Region around a planet (such as Earth) or a natural satellite that possesses a magnetic field (see geomagnetic field ) where magnetic phenomena and the high atmospheric conductivity caused by ionization strongly influence the behaviour of charged particles (see electric charge ).
A planet or moon's magnetic field, like its gravitational field, becomes weaker with distance from the body. The solar wind sweeps a body's magnetosphere out away from the Sun in a "tail" trailing well beyond it.