Although the existence of the Earth's magnetic field has been known since ancient times, the fact that it does not extend indefinitely into space, but is confined to a cavity, is a discovery of the twentieth century. This cavity is created by the solar wind, which shapes the Earth's field into a comet-like structure as shown in Fig. 10.1. The region in which the Earth's field is confined is called the magnetosphere, a term coined by T. Gold in 1959 before the true shape of this region had become known (Gold, 1959). Upstream of the magnetosphere there is a detached bow shock, which occurs because the solar wind flows past the Earth at a speed approximately eight times greater than either the Alfvén or sound speeds. The bow shock is located about 15 RE sunward of the Earth, where one Earth-radius (1 RE) is about 6,370 km. In front of the bow shock is a region called the fore shock, which contains energetic particles and waves. In MHD terms, the bow shock and fore shock together constitute a fast-mode MHD shock, while the bow shock itself may be thought of as a subshock (Kennel, 1988), where the solar wind is compressed. The region downstream of the bow shock is called the magnetosheath, and it is here that the shocked solar wind is deflected around the magnetospheric cavity.

The concept of the magnetosphere was first introduced by S. Chapman and V. Ferraro (1931), who thought the Earth's magnetic field would be confined to a cavity whenever the Earth is impacted by a plasma cloud ejected from the Sun.