Colour

(Section 3.2.4.3.2)

The colour of the polar aurorae is dependent on the particular atmospheric gas that emits the light, its electrical state and the energy of the solar particles. The brightest and most common auroral light is white with a greenish or greenish-yellow tinge, emitted by atomic oxygen at about 100 km above the ground. At an altitude of about 150 km, single oxygen atoms produce a diffuse red glow. Pinkish light on the lower fringes of arcs and bands comes from atomic nitrogen as low as about 60 km. Molecular nitrogen emits bluish-purple light at the highest levels.