ASTROPHYSICS:
Cluster Reveals Earth's Rippling Magnetic Field

Barbara Casassus and Alexander Hellemans*


 PARIS--Four satellites flying in unison have revealed a hidden wild side to Earth's magnetosphere, the magnetic field enveloping the planet that acts like a gigantic deflector shield against blasts of solar radiation. The unprecedented view, unveiled here last week at European Space Agency (ESA) headquarters, could help scientists devise better defenses against crippling magnetic storms.

 ESA launched the quartet of identical spacecraft last summer, 4 years after the original set of satellites was lost in an explosion seconds after lift-off (Science, 28 June 1996, p. 1866). The satellites of the resurrected mission--nicknamed Salsa, Samba, Rumba, and Tango--each carry 11 instruments designed to produce the first three-dimensional maps of the magnetic fields and plasmas surrounding Earth.

 Project scientists are thrilled with the data so far. "We can see things we couldn't possibly see before," says André Balogh of Imperial College in London, the principal investigator of the fluxgate magnetometer experiment. The magnetometer has two sensors on each craft that measure the intensity and orientation of Earth's magnetic field lines. Outside experts also are impressed. "I am surprised the team has been able to extract such exciting observations so soon after launch," says Alan Gabriel of the Institut d'Astrophysique Spatiale in Orsay and president of the French sun-Earth research program.



Figure 1
Sunstruck. A quartet of satellites carries dozens of instruments to monitor various solar phenomena and their impact on Earth.
CREDIT: ESA


The Cluster spacecraft began gathering data soon after crossing the magnetopause--the outer edge of the magnetosphere, where the influence of the sun's magnetic field takes over--on 8 November. Chancing upon one of the most violent solar storms in 25 years, the spacecraft watched a barrage of particles from the sun, carried on a supercharged solar magnetic field, compress the magnetosphere to about half its usual size. It was the first time that this phenomenon has been measured in detail, says Cluster project scientist Philippe Escoubet of the European Space Research and Technology Centre in Noordwijk, the Netherlands.

 More data came pouring in last month when the satellites crossed a polar cusp, a funnel-shaped gap in the magnetosphere through which charged solar particles reach the atmosphere and set off the northern and southern lights. Refuting the classic view of polar cusps as relatively stable, the satellites found the northern cusp gyrating wildly like a top, moving at speeds of up to 30 kilometers per second.

 The Cluster spacecraft have found that the magnetopause, thought to be smooth, is actually corrugated and undulates like an ocean wave buffeted by wind, says Nicole Cornilleau-Wehrlin of the Centre d'Etude des Environnements Terrestres et Planétaires in Vélizy. "For years, we had been trying to find out what happens to this shield," says Cornilleau-Wehrlin, whose instruments on the Spatio-Temporal Analysis of Field Fluctuations experiment detected waves in the magnetosphere that extended for 1000 kilometers and rippled along the magnetopause away from the sun--the first proof that these waves exist, says Escoubet: "That was not possible with a single spacecraft."

 Cluster's findings could soon have some practical benefits as well. The sun is entering the peak of its 11-year cycle of activity, which is expected to bring powerful solar flares that trigger magnetic storms in Earth's atmosphere. Such storms can disrupt radio and satellite communications. "Cluster is well positioned at the most complicated phase of the solar cycle to try and work out what the solar storms do to the magnetosphere," says Balogh. A better understanding of these processes, he says, could lead to the development of early warning systems that would enable satellite operators to shut off their equipment before electrical circuits are damaged.

Casassus is a writer in Paris; Hellemans is a science writer in Naples.

Volume 291, Number 5508, Issue of 23 Feb 2001, pp. 1466-1467. 
Copyright © 2001 by The American Association for the Advancement of Science.