Where does the wind blow? In the Earth’s thermosphere, it’s hard to know.
The thermosphere is what most people would refer to as outer space. Beginning at roughly 85 kilometers above the Earth’s surface and extending up to 1000 kilometers, this region is the domain of satellites and space stations. However, even here, there is atmosphere—albeit vanishingly thin. Its temperature reaches 2500°C at times, but with so few air molecules, you’d hardly feel warm, and although the wind blows, it carries the force of barely a whisper.
Measuring these winds is a considerable challenge, as scientists can’t simply put out a windsock. Instead, they exploit the Doppler effect. At these altitudes, the air glows faintly, releasing energy through chemical reactions—similar to a glow stick. As it blows, it shifts colors—ever so slightly bluer or redder as it travels toward or away from an observer.
To measure these shifts, scientists point ground-based interferometers skyward, using two different types. The first is called a narrow-field Fabry-Perot interferometer (NFPI). As the name suggests, it has a very narrow field of view—1° across or less—and must be repointed across the sky repeatedly to build up a map of the thermospheric winds. In contrast, the scanning Doppler imager (SDI) moves its optical elements and can capture light from all across the sky—but with a resolution less precise than a single measurement from an NFPI. Understanding the capabilities of these two methods—and how well they agree—is crucial for a comprehensive picture of winds in the thermosphere.
In a new study, Dhadly et al. compare these two methods for the first time, with three NFPIs and two SDIs located at various stations in Alaska. They tuned all of them to home in on the red light emitted by oxygen atoms at altitudes of roughly 200–300 kilometers, measuring its shift in color. They found that the data from the two methods agree remarkably well—both in their raw form, which only indicates the wind’s movement along the instruments’ lines of sight, and after numerical processing to map out vector wind flows.
The team also discovered wind “gusts”—small-scale fluctuations in the wind speeds. The team thinks these are waves of air sloshing through the thermosphere, like swells on the surface of the sea. One explanation for them is that they propagated upward from deeper in the atmosphere. Another culprit might be displays of aurora: The charged particles from space that bombard the atmosphere to create dazzling curtains of light may also be churning up these thermospheric waves.
Although the SDI maps have a much wider field of view, the more focused NFPI proved more sensitive to these gusts. The team notes that scientists will need to find a middle ground to fully map these small-scale fluctuations—either with a cluster of SDIs modified to have a narrower field of view or with a broad network of NFPIs. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA021316, 2015)
—Mark Zastrow, Freelance Writer
Citation: Zastrow, M. (2016), Scientists detect wisps of wind in space, Eos, 97, doi:10.1029/2016EO044199. Published on 28 January 2016.