Magnetotail

n. region of a planetary magnetosphere which is propelled in the direction of the solar wind

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 Jupiter, Saturn, Uranus and Neptune. Mercury and Jupiter's moon Ganymede are magnetized, but too weakly to trap 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.
See more at Wikipedia.org...