ESA Science & Technology - Cluster will become the first multi-scale mission

Image 1: original dayside near-Earth targeted regions	Image 2: original nightside near-Earth targeted regions

The Cluster and SOHO missions constitute the first cornerstone of ESA's Horizons 2000 Programme. Together they allow the investigation of energy flow from the Sun to the Earth, and the study of plasma processes in the Earth's magnetosphere driven by solar wind input.

The four Cluster spacecraft were launched in pairs on two Soyuz rockets in July and August 2000, into a polar orbit targeting some of the most important near-Earth regions: solar wind, bow shock, magnetosheath, cusp, magnetopause, plasmapause and magnetotail.

Cluster is the first spacefleet, flying in close formation, to study the magnetosphere. The four spacecraft are needed to provide a three-dimensional picture of the plasma and to separate spatial and temporal features. By providing its unique four-point measurements in the Earth's magnetosphere, Cluster is revolutionising our understanding of the dynamics of space plasmas.

Discoveries made by Cluster have demonstrated the critical importance of making measurements on different spatial scales. Using the thrusters onboard each spacecraft, the inter-spacecraft separation distance has been changed already several times, ranging from 100 km to 5000 km.

Originally funded to operate for two years, and now in an extended mission phase until December 2005, the spacecraft and payload continue to perform well and are expected to do so for several years to come.

In order to fully realize the potential of the mission, an extension of four years has been granted to the Cluster team, with a spacecraft and payload review at the end of the second year. During this period, the four spacecraft will be used to provide the first measurements of space plasmas simultaneously at multiple scales on both the dayside (Image 1) and the nightside (Image 2). The evolution of the orbit will also allow Cluster to explore important magnetospheric regions that could not be sampled earlier in the mission (Image 3 and Image 4).

Image 3: orbit evolution allows new targeted regions	Image 4: orbit evolution allows new targeted regions

Cluster 4th anniversary of scientific operations

On 1 February 2005, Cluster celebrated its 4th anniversary of scientific operations. As detailed in the scientific highlights (see overview below), since 2001, Cluster has made breakthrough discoveries in many regions of the near-Earth environment. At the end of 2004, more than 250 refereed papers, based on Cluster data, have been published in scientific journals, including Nature, Physical Review Letters, Annales Geophysicae, Geophysical Research Letters and the Journal of Geophysical Research. A growing number of Cluster papers are published in the scientific literature, reaching 79 refereed papers over the last six months, and counting.

Science Highlights after 4 years of scientific operations

The highlights presented below are only a few of the many results which show that Cluster is delivering fundamental results in space plasma physics. A dedicated story will soon be posted to present a much more complete overview of the Cluster achievements. More detailed explanations together with pictures, 2D and 3D animations can be obtained by clicking on each of the following links (last item is not available yet).

• Determination of the thickness of the Earth's bow shock
• First observation of 3D magnetic reconnection
• First detection of Kelvin-Helmholtz vortices at the edge of the magnetosphere
• Spatial-scale determination of bursty bulk flows in the magnetotail
• Remote sensing of electromagnetic emission sources by triangulation
• First measurement of current density in thin magnetotail current sheets
• The exterior cusp spatial structure
• Waves identifications (k-filtering)

Why an extension?

The near-Earth targeted regions of the four Cluster spacecraft were originally: the solar wind, bow shock, magnetosheath, cusp, magnetopause, plasmapause and magnetotail regions (Image 1).

Launched in 2000 and initially funded for two years, Cluster is now in its first extended mission phase of 3 years. At the end of 2005, the original targeted regions will be sampled at a range of scales from 100 km to 5000 km, covering their kinetic physics, dynamics and medium scale morphology.

By using the thrusters onboard each spacecraft, a 10 000 km separation in the magnetotail will be achieved during summer 2005. However, without an extension of the mission, the polar cusp and the solar wind regions would not have been sampled at a 10 000 km scale during the first 6 months of 2006.

In the course of this new extension, the inter-spacecraft separations will be altered: three of the four spacecraft will keep a separation distance of 10 000 km, while the fourth spacecraft will be moved to orbit at a closer distance from one of its space companions (Image 1). This will result in measurements at both small and large scales simultaneously, making it possible to study the links between kinetic processes and large scale morphology. Cluster will then become the first multi-scale mission ever launched in space.

The Cluster polar orbit is designed for the study of the cusp and the magnetotail, both critical regions of the magnetospheric system. The evolution of the orbit is carrying the spacecraft away from these regions and towards new regions that are also vital for understanding energy flow through the system: the subsolar magnetopause and the auroral acceleration zone (Image 3) and the tail current disruption region (Image 4). By taking advantage of this natural evolution, the extension will for the first time allow four-point measurements of these regions.

"Multi-scale and the change of orbit will bring new science to be done with Cluster", underlined Philippe Escoubet, Project scientist of the Cluster mission at ESTEC, European Space Agency, The Netherlands.

Worldwide interest

The Cluster mission is a project of international collaboration between ESA and NASA, and the mission involves a larger scientific community. 257 scientific Co-Investigators from 75 research laboratories, spread over 24 countries, are associated with Cluster, including researchers from China, Russia, Japan and India. Every 6 months, a Cluster workshop, gathering an average of 120 scientists, is organized to assimilate the complex Cluster data sets and multi-spacecraft data analysis tools. The 9th Cluster workshop will take place in Meudon, France, from 7-11 March 2005.

Public access to Cluster data

The importance of the Cluster data to space plasma physics is underlined by the actual development of the Cluster Active Archive (CAA), funded by ESA, NASA and the Member States. From mid-2005, this archive will provide free access to the full, calibrated, high resolution Cluster data sets. Future data will be added to the archive during the course of the mission. CAA will be a unique research tool for the worldwide magnetospheric science community for many years to come.

Complementarities with other geospace missions

The Extension of the mission to end 2009 will also allow an extended collaboration with Double Star and other Earth-orbiting spacecraft, and result in an overlap with NASA's multi-spacecraft THEMIS mission to study substorms in the Earth's magnetotail.

Key facts of the Cluster mission

• 1st four-spacecraft mission, flying in close formation, to study the Earth's magnetosphere and its interaction with the Sun
• 44 instruments (11 per spacecraft)
• Elliptical polar orbit, 19 000 to 119 000 km, 57 hour period
• Flexible inter-spacecraft distance
• Spacecraft operations centre: European Space Operations Centre (ESOC), Germany
• ESA ground stations: VILSPA and Maspalomas, Spain and during the extensionPerth, Australia
• Science operations centre: Joint Science Operations Centre at RAL, Didcot, UK

Web story author
Arnaud Masson, SCI-SH division, RSSD, ESA, The Netherlands.
Tel: +31-71-565-5634

Web story editor
Philippe Escoubet, SCI-SH division RSSD, ESA, The Netherlands.
Tel: +31-71-565-3454