# Why are red auroras rare? Why do oxygen atoms (not oxygen molecules) cause auroras while molecular nitrogen cause auroras instead of atomic nitrogen?

Firstly, since there are a lot of oxygen atoms at a height of 200km, and less energy is needed to produce a red aurora than a green aurora, so it should be easier to produce red auroras than green auroras and we should see more red auroras than green auroras, but we see more green aurora in reality. Why are red auroras rare? Is it true that red auroras need 200s for the 630 nm transition?

Secondly, why do oxygen atoms (not oxygen molecules) cause auroras while molecular nitrogen cause auroras instead of atomic nitrogen?

• The first aurora I saw in the southern hemisphere was a red aurora. However, I haven't seen many auroras down here.
– Fred
Sep 22, 2021 at 17:04
• Don't forget that human eyes are more sensitive to green than to red, and in low-light conditions even more to cyan than to green. So red auroras might be frequent but not bright enough to see. Sep 23, 2021 at 13:23
• To Fred, where are you located? Sep 28, 2021 at 2:52
• To Ruslan, Why are we more sensitive to green light when 65% of our cone cells are red cone cells? Sep 28, 2021 at 2:53
• Atoms or molecules of anything up there cause auroras. The question is, which sources give results that land in our visible frequency range? Oct 5, 2021 at 16:24

In fact multiple species of oxygen or nitrogen can create emissions at various wavelengths. There can be aurorae over a wide range of wavelengths, we just see only some of them that happen to be in the visible range. So, it's all a matter of which species hit the relatively narrow visible target.

With respect to oxygen, Rezaei et al. 1 provide this spectrum, in which atomic oxygen produces an emission near the red end of the visible range, while ions of both monatomic and diatomic oxygen give less intense emissions of various colors from blue to orange. Note that no emission is listed for neutral $$\text{O}_2$$.

With nitrogen, similar results from Hosseini et al. 2 show an emission in the violet or blue range from ionized diatomic nitrogen, but none from any monatomuc nitrogen species. Note that in this spectrum the red eemission from atomic oxygen is seen, apparently from a small amount of this imputity in the gas sample.

References

1. Rezaei, Fatemeh & Abbasi-Firouzjah, Marzieh & Shokri, Babak (2014). "Investigation of antibacterial and wettability behaviours of plasma-modified PMMA films for application in ophthalmology", Journal of Physics D Applied Physics, 47, 085401. https://doi.org/10.1088/0022-3727/47/8/085401.

2. Hosseini, Seyed Iman & Mohsenimehr, Soad & Hadian, Javad & Ghorbanpour, Mansour & Shokri, Babak (2018). "Physico-chemical induced modification of seed germination and early development in artichoke ( Cynara scolymus L.) using low energy plasma technology", Physics of Plasmas, 25, 013525. https://doi.org/10.1063/1.5016037.

• How are your two answers different? Why not just edit the first answer if you wanted to add, remove or clarify something?
– Fred
Mar 1 at 2:18
• Sorry, it was an accident. Will delete the other one. Mar 1 at 2:26