06 Sep 2010

Illustration of the four Cluster spacecraft. Credit: ESA

"Cluster is the first space mission with four identical spacecraft flying in formation to study this Sun-Earth interaction," said Matt Taylor, ESA's Cluster project scientist.

"Physical quantities, such as electrical currents, flow vortices and the motion of boundaries, can only be measured with a minimum of four spacecraft and Cluster's capability in this area has provided a leap forward in our understanding of the interaction of the solar wind with Earth."

As their orbits have evolved and the distances between them have altered, the spacecraft have been able to study in great detail many different regions of near-Earth space. During a decade of discovery, Cluster has provided numerous new insights into the fundamental processes that influence not only near-Earth space, but the Universe as a whole.

Illustration of black aurorae conditions. Credit: ESA

Cluster discovered that the dark patches correspond to 'holes' in the ionosphere that grow in size as more and more electrons shoot upwards into space. This is the opposite to what happens during a normal aurora, where electrons spiral down into the atmosphere and collide with ionospheric particles. Cluster found that all available electrons are 'sucked' out of the ionosphere within a few minutes. After that the black aurora disappears.

The breakthrough came when a particularly strong solar shock wave hit the magnetosphere in 2004. Over a period of 15 minutes, the spacecraft detected the transition from normal electron intensities to killer electrons levels. In the first stage of the process, electrons were accelerated by the shock wave compressing Earth's magnetic field. Then the magnetic lines wobbled, creating something like a very large-scale, low frequency laser, which further accelerated the electrons until they reached 'killer' energies.

Cluster data led to the first 3-D picture of what happens at the heart of the process - the magnetic 'null' point. The new insights showed that the magnetic field can be twisted into 500 km-wide tubes.

Whirlpools of plasma in Earth's magnetosphere. Credit: ESA

Computer simulations show that the vortices inject plasma into the magnetosphere by forcing magnetic reconnection to take place. This opens passageways that allow the particles to cross the usually impenetrable boundary of the magnetosphere.

Oscillations observed in the magnetotail by Cluster and Double Star. Credit: ESA

Because of the geometrical arrangement of the satellites at the time, the oscillations were detected closer to Earth than ever before - only 70 000 km away. At such proximity, the oscillations probably send particles into the atmosphere, sparking colourful auroras.

Despite years of continuous exposure to the harsh radiation environment around Earth, all of the major systems on the four satellites are still functioning well. The mission has been extended until December 2012, subject to confirmation in late 2010, and there is a proposal under consideration to continue until 2014 . This means that it will have observed an entire solar cycle. Meanwhile, Cluster is being opened up to the wider scientific community after a recent announcement of opportunity for multi-instrument observations by Guest Investigators.

"During the last 10 years, the Cluster data have enabled many discoveries and breakthroughs in space physics, thanks to a very active scientific community and to free access to all high resolution data products," said Philippe Escoubet, ESA's Cluster mission manager.

"Cluster has driven the direction of how space physics will be studied in the future. Future Sun-Earth connection missions are all foreseen to be composed of multiple satellites."