we're doing a northern lights cruise mid february, purely coincidental that it happens to be a good year for the lights this year as we didn't know when we booked. we have to fly to somewhere in norway to pick up the ship and have a fair few trips ashore booked, so have been wondering much the same myself, about what to wear. think we're going to go for layers which we can take off if/when we get too warm rather than stuff the suitcases full. we both have a padded down jacket thing which we'll be wearing too, along with our walking trainers (not taking walking boots). so for the flight i reckon it'll be jeans, t-shirt, brushed shirt, thin fleece, jacket. lots of thermals, thick socks and gloves in the suitcase.
hope you have a great time and get at least a glimpse of the lights :)