The Earth, like all bodies in the solar system, is plunged into the solar wind, a completely ionized, non-collisional plasma basically formed of protons and electrons. This stream of particles is propagated radially from the Sun at a supersonic velocity of about 400 km/s. A bow shock forms in the vicinity of an obstacle, in this case our planet. Earth has a magnetic field that is nearly dipolar and pushed out of shape by the pressure of the solar wind, flattened in the direction of the Sun (on the dayside) and stretched in the direction opposite the Sun (on the nightside). The cavity thus created around the planet, the magnetosphere, has a boundary with the solar wind known as the magnetopause. It is bounded by the ionosphere, a partially ionized gas layer from the atmosphere. The magnetosphere is also filled with plasma from the ionosphere and the solar wind. Non-collisional and completely ionized like the solar wind, it has different characteristics and its dynamic range is controlled by the planet's magnetic field. In short, the solar wind - magnetosphere system includes differentiated regions and - the physics of these different environments, especially of their interactions, is far from being fully understood.
Until now, magnetospheric research space missions were seriously handicapped by only being able to fly one satellite in a given region. This strategy, which is well suited to exploration, i.e. the mapping of a supposedly stationary environment over time, is illusory for turbulent processes developing in a rapidly changeable environment, as during the interaction between the solar wind and the Earth's magnetic field. The European Space Agency's Cluster project proposes an original concept to overcome this difficulty, comprising 4 identical satellites in a tetrahedral shaped formation. With its 4 satellites, Cluster can be used to study for the first time in three dimensions the physical processes prevailing in regions interfacing between the solar wind and the magnetospheric plasma and to separate spatial effects from temporal effects.
The magnetopause surrounding the region where Earth's magnetic field is dominant theoretically forms a tight boundary, but the plasma from the solar wind partially penetrates the magnetosphere. How? We can study the transfers of masses, energy or kinetic moment, via the magnetopause, but also the other boundaries: the shock wave, polar cusps and the regions of the tail of the magnetosphere. In non-collisional plasmas, like the solar wind and magnetospheric plasma, only waves or turbulence can ensure particle thermalization or energy exchange between different particle populations. A homogenous set of parameters is required for these studies: electromagnetic measurements (magnetic and electrical fields and their fluctuations, waves), environment composition (particles), density, mean speed and pressure of the plasma (the environment is totally ionized) and - and essential parameter - the gradients of these quantities which are the driving forces of the magnetospheric system.
The list of science objectives below, associated with the key regions crossed by Cluster, is taken from ESA's phase A study (December 1985). As Cluster II is still the first mission composed of 4 identical, co-ordinated measuring points, these objectives remain relevant:
- Physics of boundary regions between two cosmic plasmas and processes which transfer mass, momentum and energy across the boundary, such as magnetic reconnection (dayside magnetopause and polar cusps)
- Plasma acceleration during large-scale reconfiguration of plasma and electromagnetic fields (geomagnetic tail)
- Study of turbulence, vortex formation and eddy diffusion (solar wind, polar cusps, magnetopause and plasma sheet boundary layers)
- Structure and properties of collisionless shock waves, associated particle acceleration and wave generation (bow shock, interplanetary shocks)
- Microstructure of plasma and fields in the solar wind as an example of a stellar wind.
The increased onboard storage capacity of Cluster II compared to Cluster will provide improved orbital coverage with the ability to record data from an entire orbit. Consequently, several complementary studies can be approached in a new way thanks to a multipoint view of the internal magnetosphere, although the tetrahedral configuration is out of shape at low altitudes: analysis of electromagnetic sources, mass transfers, processes linked to auroral activity (internal magnetosphere, plasmasphere, auroral lines).
Inter-satellite distances must be adapted to the characteristic dimensions of the key regions crossed (readjustment is scheduled every 6 months) and the choice of the region of the orbit where the fleet will be in tetrahedral formation must be optimized (see the graph).
The Cluster project was officially launched in February 1983 at the European Space Agency (ESA) at an Assessment Study start-up meeting bringing together European scientists and engineers (including a strong French and German scientific contingent). This phase was preceded by numerous studies conducted mainly in France since the end of the 1970s.
The project then successfully progressed through all decision-making steps to finally become, along with the SOHO solar project, the first cornerstone of the ESA Horizon 2000 programme.
The choice of payload was officially approved by the ESA Scientific Programmes Committee in 1988.
The payload was unfortunately lost on 4 on June 1996 when the Ariane 5 launch vehicle failed its first qualification flight.
Following the resolution adopted by its Scientific Programmes Committee on 3 April 1997, ESA, with the agreement of the international scientific community, decided to launch another Cluster mission called Cluster II, with 4 satellites and instruments identical to those in the initial mission, with a view to fulfilling the scientific objectives planned in the first cornerstone of the Horizon 2000 programme.