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A magnetosphere is the volume of space around an astronomical object that is controlled by that object's magnetic field. The Earth's magnetosphere is the cavity formed by the Earth's magnetic field in the flow of plasma from the Sun known as the solar wind. The interaction with the solar wind deforms the Earth's basically dipolar magnetic field, compressing the field lines on the day side and stretching them out to form a long comet-like tail (the magnetotail) on the night side. On the day side, the magnetosphere extends out to a distance of approximately 10 Earth radii (under quiet conditions), while the magnetotail extends several hundred Earth radii in the antisunward direction. The magnetosphere contains various large-scale regions, which vary in terms of the composition, energies, and densities of the plasmas that occupy them. The sources of the plasmas that populate these regions are the solar wind and the Earth's ionosphere; the relative contributions of these two sources to the magnetospheric plasma vary according to the level of geomagnetic activity. (See the glossary entry on the DST index, which is one of the measures of geomagnetic activity.)

The Earth's magnetosphere is a highly dynamic structure that responds quite dramatically to changes in the dynamic pressure of the solar wind and the orientation of the interplanetary magnetic field (IMF). Its ultimate source of energy is the interaction with the solar wind. Some of the energy extracted from this interaction goes directly into driving various magnetospheric processes, while some is stored in the magnetotail, to be released later in substorms. The principal means by which energy is transferred from the solar wind to the magnetosphere is a process known as "reconnection," which occurs when the IMF is oriented antiparallel to the orientation of the Earth's field lines. This orientation allows interplanetary and geomagnetic field lines to merge, resulting in the transfer of energy, mass, and momentum from the solar wind to the magnetosphere. The viscous interaction of the solar wind and the magnetosphere also plays a role in solar wind/magnetosphere coupling, but is of secondary importance compared with reconnection.