ESA Science & Technology11-Jul-2005 09:58:40

Science Results

Plasma Waves

Artist's impression of a wave on the magnetopause passing by the four Cluster satellites

Colour diagram showing electric field measured by 4 spacecraft and shapes of waves

The Cluster observations 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.

At the end of the year 2000, 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 other data from the Electric Field and Wave (EFW) experiments, it is the first confirmation that a succession of crests and troughs is traveling 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 kms-1 - equivalent to traveling from London to Paris in 2.5 seconds. These results were published in Gustafsson et al, Annales Geophyicae, 2001.

Plasmas waves are also observed in the magnetotail. Such waves were observed on 3 August 2001. Cluster observed that these waves were typically 4 Earth radii in size and moving at around 20 kms-1 across the tail in the dawn-dusk direction. These results were published in Zhang et al, Geophys. Res. Lett., 2002.

Schematic interpretation of the multiple crossings between 0847
and 0952 UT on August 3, 2001. View toward the Earth. A wave
moving in the dawn-dusk direction is superimposed on a extremely
twisted and warped neutral sheet. The numbers indicate the crossings
of the neutral sheet by the spacecraft (from Zhang et al, Geophys.
Res. Lett., 2002).

For further information please contact: