= Pickup ion
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 $V$, with <magnetic field> perpendicular to the flow, a newly ionized atom initially at rest relative to the <Sun> has speed $V$ 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 $2V$ 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.<Direct observation of He+ pick-up ions of interstellar origin in the solar wind by E. Möbius and others>[Nature 318, 426–429, abstract]{ref} These velocity and composition measurements distinguish newly picked-up material from the main thermal flow.
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