A very useful article by Dr. Robert Berdan. One thing not mentioned that might be of critical importance, however, is NOT to use any type of filter when taking exposures of the northern lights.
I shot some northern lights two nights ago, and was alarmed to find every image had a series of perfect, concentric rings dead center of every shot. I thought that it must be a sensor malfunction, because the rings were so perfect as to appear to be man made artifacts. They appeared in every image.
A Google search revealed that use of any filter, even a clear skylight filter (which I was using) results in a very unusual phenomenon wherein the specific light from the spectral green of the northern lights creates a light refraction between the lens surface and the fliter, creating a light beam bounce as though the two surfaces were mirrors in parallel, creating the image of concentric rings through the middle of your hear-earned timed exposures.
I was lucky, my shots were taken about an hour from home, but if you laid on big money for a northern trip and came home to find these rings in all your shots (on the rear display screen of my Canon 7D it seemed like the plastic of the display screen was going a wee bit of an odd reflection; in 6 years of shooting digital, I've never seen anything like it.
Borrowing from the outstanding Alaska Photograhy Blog: