres, around other stars.

IBEX isn’t the only spacecraft sniffing around the boundaries of the heliosphere. The two Voyager probes, launched in 1977, are also helping map the solar system’s home bubble. Recent data beamed back from the craft suggest that the heliosphere is asymmetrically squished by a stronger magnetic field in the south, says Voyager chief scientist Edward Stone, an astrophysicist at Caltech.

Now, Stone says, the craft are tiptoeing up to the heliopause, with Voyager 1 leading the way at 120 times the distance between the Earth and Sun. “We don’t know exactly where the heliopause is,” Stone says, noting that this newest study suggests the bubble is 12 percent larger than expected. “It could be another two or three years.”

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