Earth Atmosphere "Breathes," Thanks to Solar Winds

Victoria Jaggard in San Francisco
National Geographic News
December 16, 2008
Earth's upper atmosphere "breathes," expanding and contracting due to previously unnoticed influences from the sun, say scientists who have measured the phenomenon for the first time.

The steady rhythm of "breaths" is linked to the ebb and flow of geomagnetic activity that is driven by intense solar winds—charged particles streaming outward from the sun—according to recent satellite data.

The breathing cycle seems to reach its peak when solar features called coronal holes are facing Earth. These dark spots in the sun's corona—a sort of solar atmosphere—are areas where the sun's magnetic field has been blown open by pressurized solar wind, sending the "winds" toward Earth at high speed.

As the fast winds streaming from coronal holes approach Earth, they cause gases in our upper atmosphere to heat up and expand, then cool down and contract, changing the upper atmosphere's density.

(Related: "Sun's Mysterious Waves Found; May Be Solar Wind Source" [December 6, 2007].)

A Drag on Satellites
Flying through the resulting "hills and valleys of density," orbiting satellites experience more or less drag, respectively, noted research team member Jeff Thayer of the University of Colorado, Boulder, during a meeting of the American Geophysical Union in San Francisco.

Like runners trying to sprint through a lake, satellites faced with denser atmosphere will slow down and need more energy to keep moving in a given orbit.

The changes in drag affect a satellite's ability to stay on course, requiring more fuel and complicated orbital adjustments to keep on a predictable and safe path.

Better understanding of the Earth's breathing effect is therefore crucial, because the upper atmosphere is heavily populated with spacecraft and debris, Thayer said.

The region includes the orbits of the space shuttle, the International Space Station, more than 800 working probes, and about 10,000 pieces of space junk.

Outer Gaseous Shell

The Earth's upper atmosphere, also called the thermosphere, "is the outer gaseous shell of the planet's atmosphere that exchanges energy with the space-plasma environment," Thayer said.

In other words, it's the region that has direct interactions with the sun's atmosphere.

"From Earth's perspective, we're embedded in the sun's upper atmosphere," he said.

Starting at about 5 miles (85 kilometers) above Earth's surface, the thermosphere has been known to experience expansion and contraction every 27 days, the time it takes the sun to rotate on its axis.

But until now few scientists had looked at shorter-term changes in atmospheric density.

Thayer and colleagues found fluctuations in density roughly every five, seven, or nine days that correspond to changes in geomagnetic activity, atmospheric composition, and infrared radiation—all signs that extreme solar weather is at work.

The team thinks coronal holes help explain the pattern of the breaths.

"Think about a searchlight" that takes 27 days to complete a rotation, Thayer said. "If you have one searchlight … and you mark down every time you saw brightness, you'd see it every 27 days.

"Now if you had three searchlights [set up] so they're all evenly distributed and you rotate those at 27 days, you would see … a bright light every 9 days."

Similarly, four lights would equal bright sweeps every 6.75 days, while five lights would yield passes every 5.4 days—the periods found in the new study.

Thayer and colleagues' data was published earlier this year in the journal Geophysical Research Letters.

Ringing Atmosphere

David Klumpar, a solar physicist at Montana State University in Bozeman, said that activity from the sun is constantly and randomly "tapping" Earth, creating oscillations in the our planet's natural frequencies like a spoon tapping on a glass of water.

"I would imagine that Thayer [and colleagues] are finding some of those natural frequencies," said Klumpar, who was not involved in the work.

"The atmosphere is ringing, like a water glass when 'tapped' by solar eruptions."

David Hathaway, a solar physicist at NASA's Marshall Space Flight Center, said that, without having seen the actual data, "I'm a little surprised."

He noted that solar scientists have long known about the 27-day cycles of geomagnetic activity related to the sun's rotation.

"But on a nine-day cycle, I'm not sure what it would be."

He added that the last two years would indeed have seen most of the atmospheric disturbances coming from coronal holes.

The real question with the shorter-term bumps noticed by Thayer's team would be how significant they are above the "noise"—expected and random variations in atmospheric conditions.

For now, he said, the idea raises more questions than it does answers.

Study co-author Thayer agrees that more work needs to be done, including getting solar physicists involved in analyzing the data.

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