The highly ionized solar wind blows around our planet,disrupting satellites and endangering unprotected astronauts. A flotilla offour satellites have recently measured random variations in the solar wind'spropagation, providing the first definitive detection of turbulence in space.
The observation could improve space weather forecasts, aswell as help improve models of turbulent flow in ionized gas, called plasma.
Turbulence is quite common on Earth, as any frequent airplanepassenger can attest. But even physicists get a little queasy when tryingto explain the nature of this choppy, swirling flow.
"One cannot predict future behaviors with satisfactoryaccuracy," says Yasuhito Narita of the Institute of Geophysics andExtraterrestrial Physics in Braunschweig, Germany. "Even a small deviationor uncertainty in the initial state will end up with a completely differentstate."
It's a bit of the butterfly-tornado connection from chaostheory. Without predictive mathematical equations for turbulence, scientistsusually resort to statistical descriptions, like how much does the pressure or velocityvary over a certain distance.
Researchers have done such detailed observations of theturbulence in wind tunnels and water pipes. Making similar measurements inspace has been harder to do. Still, astrophysicists have inferred the presenceof turbulence inside stars, among interstellar clouds, in black hole accretiondisks and around Jupiter's red spot.
Single satellites have also studied the solar wind and havedetected turbulent signals in the way this plasma flow changes with time.However, to make direct comparisons to models, researchers had to assumesomething about the size of wind variations.
To avoid this ambiguity, multiple sensors are needed tomeasure the wind's properties at several points. This is exactly what theCluster suite of satellites was designed for.
"One needs at least four spacecraft to obtain thespatial resolution in three dimensions," Narita told Space.com."Cluster spacecraft provide a minimal set of the measurement points forthis purpose."
The four identical Cluster satellites orbit the Earth in apyramid formation, collecting electric and magnetic field data. Of specialinterest is the Earth's protective magnetosphere, where the planet's magneticfield 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 theleading-edge of the magnetosphere. At this bow shock, reverberating shock wavescause ripples and eddies in the solar wind propagation: a prime place to lookfor turbulence. Analyzing the magnetic field intensities recorded by eachsatellite, Narita and his colleagues were able to pinpoint changes in the windspeed. From this, they determined how the solar wind's energy varied overdistance, as detailed recently in the journal Physical Review Letters.
The results largely matched energy fluctuations seen inEarth-bound fluid turbulence, making this the first "definitive"detection of space turbulence, said Melvyn Goldstein of Goddard Space FlightCenter. He has worked on previous studies that gave hints of the samesimilarity.
That the solar wind behaves like the cream swirling in yourcoffee is surprising, since the low-density solar wind has almost noviscosity--an important component in fluid turbulence.
"For turbulence to develop in space, there must be somephysical processes that can replace the role of viscosity," Narita says.
This viscosity replacement may be some complicatedelectromagnetic interaction between the solar wind's ionized particles.Goldstein says much of the current work is aimed at understanding how thisplasma behaves in relation to the nearby magnetic fields.
Better characterization of solar wind turbulence could helpscientists predict space weather, which affects the radiation level forastronauts and spacecraft, Narita says.