Sweden is a northern country. It has significant territory inside the Arctic Circle, and aurora can often be seen in winter.
Aurora is caused by the interaction between Earth’s magnetic field, atmosphere, and solar wind. Aurora borealis is the aurora of the north; it usually appears at latitudes of around 70-80 degrees north, although occasional geomagnetic storms expand this area south, in which case the aurora may be seen in much of Sweden, including Stockholm.
The area around Kiruna (67.8 degrees north) in the far north of Sweden is a good place to watch aurora. On a clear winter night with strong solar activity, splendid auroras can be observed. You can join one of the tours that will bring you to a good observation spot and keep you refreshed.