The Hessdalen lights

Lights reported over a 12-kilometre section of Norway's Hessdalen valley, in the rural centre of the country. Sightings peaked between 1981 and 1985.

Wikipedia says lights have been seen in the Hessdalen valley at intervals from the 1930s at the latest. Life in Norway, a Norway guide site, dates the first sighting of the modern lights to 8 December 1981, when Ruth Marry and Åge Moe watched from their kitchen window what they called a ball of fire in the evening sky.

Project Hessdalen, based at Østfold University College, says sightings rose sharply from 1981 to 1985, reaching up to 20 a week. It now logs about 20 a year and says the count may be much higher.

In 2012 Gerson Paiva and Carlton Taft proposed a model in the journal Meteorology and Atmospheric Physics. They argued that the lights' odd spectrum, flat on top with steep sides, fits bremsstrahlung radiation from a dense ionized gas, shaped by its optical thickness.