<p>Close, but not quite right - the blue light is indeed emission from CO<span class="math-container">$^+$</span>, but it's from the CO<span class="math-container">$^+$</span> 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:</p>
<p><a href="https://i.sstatic.net/54Zqr.jpg" rel="noreferrer"><img src="https://i.sstatic.net/54Zqr.jpg" alt="Spectrum of Comet C/2016 R2 (Pan-STARRS)" /></a></p>
<p>Spectrum of Comet C/2016 R2 (Pan-STARRS), Figure 2 from <a href="https://ui.adsabs.harvard.edu/abs/2018ApJ...854L..10C/abstract" rel="noreferrer">Cochran and McKay (2018).</a></p>
<p>There's more on the physics of comet tails from Chris Mihos <a href="http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/Comets/tails.html" rel="noreferrer">here</a>.</p>

CC BY-SA 4.0
2020-07-13T02:22:24.743Z
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](https://electronics.stackexchange.com/a/414244/102305), [2](https://astronomy.stackexchange.com/q/17811/7982), [3](https://space.stackexchange.com/a/36106/12102)) look blue or violet. *Thanks!*
uhoh
CC BY-SA 4.0
2020-07-13T03:24:09.400Z
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.
Eric Jensen
CC BY-SA 4.0
2020-07-13T09:09:24.870Z
Interesting, *thanks!*
uhoh
CC BY-SA 4.0
2020-07-13T12:15:50.580Z
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...
Leos Ondra
CC BY-SA 4.0
2020-07-13T12:33:22.277Z
NEOWISE has a spectrum too :) https://groups.io/g/cometes/message/410
pela
CC BY-SA 4.0
2020-07-13T12:53:13.397Z
Neat!  Translating to English with Google, part of the text says “I have identified a few groups of lines, in the photos of amateurs we can clearly see an ionic tail of blue color, on the spectrum, I do not see the lines characteristic of CO+.”  This could partly depend on how bright the ion tail is relative to the rest of the comet’s emission.
Eric Jensen
CC BY-SA 4.0
2020-07-12T23:52:45.010Z
2020-07-12T23:52:45.010Z
Eric Jensen
<p>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?</p>

CC BY-SA 4.0
2021-12-25T17:58:22.713Z
To understand Eric's excellent answer, please understand that the molecule will have its excitation states...
Sean
<comets><spectroscopy><plasma-physics>
Blue color of ion (plasma) comet tails
Leos Ondra
# Blue color of ion (plasma) comet tails

(Asked by: username_0 on 2020-07-12)

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?

**Tags:** comets, spectroscopy, plasma-physics
> **Comments:**
> **username_1**: To understand Eric's excellent answer, please understand that the molecule will have its excitation states...  

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## 1 answers

### Answer by username_2 on 2020-07-12. Score: 12 | Accepted answer

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)](https://i.sstatic.net/54Zqr.jpg)](https://i.sstatic.net/54Zqr.jpg)

Spectrum of Comet C/2016 R2 (Pan-STARRS), Figure 2 from [Cochran and McKay (2018).](https://ui.adsabs.harvard.edu/abs/2018ApJ...854L..10C/abstract)

There's more on the physics of comet tails from Chris Mihos [here](http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/Comets/tails.html).

> **Comments:**
> **username_3**: 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](https://electronics.stackexchange.com/a/414244/102305), [2](https://astronomy.stackexchange.com/q/17811/7982), [3](https://space.stackexchange.com/a/36106/12102)) look blue or violet. *Thanks!*  
> **username_2**: 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.  
> **username_3**: Interesting, *thanks!*  
> **username_0**: 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...  
> **username_4**: NEOWISE has a spectrum too :) https://groups.io/g/cometes/message/410  
> **username_2**: Neat!  Translating to English with Google, part of the text says “I have identified a few groups of lines, in the photos of amateurs we can clearly see an ionic tail of blue color, on the spectrum, I do not see the lines characteristic of CO+.”  This could partly depend on how bright the ion tail is relative to the rest of the comet’s emission.  

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