The picture was taken by the Bäsingen lake outside the town of Avesta, in the county of Dalarna.
It is rare to see the lights so far south in Sweden. Magnetic storms and increased solar activity can make them visible further south, making them visible throughout the country.
The Northern Lights appear when electrons from space (mostly from the sun) are drawn through the atmosphere by the Earth's magnetic fields. An equivalent light is seen at the South Pole as well.