A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus and Neptune. Jupiter's moon Ganymede is magnetized, but too weakly to trap solar wind plasma. Mars has patchy surface magnetization. The term "magnetosphere" has also been used to describe regions dominated by the magnetic fields of celestial objects, e.g. pulsar magnetospheres.
The magnetosphere of Earth is a region in space whose shape is determined by the extent of Earth's internal magnetic field, the solar wind plasma, and the interplanetary magnetic field (IMF). In the magnetosphere, a mix of free ions and electrons from both the solar wind and the Earth's ionosphere is confined by magnetic and electric forces that are much stronger than gravity and collisions.
In spite of its name, the magnetosphere is distinctly non-spherical. On the side facing the Sun, the distance to its boundary, which varies with solar wind intensity, is about 70,000 kilometers, 10 to 12 Earth radii or RE, where 1 RE = 6371 kilometers; unless otherwise noted, all distances here are from the Earth's center.
The boundary of the magnetosphere ("magnetopause") is roughly bullet shaped, about 15 RE abreast of Earth and on the night side (in the "magnetotail" or "geotail") approaching a cylinder with a radius 20 to 25 RE. The tail region stretches well past 200 RE, and the way it ends is not well-known.
The outer neutral gas envelope of Earth, or geocorona, consists mostly of the lightest atoms, hydrogen and helium, and continues beyond 4 to 5 RE, with diminishing density.