that region surrounding a planet in which the planet's magnetic field is stronger than the interplanetary field: the solar wind gives it a cometlike shape with the tail extending from the night side of the planet for vast distances
See magnetosphere in American Heritage Dictionary 4
(măg-nēˈtō-sfîrˌ)
noun
An asymmetrical region surrounding the earth, extending from about one hundred to several thousand kilometers above the surface, in which charged particles are trapped and their behavior is dominated by the earth's magnetic field.