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I've been reading several publications about Titan's atmosphere and chemistry, and the term "pickup ion" kept appearing here and there (see this publication for example).
I looked it up and found the Wikipedia entry, but could not understand the essence of the term (only later I noticed that apparently even Wikipedia is not entirely satisfied with this entry, since it is regarded as "too technical for most readers to understand.").

Other Google results just give other publications where the term is used, but I could not find any definition there too. I also could not find any questions about it here.

So if anyone could provide a concise and clear explanation, I'd be grateful. I am a 1st-year chemistry M.Sc. student, so you can adjust the explanations accordingly.

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So most of the particles we normally detect (or just get hit by) in space within the solar system originate from the Sun. Judging by the wikipedia article, pickup ions are all the rest -- atoms of gas from the interstellar medium which made it through the Suns magnetic field and past the outward flowing gas, and then somehow picked up electrical charge and got accelerated into the solar system, or atoms which were knocked off, or escaped from, moons, comets, asteroids, etc.

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Pick-up ion, in this context refers most likely to the atoms and molecules originating in moon/planet's atmosphere, which are ionized by solar wind particles and subsequently accelerated by the moving magnetic field "frozen" in solar wind. One possible outcome of this interaction is atmosphere of this body being lost to the interplanetary space, thus the term 'pick-up' (by solar wind).

More detailed description of the process can be found, for example, in the Jarvinen, R., and E. Kallio (2014), Energization of planetary pickup ions in the solar system, J. Geophys. Res. Planets, 119, 219–236 (open access) or https://doi.org/10.1063/1.3529973, https://doi.org/10.1063/1.4723595 etc. (these two later papers are not open, though).

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  • $\begingroup$ The usual ionization mechanism is solar photons, usually UV. Indeed most of the molecules or atoms that get picked up come from an atmosphere, but some can come from surfaces too. Mechanisms like sputtering can eject individual atoms or molecules from a surface. $\endgroup$ – Tom Spilker May 1 '18 at 19:59

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