Aurora Borealis (northern lights) over the forests of the Kananaskis Country on 09/07/2012 in very early hours. The long filament of red and green colours lasted for about 45 mins and it stretched from constellation of Big Dipper to Lyra (Alberta, Canada).
Auroras 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, or by collision with another atom or molecule:
- oxygen emissions
green or brownish-red, depending on the amount of energy absorbed
- nitrogen emissions
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