How come the Aurora Borealis so green???
The green color is due to light emitted at a wavelength of 557.7 nanometers by oxygen in its atomic form – i.e. existing as single atoms rather than as O2 molecules. The reason that this particular optical emission is so bright (relative to the others) is complicated. It is partly because there is a lot of atomic oxygen in the upper atmosphere. But it is also due to the quantum mechanics of the possible atomic transitions – this transition turns out to be the one that is most likely to occur.
