There’s a new theory about how STEVE, everybody’s favorite aurora-like phenomenon, gets its otherworldly glow. 

When STEVE was first observed by amateur aurora spotters in 2018, researchers thought it was a new form of aurora borealis, otherwise known as the northern lights, but a deeper look revealed that this celestial phenomenon is actually dominated by white light, indicating it must produce light in a different way to typical aurorae. How exactly it pumps out this light, however, remained a bit of mystery.

Now, scientists have got a new theory. Reporting in the journal Geophysical Research Letters, scientists from the University of California Berkeley say the light might be generated by fast-flowing rivers of solar wind sparking chemical reactions high up in Earth’s atmosphere. 

Your typical aurora borealis produces swirling ribbons of purple, mauve, and green light in the night sky through the solar wind, a high energy gas of charged particles and electrons emitted by the Sun, smashing into Earth’s magnetosphere. The charged gas transfers much of its energy into the oxygen and nitrogen molecules found in the atmosphere, exciting them and making them release a burst of energy in the form of light.

However, STEVE seems to be a slightly different story. Earth’s atmosphere is mostly made of nitrogen and oxygen gas that are bound together by pairs of nitrogen and oxygen atoms (N₂ and O₂), but in the upper atmosphere where STEVE occurs, single atoms of oxygen are often found (O).

When fast-moving streams of solar wind hit the upper atmosphere, they can split nitrogen molecules (N₂), which then combine with single oxygen atoms to form nitric oxide (NO). The nitric oxide then picks up other free oxygen atoms floating around the upper atmosphere to create nitrogen dioxide (NO₂), a chemical reaction that produces a broad spectrum light creating a white-purple glow.

This, for now, is just a theory. Although the chemistry appears to line up nicely with observations of STEVE, the researchers concede they have not tested their idea in the real world. With that in mind, they say they're perfectly happy to be proven wrong about their theory. 

“It’s just exciting for me to find something where, you could ask a very simple question about it, like, ‘What is this?’ And it turns out the answer to that question is very nuanced and exciting and may indicate some new physics,” lead author Dr Brian Harding, a space physicist at the University of California Berkeley, said in a statement. 

"It would be more exciting if this were wrong, then we're back at square one, and nature had confounded us again," added Harding.