Placing a cornerstone

The original Cluster mission and the Solar Heliospheric Observatory (SOHO) together comprised the Solar Terrestrial Science Programme (STSP), the first 'Cornerstone' of ESA's Horizons 2000 Programme. These two missions were selected at the same time to investigate the relation between the Sun and the Earth's environment.

Since its successful launch in December 1995 SOHO has been providing fascinating new information on the effect of the Sun and the solar wind on the Earth's magnetosphere. The approval of the complete Cluster II mission means that the original STSP objectives can be fulfilled, especially now that SOHO's lifetime has been extended.

Together the two STSP missions will answer questions about an entire chain of processes: right from the Sun's very interior down to the Earth's magnetosphere. Their results will also be combined with a number of other missions within the framework of the IACG, the Inter-Agency Consultative Group.

Goals

The goals of the Cluster II mission are identical to those of the original Cluster mission lost in June 1996 and the instrument complement remains the same. The Cluster II mission is an in-situ investigation of the Earth's magnetosphere using four identical spacecraft simultaneously. It will permit the accurate determination of three-dimensional and time-varying phenomena and will make it possible to distinguish between spatial and temporal variations.

Reference to specific Cluster II instruments in the following sections is by way of illustration only. All instruments will be making measurements in all key plasma regions.

The interaction between the solar wind and the magnetosphere is a key element in the Solar Terrestrial Science Programme (STSP). One example of this interaction is the direct entry of solar wind particles through the polar cusps. The polar cusps are two magnetic funnels, one in each hemisphere, which focus the solar wind particles (rather like a telescope) on photons. The solar wind particles enter the exterior cusp, which has a diameter of approximately 50 000 km, and then follow the converging magnetic field down to the ionosphere where the cusp size is around 500 km. This converging magnetic field allows the study of a very large area of the magnetopause through a limited region of space inside the cusps.

Another example of solar wind/magnetosphere interaction is the acceleration of plasma in the magnetotail during substorms. The magnetotail is a large reservoir of both solar wind and ionospheric particles which, under some circumstances, for instance the reversal of the interplanetary magnetic field from north to south, releases a large quantity of particles towards the Earth. Both mechanisms - particles entering the polar cusps and the substorms - produce aurorae when the participating particles (electrons and ions) hit the neutral gas of the atmosphere. When these particles are particularly energetic they can have a dramatic effect on human activities, disrupting electrical power and telecommunications or causing serious anomalies in the operation of satellites, especially those in geostationary orbit.

Cluster II is determining the physical processes involved in the interaction between the solar wind and the magnetosphere by visiting key regions like the polar cusps and the magnetotail. The four Cluster spacecraft is mapping in three dimensions the plasma structures contained in these regions. The simultaneous four-point measurements also allow differential plasma quantities to be derived for the first time. For example, the density of current flowing around the spacecraft is derived from the magnetic field measurements using Ampere's law.

Cluster II's main goal is to study the small-scale plasma structures in space and time in the key plasma regions:

• solar wind and bow shock
• magnetopause
• polar cusp
• magnetotail
• auroral zone