Alex, I live in Quebec, Canada and tried to shoot the ones here on Tuesday night, but did not have a tripod with me so my images are blurry. I am not a regular Aurora shooter as I can't stay out in the cold too long but I tried again on Wednesday with a tripod but they just wouldn't come out to play. :-P
Yours looks great. As for what to do, you are doing it right! Depending on how subtle they are, you can go anywhere from ISO400-to ISO1600 but you need to be aware of noise. Newer cameras do better than older ones, obviously.
Shoot pretty open.. but perhaps not open completely to 2.0 ... stop it down to 2.8 - 4.0 so you get better sharpness. If your tripod is not very sturdy, there is usually a hook you can hang something heavy on or place something heavy on the braces between the legs.
As for how long, anywhere from 4 seconds to 30 is pretty normal; it will always depend on how bright the display is.
All in all, you did everything right and I think your photo is very beautiful! Congrats!
:-)
Maggie