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Swarm satellites. Computer illustration of the European Space Agency's (ESA) three Swarm satellites in Earth orbit. These satellites were launched together on one rocket on 22nd November 2013. Named Alpha, Bravo and Charlie, they occupy different orbits (two in a lower orbit, one in a higher orbit) and record differences in the strength of the magnetic field associated with the Earth's core, mantle, crust, oceans, ionosphere and magnetosphere. This information is necessary for modelling the Earth's geomagnetic field, and its interaction with solar winds and with other physical processes such as ocean circulation, climate and weather. The magnetic field is thought to be largely generated by an ocean of superheated, swirling liquid iron (bright orange) (KEYSTONE/SCIENCE PHOTO LIBRARY/EUROPEAN SPACE AGENCY/SCIENCE PHOTO LIBRARY)