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Mission description
From the Sun to the Earth
Our Sun constantly ejects a plasma, called solar wind, in the interplanetary medium. The solar wind is a natural laboratory for plasma physics. It is accessible to scientists by in situ measurements using space missions.
- Figure 1. The Sun-Earth connection and the Earth magnetosphere
Source ESA
The encounter of the solar wind with the Earth’s magnetic field creates a magnetic shield which is called the planet’s magnetopause, separating the Solar Wind from the Earth’s magnetosphere. Within the latter, many physical processes occur. Some of them have observable effects from ground: northern and southern aurorae. They sometimes have consequences on human activity: e.g. disturbances in power grids and telecommunication networks.
Cluster has been designed to investigate the Earth’s magnetosphere in three dimensions, in particular the interfaces between two different plasmas, as the one between the Solar Wind and the magnetosphere. It therefore consists of an array of four identical spacecraft *. The Cluster mission is associated with the SOHO mission dedicated to the observation of the Sun. These two missions form the Solar-Terrestrial Science Program (STSP) of the European Space Agency (ESA).
A failure followed by a rebirth
The Cluster mission launch was scheduled for June 4, 1996 with the first Ariane 5 launch launch which failed. Ariane 5 had to be destroyed within seconds after launch. The on board Cluster instrumentation was also destroyed.
ESA decided to re-build alike Cluster spacecraft and instrumentation . The probes of the new mission Cluster II have been launched in July and August 2000 by two Russian Soyuz and Moliniya rockets. Given the harvest of positive results since 2000 by the Cluster mission, it is extended until December 2018.
Visit the key regions
The four Cluster satellites are exploring the Earth’s magnetosphere and the Solar Wind near the Earth, and in particular the key regions of the interaction of both regions:
- The solar wind in the neighborhood of the Earth,
- The bow shock
- The magnetopause
- The polar cusps
- The magnetotail
The satellites have a polar elliptical orbit aroun Earth. The altitude of perigee and apogee are resopectively at 19000km and 119000km of altitude (Earth radius: 6371km). This allows the crossing of all these regions. The main objective of the mission is to study the physical processes occuring in these regions.
- Figure 2. Artist view of Cluster
Source ESA
Get rid of ambiguities
When a satellite measures a physical quantity that fluctuates two cases arise. These fluctuations may be caused by satellite motion in a stationary environment. They are called spatial variations. These fluctuations may instead reflect a change over time. In this case these are temporal variations.
The Cluster mission presents an innovative structure formed of four identical satellites. This allows the differentiation in the observations between spatial and temporal variations. It is important in the case of a non-stationary environment as Earth’s magnetosphere. By this, Cluster is very different from all previous space plasma missions.
3D measurements
The analysis of three-dimensional properties of the plasma and the magnetic field is the implicit motivation of the mission. The four Cluster spacecraft have exactly the same instrumentation. Thanks to their flying in tetrahedral formation a 3D analysis is possible *. They measure a physical quantity at the same time at the four corners of the tetrahedron.
Therefore, Cluster was the first space mission to estimate locally the electric current density in the plasma. The current is determined across the tetrahedron from the measurement of the magnetic field at four points. Another originality of the mission was to assess the possibility of calculating plasma density gradients.
* With three points one defines a plane. Four points are necessary to describe the three dimensions. The four identical Cluster satellites were put in a tetrahedric formation in the key region of their orbit during the first years of the mission. They can perform the same measurement simultaneously at four different points. This helps to determine the three dimensional structure of ameasured variable.
To know more about the tools that have been developed for 3D studies:
Paschmann G. et Daly, W., Analysis methods for multispacraft data, ISSI Scientific report, 1998.
Paschmann G. et Daly, W., Analysis methods for multispacraft data revisited, ISSI Scientific report, 2008.
