The Earth's magnetosphere is composed of two essential ingredients. The first is the Earth's magnetic field, generated by currents flowing in the Earth's core. Outside the Earth this field has the same form as that of a bar magnet, aligned approximately with the Earth's spin axis. The second ingredient is the solar wind, a fully ionized hydrogen/helium plasma that streams continuously outward from the Sun into the solar system at speeds of ˜300–800 km s−1. This plasma wind is pervaded by a large-scale interplanetary magnetic field (IMF), which plays a crucial role in the Earth's interaction with the solar wind. There is also a third ingredient that plays an important role: the Earth's ionosphere. The upper atmosphere is partially ionized by solar far-ultraviolet and X rays above altitudes of ˜100 km. The ionosphere forms a second source of plasma for the magnetosphere, mainly of protons and singly charged helium and oxygen.