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As the universe consists almost entirely of plasma, the understanding of astrophysical phenomena must depend critically on our understanding of how matter behaves in the plasma state. In situ observations in the near-earth cosmical plasma offer an excellent opportunity for gaining such understanding. The near-earth cosmical plasma not only covers vast ranges of density and temperature, but is the site of a rich variety of complex plasma physical processes which are activated as a result of the interactions between the magnetosphere and the ionosphere. The geomagnetic field connects the ionosphere, tied by friction to the earth, and the magnetosphere, dynamically coupled to the solar wind. This causes an exchange of energy and momentum between the two regions. The exchange is executed by magnetic-field-aligned electric currents, the so-called Birkeland currents. Both directly and indirectly (through instabilities and particle acceleration) these also lead to an exchange of plasma, which is selective and therefore causes chemical separationi. Another essential aspect of the coupling is the role of electric fields, especially magnetic-field-aligned ("parallel") electric fields, which have important consequences both for the dynamics of the coupling and, especially, for energization of charged particles.