After finding the best place that you can consider seeing the northern lights then comes the other part of what you have to wear. You have to try some of the outdoor tours that will suit the condition because the climatic conditions there are fairly mild and can go up to -30 Celsius in some places. Therefore, there is a need for you to come along with warm and comfortable clothing and pair of shoes. Always the clothes that you consider to go out with should be well layered as to enable you to regulate the temperature conditions.
The layer of your clothing that is next to your skin should contain long sleeves plus long-leg heat underwear with heavy soaks. These cloths should be 100% made from wool or synthetic material with the same quality. The second layer should be a long-sleeved shirt and you can add a jumper too with pair of cotton trousers and a pair of jeans can be fine too to keep you from the cold. The third layer can be thick jumper that is fleece or wool and a pair of lined trouser that should be wind and waterproof plus thick woolen socks. Finally, you can add on top of it a warm jacket for winter plus a hat, gloves and pair of strong winter shoes or boots. You can add a scarf that will help you cover your face when you are there.
If you want to go for photograph or hunting to the aurora borealis then you can prefer to put on light pair of gloves, because your fingers will freeze very quickly when exposed to cold.
Always keep safe and have fun!