The solar-terrestrialbattlegroundOur Earth is an oasis inspace the only haven of life yet discovered in the entire universe. Yet,just a few hundred kilometers above our heads, a continuous battle is beingfought between the forces of nature.
Like a ship in an everlastingstorm, the Earth is bombarded by swarms of particles that are ejected fromthe Sun, then sweep towards us through space at supersonic speeds. Mostof these cosmic "bullets" in the solar wind are deflected by the planetsmagnetic shield -- the magnetosphere -- and sail harmlessly by.
But Earths shield is notinvulnerable. Gusts in the solar wind can squeeze it mercilessly, pushingit back towards the planet. Weak spots in the defenses allow energeticparticles to penetrate. As they cascade down the magnetic field lines,they collide with molecules in the upper atmosphere. One spectacular resultis the auroras,beautiful curtains of light that grace the polar skies.
Not all side-effects of solarstorms are as harmless. Particularly severemagnetic storms on our nearest star can cause havoc back on Earth,with widespread power cuts, disrupted radio communications and damage tosatellites.
Scientists have been studyingthis continuous feud between the Sun and Earth for many years, first fromthe ground and then with the aid of singlesatellites.
The view from all sides
In the last few months, scientistshave begun to sample the first intriguing morsels from the feast of informationthat Cluster will accumulate over the two-year mission.
Preliminary analysis of thisearly data is already providing new insights into the interaction betweenthe electrically charged particles of the solar wind and our planets magneticfield.
The key to this scientificrevolution is the Cluster satellites ability to fly in close formationalong elongated orbits which take them between 19,000 and 119,000 km fromthe planet -- almost a third of the way to the Moon. During their passagearound the Earth, the spacecraft are sometimes inside the Earth's magneticshield and sometimes outside, fully exposed to the supersonic solar wind.
As the mini-flotilla fliesin a tetrahedral (lopsided pyramid) pattern around the planet, its suiteof scientific sensors studies the changes taking place around each spacecraft.By comparing the times of these events, scientists are able to obtain thefirst three-dimensional picture of near-Earth space.
First results
Clusters first glimpsesof the continuously fluctuating magnetic battleground came on November9, when the quartet made their first crossings of the magnetopause, theboundary between interplanetary space (where the solar wind reigns supreme)and the Earths region of magnetic domination.
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These crossings were a surprisefor the Cluster science team, since they occurred about one week earlierthan predicted.
Data from the experimentson Cluster clearly showed that gusts in the solar wind were causing themagnetosphere to balloon in and out. These rapid fluctuations in size meantthat the Cluster spacecraft were alternately inside and outside Earthsmagnetic field. For the first time, scientists were able to receive datasimultaneously from both sides of the magnetopause -- something that isonly possible with a multi-spacecraft mission.
At the peak of the sunstorm,the spacecraft were exposed to the solar wind for more than 2 hours untilthe compression of the magnetosphere decreased and the spacecraft movedback inside the Earths domain.
Since then, the Cluster spacecrafthave made regular measurements of the magnetopause. One of the earliestinteresting discoveries has been the confirmation of the existence of wavesalong this ever-shifting boundary in space.
"They are like waves thatform on the surface of a lake when the wind is blowing," said Cluster ProjectScientist, Philippe Escoubet. "The Cluster spacecraft give us for the firsttime the size and speed of these waves."
"Cluster has given us thefirst observational proof that these waves exist," said Dr. Nicole Cornilleau-Wehrlinof Velizy, France-based CETP [Center for the Study of Terrestrial and PlanetaryEnvironments] and principal investigator on the STAFF [Spatio-TemporalAnalysis of Field Fluctuations] experiment.
"Further studies will enableus to discover more about what happens and why."
A look at the bow shock
By late December, the ellipticalCluster orbits carried the quartet close to the bow shock, a huge shockwavein near-Earth space. This phenomenon, some 100,000 km from the planet,forms when particles in the solar wind slow to subsonic speeds after slamminginto Earths magnetic shield at more than 1 million km/h. Clusters batteryof instruments is beginning to record in great detail what happens at thisturbulent barrier.
Once again, gusts in thestream of solar particles caused the magnetosphere to behave like a punching-bagundergoing heavy body blows. These "punches" caused the bow shock to migratethrough space, crossing and recrossing the Cluster spacecraft at irregularintervals.
"This is the first time wehave ever seen the bow shock in such remarkable detail," said ProfessorAndré Balogh of Imperial College London, the principal investigatorfor Cluster's FGM [fluxgate magnetometer] experiment.
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Comparisons of spacecraftmeasurements will also be invaluable for in-depth studies of other partsof the magnetosphere. Of particular interest to scientists are the cusps-- funnel-shaped gaps in the Earths magnetic shield that that guide electrifiedparticles into the upper atmosphere.
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