Like the other planets, the Earth and its atmosphere are bathed in the supersonic flow of plasma from the Sun (solar wind) as well as in an intense solar photon spectrum extending well into the ultraviolet. Ultraviolet radiation continuously produces a partially ionized plasma in the upper atmosphere (known as the ionosphere), the lighter components of which expand upward, filling the inner magnetosphere and escaping through the outer magnetosphere into the solar wind. Earth has a strong magnetic field that deflects much of the solar wind around the planet at a distance much larger than Earth's atmospheric scale height. Nevertheless, the solar wind and terrestrial plasma atmosphere are strongly coupled by the magnetic field, which links the two regions and transmits stresses between them, especially along the magnetic field lines connecting the boundary layers of the magnetosphere with the auroral ionosphere. Heating of terrestrial plasma and the production of magnetic field-aligned electric fields in this region lead to the significant escape of ionospheric plasma into the magnetosphere.

The storage and dissipation of energy transmitted from the solar wind into the terrestrial plasma and gases define the various space plasma phenomena and are most dramatically manifested in geomagnetic substorms and storms. Plasma heating and particle acceleration in the magnetosphere produce a plasma much hotter than the solar corona as well as energetic particle populations in the energy range that is harmful to humans and electronic systems in space. The Earth ultimately receives the dissipated solar wind energy of the system. The atmosphere responds by expanding upward and increasing drag on low-orbiting spacecraft, while ionization changes disrupt radio communications. The varying geomagnetic field induces currents in the Earth and in electrically conducting human industrial systems such as power transmission lines and pipelines.

The Earth's magnetosphere is the most studied object in space physics, yet the establishment of a robust predictive model of its behavior remains an elusive goal. As we become a space-faring culture, we are more and more in need of a predictive understanding of the key processes that constitute the Sun-Earth connection. We can expect progress toward this goal to follow earlier developments in tropospheric meteorology rather closely. As more global observations become available, global models of the magnetosphere system will mature to the point where useful forecasting can be developed as a routine aspect of space operations.

Future missions have been designed to address the need for study of the Earth's space environment on scales ranging from the smallest features of the aurora (about 100 meters) to the global scales of the entire magnetosphere system (tens of Earth radii). The requirements for such an observational program are again analogous to those for meteorological studies of the Earth's lower atmosphere, where the relevant scales range from the size of an individual convective cell to the planetary scale, and both local measurements and global imaging from synchronous orbit have proven essential to achieving our present level of weather forecasting.