Editing Aurora (section)

=== Atmosphere ===
Auroras result from emissions of [[photon]]s in Earth's upper [[Earth's atmosphere|atmosphere]], above {{convert|80|km|mi|abbr=on}}, from [[ionized]] [[nitrogen]] atoms regaining an electron, and [[oxygen]] atoms and [[nitrogen]]-based molecules returning from an [[excited state]] to the [[ground state]].<ref>{{cite web|title=Ultraviolet Waves|url=http://missionscience.nasa.gov/ems/10_ultravioletwaves.html|url-status=dead|archive-url=https://web.archive.org/web/20110127004149/http://missionscience.nasa.gov/ems/10_ultravioletwaves.html|archive-date=27 January 2011}}</ref> They are ionized or excited by the collision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost within the atmosphere by the emission of a photon, or by collision with another atom or molecule:
;[[Oxygen]] emissions: green or orange-red, depending on the amount of energy absorbed.
;[[Nitrogen]] emissions:blue, purple, or red; blue and purple if the molecule 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 0.7 seconds to emit the 557.7&nbsp;nm green light and up to two minutes for the red 630.0&nbsp;nm emission. Collisions with other atoms or molecules absorb the excitation energy and prevent emission; this process is called [[Quenching (fluorescence)|collisional quenching]]. Because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities, such collisions are rare enough to allow time for oxygen to emit red light. Collisions become more frequent progressing down into the atmosphere due to increasing density, so red emissions do not have time to happen, and eventually, even green light emissions are prevented.

The change in auroral colour with altitude is therefore explained—oxygen red is predominant at high altitudes, followed by oxygen green and nitrogen blue/purple/red, then finally other hues of nitrogen blue/purple/red where particle collisions prevent oxygen from emissions. Green is the most common colour. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue.

Precipitating protons generally produce optical emissions as incident [[hydrogen]] atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes.<ref>{{cite web|url=http://auspace.athabascau.ca/handle/2149/518|title=Simultaneous ground and satellite observations of an isolated proton arc at sub-auroral latitudes|publisher=Journal of Geophysical Research|date=2007|access-date=5 August 2015|archive-date=5 August 2015|archive-url=https://web.archive.org/web/20150805154623/http://auspace.athabascau.ca/handle/2149/518|url-status=live}}</ref>