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Swarm  ESA's magnetic field mission Swarm
The Swarm concept consists of a constellation of three satellites in three different polar orbits between 400 and 550 km altitude. High-precision and high-resolution measurements of the strength and direction of the magnetic field will be provided by each satellite. In combination, they will provide the necessary observations that are required to model various sources of the geomagnetic field. GPS receivers, an accelerometer and an electric field instrument will provide supplementary information for studying the interaction of the magnetic field with other physical quantities describing the Earth system – for example, Swarm could provide independent data on ocean circulation.
Magnetic fields play an important role in many of the physical processes throughout the Universe. The Earth in particular has a large and complicated magnetic field, the major part of which is produced by a self-sustaining dynamo, operating in the fluid outer-core.
However, measurements taken at or near the surface of the Earth are the superposition of magnetic field originating from the outer core as well as the fields caused by magnetised rocks in the Earth’s crust, electric currents flowing in the ionosphere, magnetosphere and oceans, and by currents induced in the Earth by time-varying external fields.
Magnetic field changes in internal as well as external origin occur on a variety of time scales, and separating them relies on their different temporal variations. For example, over the last 150 years it has been observed that the axial dipole component of the Earth’s magnetic field has decayed by nearly 10%. This fast decay rate is characteristic of magnetic reversals which occur on average about once ever half million years. Geographically, this recent dipole decay is largely due to changes in the field beneath the South Atlantic Ocean, connected to the growth of the South Atlantic anomaly. Within the Earth’s interior the core field and, in particular, its temporal changes, known as ‘secular variation’, are among the few means available for probing the properties of the outer core. This secular variation directly reflects the fluid flow in the outmost core and provides a unique experimental constraint on ‘geodynamo theory’. However, the only part of the core field that varies on time scales longer than around one year is observable at the Earth’s surface. Studies of the electromagnetic core-mantle coupling require a better knowledge of the electrical conductivity of the lowermost mantle – this can be obtained from the analysis of ‘jerks’, which are sudden changes in the secular variation that last for 1 or 2 years. An improved determination of the core’s contribution to the Earth’s angular momentum budget will allow for a better estimation of changes in atmospheric and ocean circulation patterns. It is clear that the nature of the Earth’s magnetic field is complicated. It is also therefore clear that there is the need for a comprehensive separation and understanding of the external and internal processes that contribute to the Earth’s magnetic fields – the Swarm mission aims to address such needs as well as allowing for new and exciting studies of the lithospheric field. The magnetic field is also of importance for the Earth’s external environment. While it is known that the air density in the thermosphere is related to geomagnetic activity, recent results from the German CHAMP mission have indicated that air density is locally affected by geomagnetic activity in a specific way that is still to be explored and understood. Furthermore, the magnetic field acts as a shield against high-energy particles from the Sun and outer Space. Continuous space-borne monitoring of the magnetic field at low Earth orbit, and the derivation of field models play an important role in predicting radiation hazards within the space environment. Last update: 27 October 2011
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