What determines aurora colors?
The light from an aurora comes from photons emitted by electrons in atoms of the upper atmosphere which have been bumped up to excited levels by collisions with the solar particles. These excited electrons then emit photons when they jump back down to lower levels. Each gas emits a spectrum of characteristic colors; so the color depends on the type of gas, and the typical de-excitation energy. Excited oxygen tends to make red, green or yellow light; nitrogen can produce a reddish light.
