The goal of the Swarm mission is to make the most thorough study ever undertaken of the Earth's magnetic field and ionosphere. The constellation of three satellites will measure the strength, direction and variations in the Earth's magnetic field and will supplement this with measurements of the electrical field and the density and winds of the thermosphere.
The principal objectives are to improve our characterization and understanding of:
- the principal magnetic field and the dynamo mechanism that generates it in the Earth's core,
- the dynamics of the core and the way it interacts with the mantle,
- the magnetic sources of the magnetic field in the lithosphere,
- the heterogeneities of electrical conductivity in the mantle,
- the electrical currents circulating in the ionosphere and the magnetosphere, and the way they are affected by the Sun,
- the role of the magnetic field and of the coupling between the ionosphere and the magnetosphere in injecting energy into the thermosphere,
- the magnetic signature produced by the tides and ocean currents.
In addition, data on the satellites' orbits and from their accelerometers will improve our knowledge of the Earth's gravity field. Swarm is the first mission of this type, consisting of several satellites. It takes over from the German CHAMP satellite (2000-2010), which carried a comparable set of instruments, and the Danish Ørsted satellite, launched in 1999, whose scalar magnetometer continues to provide scientists with data.
All this data, together with that from ground-based observatories that measure variations in the magnetic field around the world, will further enhance the results of the Swarm mission, especially as regards long-term changes to the principal magnetic field.
Identifying and understanding the different sources of the Earth's magnetic field
To thoroughly investigate the Earth's magnetic field, it is vital to adopt an observation strategy that will identify the contributions made by the different sources.
With this in mind, the Swarm mission was designed as a constellation of three satellites.
As a result it will be possible to study the signal generated by the core, the mantle, the lithosphere, the ionosphere and the magnetosphere, and perhaps even by the ocean currents.
By studying the principal field, it will be possible to "watch" the dynamics governing the core and the way in which the geodynamo currently works, in order to improve our ability to forecast changes to the principal field (and thus changes to the famous South Atlantic Anomaly), perhaps over a few decades.
Data from Swarm will also help build better three-dimensional models of the electrical conductivity of the Earth's mantle. These models are important for understanding the nature and the thermodynamic status of the rocks making up the mantle, to supplement information provided by studies of seismic waves and the Earth's gravity field.
The lithosphere is the rigid outermost shell covering the Earth's surface, including the Earth's crust and part of the upper mantle. Down to a certain depth at which the high temperatures make magnetization impossible, the rocks of the lithosphere are often magnetized. The distribution and strength of this magnetization provide precious information about geology, tectonic activity and the history of the magnetic field. Analysis of the magnetization of the ocean bed, for example, has provided key information for helping to understand the history of these beds and to determine the sequence of reversals of the Earth's magnetic field. By analyzing the signal from these sources, Swarm will fill gaps in our knowledge, which currently still prevent us from fully mapping this particularly useful signal at all spatial scales.
The ionosphere and the magnetosphere
The ionosphere is the upper part of the atmosphere, partly ionized by ultraviolet solar radiation. It extends from an altitude of 60 km up to 800 km. By its very nature, the ionosphere is therefore a very good electrical conductor when in daylight, especially in certain places such as along the magnetic equator. It dilates every morning and contracts every evening, moving within the principal magnetic field, causing electrical currents to appear with a corresponding magnetic signature. Much more complex and irregular currents also run through it, generated by the magnetosphere along the lines of the principal magnetic field.
All these currents produce magnetic signals, sometimes very strong, that the Swarm satellites will detect. They will also detect the electric currents associated with the circulation of charged particles much further away in the magnetosphere (at a distance of typically 3 to 5 times the radius of the Earth). The constellation design adopted for Swarm is also particularly well-suited to the task of identifying and analyzing the signals produced by these currents.
By carrying an accelerometer and taking in situ measurements of the electrical characteristics of ionospheric plasma, Swarm will also be able to study the impact of these currents in terms of the energy they inject into the thermosphere, for example.
Understanding the way in which the coupled electrical currents between the ionosphere and the magnetosphere work, and the influence of solar activity on these currents, is of great importance for a better understanding of the Earth's near environment.
There is more at stake in understanding the Earth's magnetic field than mere scientific curiosity.
The Swarm mission will therefore be attempting to solve many scientific mysteries and it is obvious that a detailed study of the Earth's magnetic field remains one of the key areas for studying the overall Earth system. But it is also important to emphasize the need to further our understanding in this field, for wider applications such as space technology, navigation and guidance systems, or the exploitation of resources. It is also important to be able to predict changes at the scale of the lifetime of systems exposed to powerful magnetic storms (such as high voltage power lines) or radiation (such as satellites), especially as the South Atlantic Anomaly is continuing to expand and intensify. This will make it possible to take precautionary measures and to develop appropriate technologies.
For more information about the Earth magnetism, see the following links:
- The Earth's magnetic field, a shield that protects the planet
- Le Champ Magnétique : Comment le Mesurer ? A quelles applications sert-il ? (in French)
A constellation of satellites
The SWARM mission consists of a constellation of 3 identical satellites dedicated to the study of the Earth's magnetic field.
To optimize the use of the 3 satellites and to obtain the required results, the SWARM constellation will be positioned as such:
- 2 satellites flying side by side (separation in longitude at the equator of about 150 km) in low polar orbit at an altitude of 460 km at the beginning of life, following an inclination of 87.35°.
- 1 satellite in high polar orbit at an altitude of 530 km at the beginning of life, with a 87.95° inclination.
- 2 orbital planes with two quasi-polar inclinations allowing a drift in local time.
- Quasi-global coverage.
Swarm in figures
3 identical satellites
Dimensions: 9.1 metres long (including a 4-metre deployable boom), 1.5 metres wide and 0.85 metres high.
Total mass of each satellite: 473 kg at launch.
Nominal mission duration: 4 years.
Orbit: The satellites are on near-polar orbits. Two of them orbit side-by-side, with their orbits decaying naturally from an initial altitude of 460 km to 300 km by the end of the mission. The third maintains an altitude of 530 km, gradually producing a relative drift in local time.
Payload: Vector Field Magnetometer (VFM) coupled with a star-tracker camera (STR), Absolute Scalar Magnetometer (ASM), Electric Field Instrument (EFI), Accelerometer (ACC), Laser Retro-Reflector (LRR) and GPS receiver.
Power consumption: instruments: 50 W; platform units: 140 W
Volume of data provided per day and per satellite: 1.8 Gbits
Data storage 2 x 16 GBytes
Cost of the mission (ESA): 229.6 million Euros
Total cost of French participation: 22.3 million Euros