"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.

Detailed data plots relating to this article Related Links
Detailed data plots relating to this article
ESA Cluster II
The instruments onboard Cluster
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