Cluster Spacecraft Detects Elusive Space Wind Above Earth's Atmosphere

The idea of space wind has been around for the last 20 years. Yet scientists have been unable to prove its actual existence. Now, researchers have detected this plasmapheric wind, so-called because it contributes to the loss of material from the plasmasphere, and have been able to show that it actually is present above Earth's atmosphere.

The plasmasphere is a doughnut-shaped region extending above Earth's atmosphere. It's essentially a region filled with charged particles that takes up the inner part of Earth's magnetosphere, which is dominated by the planet's magnetic field. About 20 years ago, researchers predicted a space wind that carried material from the plasmasphere outward, the result of an imbalance between the various forces that govern plasma motion.

Actually detecting this wind wasn't easy, though. The researchers analyzed the properties of the charged particles located in the plasmasphere, using information collected there by ESA's Cluster spacecraft. They then developed a filtering technique to eliminate noise sources and to look for plasma motion along the radial direction, either directed at Earth or outer space.

It turns out that there was a steady and persistent wind carrying about a kilo of the plasmosphere's material outward each second at a speed of over 5,000 km/hr. This motion was present at all times, even when Earth's magnetic field was not being disturbed by energetic particles coming from the sun.

"The plasmaspheric wind is a weak phenomenon, requiring for its detection sensitive instrumentation and detailed measurements of the particles in the plasmasphere and the way they move," said Iannis Dandouras of the Research Institute in Astrophysics and Planetology in France in a news release. "The plasmaspheric wind is an important element in the mass budget of the plasmasphere, and has implications on how long it takes to refill this region after it is eroded following a disturbance of the planet's magnetic field. Due to the plasmaspheric wind, supplying plasma-from the upper atmosphere below it-to refill the plasmasphere is like pouring matter into a leaky container."

The plasmasphere plays an important role in governing the dynamics of Earth's radiation belts. These belts present a radiation hazard to satellites and astronauts travelling through them. The plasmasphere's material also introduces a delay in the propogation of GPS signals passing through it. Because of all of these factors, understanding the region is crucial for further developments.

The findings are published in the journal Annales Geophysicae.