Aurora Borealis
From Srafopedia
The Northern Lights.
Aurora in the Boreal (Beyond the north limit of tree growth) that makes the Stratosphere glow in intense colours of all shades (e.g. green, rose and crimson). The aurora is created by electrons caught in the Earth's magnetic field (which is concentrated over the North and South poles) colliding with atoms of the upper atmosphere, causing them to excite and emit energy, typically at heights of 80-150 kilometres. The majority of light given off appears green due to the emission of atomic oxygen, although atomic nitrogen is also present, which grows and fades faster, showing the dynamic nature of the Aurora.
Relevance to His Dark Materials
Lord Asriel is able to take a photogram of a world beyond the Aurora Borealis because the latter acts as a gap that lets photons (light particles) pass between the universes. When the Aurora Borealis slide is shown with the projector through an “emulsion treatment”, a “City in the Sky” is seen, giving evidence of the existence of other worlds.
[ TGC/NL, Chapter 2]
Lyra at first calls the Aurora Borealis the "Roarer."
