Cluster spacecraft surf the plasma waves
13 July 2001
ESA's four Cluster spacecraft continue to provide groundbreaking new information about the interaction between our nearest star - the Sun - and planet Earth.The latest breakthrough comes from recent analysis of data from the Electric Field and Wave (EFW) experiments on the Cluster quartet. As they sail through the sea of plasma (an electrified gas containing electrons and protons) that fills near-Earth space, the instruments enable scientists to create the first three-dimensional view of this turbulent region.
What they see is wave after wave travelling around the magnetopause - the flexible outer boundary between the Earth's magnetosphere (the region dominated by Earth's magnetic field) and the vast expanse of interplanetary space that is dominated by the electrons and protons of the solar wind.
This is not the first time that evidence for plasma waves has been found at the magnetopause. Earlier this year, scientists using data from the STAFF instrument on Cluster found the first observational proof that occasional waves exist.
Now, after analysis of the EFW data, scientists from the Swedish Institute of Space Physics have followed this initial success with confirmation that these waves are not single, isolated events, but a continuous flow of crests and troughs.
A never-ending series of fast-moving waves seems to be generated when the electrically charged particles in the solar wind sweep around the magnetosphere - the magnetic bubble that surrounds our Earth.
Like ships sailing a stormy sea, the Cluster's mini-flotilla has surfed the fast-moving plasma and recorded a series of waves sweeping past all 4 spacecraft. By comparing data from each satellite, the scientists have shown that the waves move along the magnetopause, away from the Sun, with a velocity of about 145 km/s - equivalent to travelling from London to Paris in 2 1/2 seconds.
"It seems that they are caused by an interaction between waves in the solar wind plasma and the outer edge of the Earth's magnetic field," said EFW scientist Andris Vaivads from the Swedish Institute of Space Physics in Uppsala, Sweden.
"It is rather like the wind blowing across a lake," said Cluster Project Scientist Philippe Escoubet. "When the air flows over the water, waves form and travel across the surface of the lake."
"However, we still have a lot to learn about the waves at the magnetopause," he added. "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."