The highly ionized solar wind blows around our planet, disrupting satellites and endangering unprotected astronauts. A flotilla of four satellites have recently measured random variations in the solar wind's propagation, providing the first definitive detection of turbulence in space.
The observation could improve space weather forecasts, as well as help improve models of turbulent flow in ionized gas, called plasma.
Turbulence is quite common on Earth, as any frequent airplane passenger can attest. But even physicists get a little queasy when trying to explain the nature of this choppy, swirling flow.
"One cannot predict future behaviors with satisfactory accuracy," says Yasuhito Narita of the Institute of Geophysics and Extraterrestrial Physics in Braunschweig, Germany. "Even a small deviation or uncertainty in the initial state will end up with a completely different state."
It's a bit of the butterfly-tornado connection from chaos theory. Without predictive mathematical equations for turbulence, scientists usually resort to statistical descriptions, like how much does the pressure or velocity vary over a certain distance.
Researchers have done such detailed observations of the turbulence in wind tunnels and water pipes. Making similar measurements in space has been harder to do. Still, astrophysicists have inferred the presence of turbulence inside stars, among interstellar clouds, in black hole accretion disks and around Jupiter's red spot.
Single satellites have also studied the solar wind and have detected turbulent signals in the way this plasma flow changes with time. However, to make direct comparisons to models, researchers had to assume something about the size of wind variations.
To avoid this ambiguity, multiple sensors are needed to measure the wind's properties at several points. This is exactly what the Cluster suite of satellites was designed for.
"One needs at least four spacecraft to obtain the spatial resolution in three dimensions," Narita told Space.com. "Cluster spacecraft provide a minimal set of the measurement points for this purpose."
The four identical Cluster satellites orbit the Earth in a pyramid formation, collecting electric and magnetic field data. Of special interest is the Earth's protective magnetosphere, where the planet's magnetic field deflects the ionized solar wind, like air hitting a car's windshield.
A big shock
On Feb. 18 2002, the Cluster quartet ventured out into the leading-edge of the magnetosphere. At this bow shock, reverberating shock waves cause ripples and eddies in the solar wind propagation: a prime place to look for turbulence. Analyzing the magnetic field intensities recorded by each satellite, Narita and his colleagues were able to pinpoint changes in the wind speed. From this, they determined how the solar wind's energy varied over distance, as detailed recently in the journal Physical Review Letters.
The results largely matched energy fluctuations seen in Earth-bound fluid turbulence, making this the first "definitive" detection of space turbulence, said Melvyn Goldstein of Goddard Space Flight Center. He has worked on previous studies that gave hints of the same similarity.
That the solar wind behaves like the cream swirling in your coffee is surprising, since the low-density solar wind has almost no viscosity--an important component in fluid turbulence.
"For turbulence to develop in space, there must be some physical processes that can replace the role of viscosity," Narita says.
This viscosity replacement may be some complicated electromagnetic interaction between the solar wind's ionized particles. Goldstein says much of the current work is aimed at understanding how this plasma behaves in relation to the nearby magnetic fields.
Better characterization of solar wind turbulence could help scientists predict space weather, which affects the radiation level for astronauts and spacecraft, Narita says.