The colors of the aurora borealis are layered with violet at the bottom, green in the middle, and red at the top, which signifies the structure of the atmosphere. Just as different chemicals create different colored flames (e.g. propane always producing a blue flame etc.), so too do photons bouncing around in the atmosphere off of different elements present. Nitrogen glows violet in a band at the bottom and oxygen glows green and red, creating a curtain 400 kilometers in length