I like my results when I have shot between 20-24mm. I was surprised that I could record a great Aurora photo at shutter speeds varying between 10 seconds and 30 seconds. If you have a long display it is fun to experiment. With a shorter shutter speed you can capture the lights in a dramatic shape. With a shutter speed above 25 seconds, you will capture more of the colors that are present, the reds and purples in addition to the green. Set the focus scale of your lens just a hair to the right of the infinity mark. ISO is something you can play with also, I have had success between 800-1200. I use remote shutter release with mirror up release mode.

The 20mm will be great for Aurora, and what else you need depends on what else you want to photograph.

December isn't exactly the best time to see the Aurora, but with a little luck you could get something spectacular. The biggest problem will be getting away from the lights of town. You can go down the Parks Highway towards Nenana. That puts you up on a ridge of hills and should be interesting. Another place is Murphy Dome. If you drive out Sheep Creek Rd to the Goldstream valley, Murphy Dome Road goes directly west of Fairbanks to the top of the hills in that direction. It's a nice view of a nicer part of Alaska! Or you can go north of town out the Steese Highway. That direction you can set up to get the Trans Alaska Pipeline into the foreground if you like. Or you can go to Pedro Dome and be up 3000 feet. Those are probably the three best locations, but they are not the only ones. They are also far enough from town that if it is cold (to you) you should find a local that knows what to do if there are any problems. The last thing you want is to be stuck on top of Murphy Dome, all alone