Reading through "Simulating the Aurora borealis" by Baranoski et al. provided me with a good grounding for what is physically happening with the Aurora borealis. The causes of the beautiful colors are well known. What interested me is exactly what colors we observe.
The researchers provide light wave lengths that they say are caused by the excitation of atomic and molecular oxygen and nitrogen. They talk about an atomic oxygen "green line" at 557.7 nm, an atomic oxygen "red line" at 630.0 nm, and an ionized nitrogen "blue band" at 427.8 nm. Using an online conversion applet, I was able to obtain approximate RGB colors for experimentation and comparison with actual photographs of Aurora borealis.
Aurora borealis colors seem to follow general patterns. Seeing a green band low in the sky blend into red as one looks up is not uncommon. Green blending into blue is fairly common. Straight blue auroras are also fairly common. I did find one photograph that was mostly red, and I have seen red and blue together (though they appeared to be different bands, rather than one band of color blending with another color).
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