I visited the Arctic several times when writing my book, Aurora, and several times since, so I have a reasonable idea of what kind of clothes you will need for a trip to the Arctic. Of course, it depends what kind of activity you are doing. If you are skiing, for example, you will get warm. But if you are standing around for hours watching the northern lights you will definitely get cold!
Northern Lights Clothing Guide
To help you prepare for a northern lights trip, I wrote a Northern Lights Clothing Guide blog for Cotswold Outdoor.