An astronaut who exits a spacecraft without a spacesuit will die very quickly
because there is no air to breathe. However, although space is often regarded
as an airless vacuum, it is by no means empty. Spacecraft such as Cluster are
built to detect and study the sparse ‘soup’ of electrified plasma — mostly
electrons and protons — that populates near-Earth space.

“The world around us is made mainly of solids, liquids and gases,” explained
Cluster project scientist Philippe Escoubet. “Plasma is none of these — it
is really a fourth state of matter. It behaves very much like a gas, but it
can conduct electricity and it can be affected by magnetic fields.”

“We find plasma in the Sun and stars — in fact, all over the Universe — so
it is very important to find out as much as we can about it,” he said.

Although plasma is observed across billions of light years of space, this
form of matter rarely occurs naturally on the Earth — with the exceptions of
lightning flashes and auroras (polar lights) in the upper atmosphere. However,
plasmas can also be created by applying an electric field to a low-pressure
gas, for example in neon or fluorescent tubes used in offices and houses, or
by heating a gas to very high temperatures.

The Cluster spacecraft measure plasma in many different regions within the
magnetosphere — the magnetic bubble that surrounds our planet. However,
the highest density of electrically charged particles is found in the
plasmasphere — a doughnut-shaped region that lies between the Earth’s two
magnetic poles.

The inner edge of the doughnut starts about 1000 km above the surface, while
its outer limit may be at an altitude of 25,000 – 40,000 km, depending how
much it is being influenced by solar wind activity.

As the four Cluster spacecraft approach the Earth every 57 hours, instruments
on board are able to study the particles, electric and magnetic fields in the
plasmasphere. Such measurements can tell scientists a great deal about the
behaviour of the high density, cold plasma it contains.

One of the experiments on Cluster that is used to study plasma is called
WHISPER. This French-led instrument measures electron density as the spinning
spacecraft sweep around the planet.

When the Cluster quartet are strung out like a string of beads near closest
approach to the Earth, they are sufficiently far apart to reveal the size of
the plasmasphere and study the particles within it.

One of the first opportunities to observe the plasma doughnut came on 17 May
2001. With the Cluster quartet approximately 16,000 km apart, the WHISPER
instrument on each satellite was able to record changes in electron density
as the spacecraft soared over the south magnetic pole and headed north.

“Our data clearly show the plasma frequency rising and decreasing as the
Cluster spacecraft enter and exit the region of high electron density,” said
WHISPER co-investigator Patrick Canu of CETP in Velizy, France. “We can see
a clear difference in the inbound and outbound timing for each spacecraft.”

“There are also significant differences in the profile of the plasma frequency
line for each pass,” he said, “the most noticeable one being the ‘shoulder’
seen by Samba around 23:00 UT.”

First to enter the plasmasphere (at around 19:30 UT or 20:30 CEST) was the
Rumba spacecraft, followed over the next one and a half hours by Salsa,
Tango and Samba. Each in turn detected the tell-tale rise in high frequency
electrons (shown by the wiggly pale blue line on the data plots). In each
case, the electric field measurements rose off the scale, indicating that
the electron count was exceeding 100 particles per cubic centimetre.

After spending approximately 90 minutes within the plasmasphere, WHISPER’s
electric field measurements began to plummet — clear evidence that the
spacecraft were speeding out of the dense plasma and back into a region
of the magnetosphere where the electron density was much lower. The Rumba
spacecraft exited the plasmasphere at around 21:30 UT. Last to cross the
outer boundary of the plasma doughnut was Samba (at around 23:30 UT).

“These results are the most detailed ever to show changes in the plasmasphere
over time,” said Philippe Escoubet.

“This is yet another example of how Cluster is revolutionising our
understanding of near-Earth space and the way our planet interacts with the
Sun,” he added.


* Cluster home page

* The instruments onboard Cluster


[Image 1:]
The four Cluster spacecraft were travelling northward along their orbit
(dashed line) and successively crossed the plasmasphere (orange region) at
around 30-60 minute intervals.

[Image 2:]
Four data plots from the Whisper electrical waves instrument on each Cluster

These frequency/time spectrograms were obtained when each of the Cluster
spacecraft, coming close to perigee, encountered the dense plasmasphere at
different times. The increase in density is shown by the rise in the plasma
frequency, a wave signature seen here as a pale blue line, which depends
only on the electron density of the plasma surrounding the spacecraft.

In the case of Rumba, the plasma frequency increased from about 10 kHz (~1
electron per cubic centimetre) at 19:00 UT to 80 kHz (~80 electrons per cubic
centimetre) at 20:00 UT before rising out of Whisper’s frequency range. Rumba
exited the plasmasphere 90 minutes later as shown by the decreasing value of
the plasma frequency between 21:15 UT and 21:45 UT.

The large spatial separation between the four spacecraft meant that their
encounters with the plasmasphere took place at very different times. Rumba
arrived first, then Salsa at about 19:45 UT, Tango at about 20:15 UT, and
finally Samba around 21:00 UT.

Differences in the density profiles can also be seen, the most striking
one being the shoulder observed by Samba at 23:00 UT, as it exited the
plasmasphere. The strong (red) emissions observed at low frequency — below
10 kHz — before and after each spacecraft encountered the plasmasphere, are
known as whistler waves. The strong (yellow/red) vertical bars observed while
the plasma frequency was increasing (for example at 20:00 UT for Salsa) are