In solar physics, heliospheric pickup ions are created when neutral particles inside the heliosphere are ionized by either solar ultraviolet radiation, charge exchange with solar wind protons or electron impact ionization. Pickup ions are generally characterized by their single charge state, a typical velocity that ranges between 0 km/s and twice the solar wind velocity (~800 km/s), a composition that reflects their neutral seed population and their spatial distribution in the heliosphere. The neutral seed population of these ions can either be of interstellar origin or of lunar-, cometary, or inner-source origin. Just after the ionization, the singly charged ions are picked up by the magnetized solar wind plasma and develop strong anisotropic and toroidal velocity distribution functions, which gradually transform into a more isotropic state. After their creation, pickup ions move with the solar wind radially outwards from the Sun. Interstellar pickup ions originate from the neutral component of the Local Interstellar Medium (LISM), which enters the heliosphere with a velocity of 25 km/s as a result of its relative motion with respect to the Sun. This neutral wind is gradually ionized and acts as the seed population for interstellar pickup ions. Inner-source pickup ions are produced by an inner-source of neutral particles. The detailed production mechanisms for these ions are currently under debate.
