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What exactly is the cause of blue light of ion (plasma) tails of comets? Somewhere I have read that the source of blue light are CO+ ions which has just acquired the missing electron and became neutral molecule CO. Is this correct?

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Close, but not quite right - the blue light is indeed emission from CO$^+$, but it's from the CO$^+$ ions themselves, with no need for recombination to CO; that (ionized) molecule has a strong set of energy transitions around 425 nm (4250 Angstroms), which is in the blue part of the visible spectrum:

Spectrum of Comet C/2016 R2 (Pan-STARRS)

Spectrum of Comet C/2016 R2 (Pan-STARRS), Figure 2 from Cochran and McKay (2018).

There's more on the physics of comet tails from Chris Mihos here.

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  • $\begingroup$ Very nice answer! +1 The spectrum shown is only about 1.5 nm wide, so it's hard to tell if this represents most of the "blue light" from the tail or just a small fraction of it, but this sounds authoritative enough to be believable. This also finally explains to me why the tails of green comets (C₂ vibronic lines, 1, 2, 3) look blue or violet. Thanks! $\endgroup$ – uhoh Jul 13 at 2:22
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    $\begingroup$ Right, this is only a zoom in on one particular band. In the paper they say that they detected “a well-developed series of bands scattered from approximately 3700 Å to 5100 Å” and that “The majority of the detected strong bands can be attributed to CO+.” This is just one of many bands they list. I don’t know how much variation there is across different comets, but in this case CO+ seems to be the dominant source of emission. $\endgroup$ – ELNJ Jul 13 at 3:24
  • $\begingroup$ Interesting, thanks! $\endgroup$ – uhoh Jul 13 at 9:09
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    $\begingroup$ Do you know if the blue color of this emission is in any way related to blue color of CO flame? I understand that during combustion CO change to quite different molecule, CO2, but after all, both molecules contain C-O bonds... $\endgroup$ – Leos Ondra Jul 13 at 12:15
  • $\begingroup$ NEOWISE has a spectrum too :) groups.io/g/cometes/message/410 $\endgroup$ – pela Jul 13 at 12:33

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