Cluster II studies Earths magnetosphere. The four spacecraft of the Cluster II mission are part of the European Space Agency (ESA)s and NASAs Solar-Terrestrial Science Program. The purpose of the Cluster II mission was to study small-scale structures in three dimensions in the Earths plasma environment, such as those involved in the interaction between the solar wind and the magnetospheric plasma, in global magnetotail dynamics, in cross-tail currents, and in the formation and dynamics of the neutral line and of plasmoids.