[chris (OP)]

The Northern Lights are usually green in photographs. Why is that? Why are they not other colours too?

Chris

[paul.fr]

I suspect that there are a multitude or answers, film type, exposure, shutter speed, and the list probibly goes on.

If i may be the first to have his idea shot to pieces. How about that the usual green as you mention, and often red colours are to do with the amount of Oxygen and Nitrogen in the atmosphere. Now i have gone and stumped myself as to why ....oh, forget i said anything.

[eric l]

Apparently, the green flares are emissions at 557.7 nm by atomic oxygen.
see :  http://en.wikipedia.org/wiki/Aurora_borealis - sub :  Auroral mechanism
and
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(from http://www.natuurwetenschappen.nl/modules.php?file=article&name=News&sid=1049)

[G-1 Theory]

The Northern Lights are usually green in photographs. Why is that? Why are they not other colours too?

Chris

Dear Chris

The mostly green color is from the amount of Oxygen in the atmosphere.

Other elements would have different color emissions. And at times there are many colors up there.

Ed