For standing around watching aurora displays (which hopefully you will be) you’ll need to layer up. Some lodges provide special all-in-one oversuits to wear. If not, or if you’re going it alone, you’ll need base layers, a good mid-layer fleece, and a down jacket. My biggest treat, if you’re not given an oversuit, is down trousers (synthetic insulated trousers also work). I love them. In combination with a down jacket it’s like walking around in your sleeping bag. Perfect!
You may also want to consider bringing a balaclava or facemask if you’ll be standing around aurora watching or photographing for a long time, or skiing in low temperatures. For skiing, you’ll also need good windproofs.