Don't be afraid to experiment with your flash when shooting the northern lights. While the flash won't affect the aurora, it will provide definition to your foreground objects and can help freeze the action of a person or animal in the frame so they aren't so blurred. You still want long exposures for the northern lights to read in your camera, but a quick flash at the start of the long exposure can provide nice foreground highlights. Since I live in Southeast Alaska, I sometimes shoot photos of the northern lights over a harbor, and the flash adds some details to the boats (and eliminates some of the blur you get when the boats bounce in the waves).