Replies

Auroras are result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule:

oxygen Green or brownish-red, depending on the amount of energy absorbed.

nitrogen Blue or red. Blue if the atom regains an electron after it has been ionized. Red if returning to ground state from an excited state.

Oxygen is unusual in terms of its return to ground state: it can take 3/4 sec to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.

This is why there is a colour differential with altitude; at high altitude oxygen - red - dominates, then oxygen - green - and nitrogen - blue/red - finally nitrogen: blue/red (collisions prevent oxygen from emitting anything).

Green is the most common color of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly pure blue.