Colors of the Aurora
Oxygen comprises 21% of the earth's atmospheric volume. At altitudes of 200-250 km, however, it reacts with the sun's particles and creates a red color. Note the red color that lines the top edge of the aurora in the following photo. This is oxygen reacting with the particles at an altitude recorded by the Space Shuttle Discovery of 250 km.
Taken by the Space Shuttle Discovery on April 29, 1991.
Other colors and the gases they represent:
Nitrogen, which is 78% of the earth's atmosphere, is responsible for the bright pink (and sometimes violet) color of the aurora at altitudes of approximately 100 km.
Hydrogen and Helium also react to emit blue and purple light. The light emitted is seen with difficulty by the human eye because there are only traces of these molecules and is best captured with highly sensitive slide film.
Neon is responsible for the rare orange flares.