ESA's four Cluster spacecraft continue to provide ground-breaking new information about the interaction between our nearest star - the Sun - and planet Earth. As they sail through the sea of plasma (electrons and protons) that fills near-Earth space, the identical instruments on the Cluster quartet are helping scientists to create the first three-dimensional views of this turbulent region.

The latest breakthrough confirms that the outer regions of the Earth's magnetosphere - the magnetic bubble that surrounds our planet - are rocked by a continuous series of rippling waves that resemble the rollers (long, swelling waves) on a terrestrial ocean.

Artist's impression of a wave on the magnetopause passing by the four Cluster satellites. Photo: ESA

Earlier this year, scientists using data from the STAFF and FGM instruments on Cluster found the first observational proof that individual plasma waves exist at the magnetopause - the outer boundary of the Earth's magnetosphere. It seemed that the waves were generated when the electrically charged particles in the solar wind were forced to flow around the magnetosphere, like the ocean swell parting around a breakwater.

Now, after analysis of data from the Electric Field and Wave (EFW) experiments, scientists from the Swedish Institute of Space Physics have followed this initial success with the first confirmation that a succession of crests and troughs is travelling around the magnetopause.

The new measurements show that Cluster's mini-flotilla has been surfing the fast-moving plasma. Like ships sailing a stormy sea, they have been ploughing through a series of enormous waves, each one measuring some 2000 km across.

Analysis of data obtained from each EFW instrument on 14 January 2001 also shows that these waves on the magnetopause are racing away from the Sun with a velocity of about 145 km/s - equivalent to travelling from London to Paris in 2.5 seconds.

"Cluster allows us to see what these waves look like, but future studies should answer the important question - 'what generates these waves?'," said EFW scientist Andris Vaivads from the Swedish Institute of Space Physics in Uppsala, Sweden. "It is not yet clear whether they are generated by the flow of the solar wind or by waves inside the solar wind hitting the Earth's magnetosphere."

"We still have a lot to learn about the waves at the magnetopause," said Cluster project scientist, Philippe Escoubet. "They seem to vary in size and speed, but we don't yet know why. Are the waves different on the dusk side of the Earth? These are questions that Cluster will help to answer in the years ahead."