This article accompanies the fable
Or, it's the Aurora Borealis. Auroras are produced when the solar wind collides with the Earth's magnetic field and the energy released forms auroras around the poles. Below is Wikipedia's Photo of the Year in 2006, taken in Alaska.
Or the Bifröst Bridge is the Milky Way. Or it's a rainbow over Bergen:
Rainbows in Scandinavia lead as often as not into the clouds, which would seem to be the right place for Asgard. Photo: