SAN FRANCISCO — Analyses of satellite data reveal that Earth’s atmosphere expands and contracts in response to short-term variations in the solar wind. Understanding this previously unrecognized phenomenon and how it affects objects traveling in low-Earth orbits will enable scientists to better track satellites, and to track the space junk that threatens them.

Besides the more than 800 satellites in low-Earth orbit, more than 17,000 pieces of space junk also circle the planet, reported Jeffrey P. Thayer, an atmospheric scientist at the University of Colorado at Boulder, December 15 at the fall meeting of the American Geophysical Union.

Researchers have long known that variations in the amount of certain wavelengths of ultraviolet light emitted by the sun cause the planet’s atmosphere to swell and shrink. The higher the amount of incoming UV radiation, the warmer the upper atmosphere becomes and the more it expands toward space, Thayer says.

Variations in the UV flux can also result from Earth’s rotation, causing daily fluctuations in the atmosphere.

And scientists have identified an 11-year cycle that’s linked to long-term variations in solar activity (SN: 1/20/01, p. 45) and a 27-day cycle that’s linked to the time it takes the sun to complete one rotation, says Thayer.

Recently, while analyzing data gathered in 2005 by a German satellite that orbits Earth at an altitude of about 400 kilometers, Thayer and his colleagues found a 9-day-long cycle in atmospheric fluctuations that lasted most of 2005. At the peak of each cycle, atmospheric density — which, at a height of 400 kilometers, is typically measured in billionths of a gram per cubic meter — would be approximately double that measured at the low point of the cycle, the researchers reported.

The nine-day fluctuation in atmospheric density at high altitude “was a surprising finding” that wasn’t correlated with any variation in solar UV flux, says Thayer.

Instead, he and his colleagues found, the short-term cycle matched variations in the speed of the solar wind striking the top of Earth’s atmosphere. High-speed blasts of solar wind originate from holes in the sun’s outer atmosphere, or corona, that are located at low solar latitudes, says Thayer. When those faster-than-average particles strike the atmosphere, the air heats up and expands to higher altitudes. As the satellites orbiting at those altitudes suddenly encounter thicker air, they slow down. Because the sun rotates once every 27 days and sported three low-latitude coronal holes for most of 2005, surges in solar wind swept by Earth, on average, every nine days.

The variations in the speed of solar wind noted by Thayer and his team also affected the chemistry of Earth’s upper atmosphere, says Geoff Crowley, an atmospheric physicist with Atmospheric & Space Technology Research Associates in San Antonio. Using data gathered by a satellite launched in late 2001, Crowley and his colleagues saw a nine-day cycle in the concentrations of monatomic oxygen (O) and diatomic nitrogen (N₂) in the upper atmosphere, created by chemical reactions the barrage of high-energy particles triggered. “This [cycle] was completely unexpected,” he reported at the meeting.

The new findings, along with full-time observations of the sun, may enable researchers to better estimate variations in the density of Earth’s upper atmosphere, says Thayer. Such estimates would in turn allow better prediction of the paths of low-flying satellites and orbiting space junk, so that warnings about possible collisions would be sharper and the number of maneuvers that satellites must make to avoid such collisions fewer.

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