The Earth is protected by a powerful magnetic field which forces the solar wind to sweep around the planet. In the process, the magnetic field is shaped into a gigantic teardrop that typically extends about 65 000 km towards the Sun and more than two million km in the opposite direction. This region is the magnetosphere and is bounded by the magnetopause. Above the magnetopause is a layer of turbulent magnetic field (the magnetosheath) which is itself bounded by the bow shock. The polar cusps are 'windows' above the northern and southern polar regions where the particles from the solar wind can penetrate the magnetic shield.