Ask A Physical Chemist: Why Are The Northern Lights Different Colours?

The Northern Lights are one of the most defining aspects of Iceland— as well as the racks of postcards/t-shirts/artwork/etc. you’ll find here. An astute observer might have noticed though that some Northern Lights have a different hue than others. Why is this?

Well, we went straight to physical chemist Dr. Helgi Rafn Hróðmarsson, aka The Cosmic Chemist, to find out the answer to this colourful question:

“The different colors of the Aurora correspond to different particles of light (or photons) that differ in energy. Different colours, different energies. These photons are produced when an atom or a molecule has absorbed energy from an external source and, as a result, scrambled its inner configuration of electrons. With their stability compromised, the atoms and mol- ecules are referred to as being in an ‘excited state.’ To alleviate their newfound disequilibrium, they relax into a more stable configuration by ejecting excess energy in the form of a photon.

“Thus, different de-excitations in different atoms and molecules can produce a variety of different colors. The usual suspect carriers of the Northern Lights’ radiation are nitrogen and oxygen, the atmosphere’s most abundant constituents.

“As an example, excited atomic oxygen can emit both red and green light and molecular (both neutral as well as ionized) nitrogen in an excited state can produce both red and blue light. The energy required for the Northern Lights originates, however, from the sun. Namely, solar winds disturb the Earth’s magnetosphere and hence, a stream of energetic, charged particles is precipitated in the Earth’s upper atmosphere where the excess energy is dissipated via collisions and excitation of the atmosphere’s constituents, nitrogen and oxygen. Thus producing the differently colored Northern Lights.”

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