Press Release

Cluster spacecraft surf the plasma waves

By SpaceRef Editor
August 30, 2001
Filed under , ,

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

Email:Andris.Vaivads@irfu.se

Dr. Mats Andre

Swedish Institute of Space Physics

Uppsala, Sweden

Tel: +46 18 471 5913,

Email:ma@irfu.se

USEFUL LINKS FOR THIS STORY

* Cluster’s new view of near-Earth space

http://spdext.estec.esa.nl/content/doc/f7/26103_.htm

* Impression of a wave on the magnetopause (mpeg video, 1.6M)

http://sci2.esa.int/cluster/videos/wave_on_magnetopause.mpg

* The instruments on board Cluster

http://sci.esa.int/content/doc/c6/1990_.htm

* EFW home page

http://cluster.irfu.se

* Cluster home page

http://sci.esa.int/cluster

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=28226&ooid=26139]
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:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=28226&ooid=28231]
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
Ä.

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=28226&ooid=28232]
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.

SpaceRef staff editor.