A pickup ion is an ion created from a neutral particle that initially moves differently from the surrounding magnetized plasma, and is then accelerated and carried along by that plasma's electromagnetic fields. In the solar wind, interstellar neutral atoms can be ionized by sunlight or charge exchange. Their new charge couples them to the interplanetary magnetic field and the solar wind's motional electric field.
For an idealized uniform flow of speed , with magnetic field perpendicular to the flow, a newly ionized atom initially at rest relative to the Sun has speed in the flow frame. It gyrates about the field while its guiding center moves with the flow. Transforming back to the solar frame gives speeds between zero and during a gyro-orbit. Subsequent scattering modifies this initial ring distribution; pickup does not mean immediate thermal equilibrium with ordinary solar-wind ions.
The 1985 discovery paper reported singly ionized helium extending to about twice the solar-wind speed, and a seasonal enhancement associated with gravitational focusing of interstellar neutrals.[ref]
Direct observation of He+ pick-up ions of interstellar origin in the solar wind by E. Möbius and others. Nature 318, 426–429, abstract
These velocity and composition measurements distinguish newly picked-up material from the main thermal flow. Articles by others on the same topic
A "pickup ion" typically refers to an ion that is formed when a neutral atom or molecule becomes ionized by the process of "pickup." This can occur in various contexts, such as in astrophysics or ionospheric studies, typically involving neutral atoms or molecules capturing energetic particles or radiation that causes them to become ionized. In astrophysics, for instance, pickup ions can be created when neutral hydrogen atoms in space collide with energetic particles, such as solar wind protons or other ions.