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Cluster Spacecraft Surf the Plasma Waves
Cluster Spacecraft Surf the Plasma Waves
30 Aug 2001
(Source: European Space Agency)

ESA Science News

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.

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."

For further information please contact:

Andris Vaivads
Swedish Institute of Space Physics
Uppsala, Sweden
Tel: +46 18 471 5904

Dr. Mats Andre
Swedish Institute of Space Physics
Uppsala, Sweden
Tel: +46 18 471 5913



[Image 1:]
Artist's impression of a wave on the magnetopause passing by the four Cluster satellites.

ESA's Cluster satellites have confirmed 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.

See a video of waves on the magnetopause passing by the four Cluster satellites - MPEG video (1.6M).

[Image 2:]
Schematic view showing bow shock, magnetopause, magnetosphere, waves. The supersonic particles (mainly electrons and protons) of the solar wind suddenly slow down when they reach the bow shock. They then flow around the Earth's magnetic field in a region known as the magnetosheath. The boundary between the magnetosheath and the Earth's magnetic bubble (the magnetosphere) is called the magnetopause. Here, the interaction between the solar wind particles and the Earth's magnetic field results in a series of waves. These waves on the magnetopause travel away from the Sun at a speed of about 145 km/s. The approximate position of the Cluster satellites is marked with ?.

[Image 3:]
Colour diagram showing electric field measured by 4 spacecraft and shapes of waves. The waves travel through space much faster than the Cluster spacecraft. As they sweep past the four closely spaced satellites, instruments detect changes in the magnetic and electric field. These regular changes show that a series of 'steep' waves is sweeping past the spacecraft. The measurements indicate that the waves are about 500 km across and that they travel through space at about 145 km/s.

Here, the colour of the four horizontal lines on each panel shows the potential of the electric field detected by each spacecraft. Red corresponds to plasma in the magnetosheath region and blue to plasma inside the magnetosphere. The grey curved line shows schematically the size and shape of the waves. The short black lines show the electric field measurements.

The top panel is a more detailed view of part of the overall observations, covering a horizontal distance of about 5000 km. The lower panel shows observations over a horizontal distance of about 40,000 km. Note that during the period of observation the satellites moved only about 550 km (it is the wave that passes the satellites).

Such high precision measurements of the electric field have been made possible by designing more sensitive probes with the help of the Swedish Institute of Space Physics.

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Last Updated: 31 Aug 2001