The northern lights in Norse sources
You will often read that the Norse saw the northern lights as light glinting off the armour of the Valkyries. It is a fine image, and it dates from 1855. In The Age of Fable, Thomas Bulfinch wrote that when the Valkyries ride out, their armour sheds ‘a strange flickering light’ that flashes up over the northern skies, ‘making what men call the Aurora Borealis’. His own footnote gives his source: Thomas Gray’s ode The Fatal Sisters, written in 1768. No Old Norse text links the Valkyries with the aurora.
The nearest passage
The closest thing in the Eddas is a scene in the first lay of Helgi Hundingsbane. Light glitters from a mountain called Logafjoll, flashes leap, and helmeted women ride ‘high under helms on heaven’s field’, their mail red with blood, ‘and from their spears the sparks flew forth’. It is a portent of battle. The northern lights are not mentioned.
The King’s Mirror
The oldest Norse discussion of the northern lights is in the King’s Mirror, Konungs skuggsjá, a book of instruction written in Norway around 1250 as a conversation between a father and his son. The son asks about the lights that the Greenlanders call norðrljós. The father begins honestly: he has no clear knowledge of them.
Then he describes them closely. They are brightest on the darkest nights and rarely seen when the moon is up. Rays dart up like flames, bright enough to hunt by, and sometimes the light dims ‘as if a black smoke’ were blown up among the rays. Anyone who has stood under an aurora will recognise it.
He offers three explanations. Perhaps fires encircle the outer ocean at the edge of the world. Perhaps it is the sun, shining up from below the rim of the earth. Or perhaps frost and glaciers in the north have grown so powerful that they radiate these flames themselves; this third one seems to him the most likely. None of the three involves a god.
What that leaves
So the one medieval Norse voice on the northern lights is a careful observer who admits he does not know, and tries to reason it out. That is perhaps less dramatic than the Valkyries, and rather more interesting.