Why are auroras different colors?

Last week, a huge solar flare sent a wave of energetic particles from the Sun surging out through space. Over the weekend, the wave reached Earth, and people around the world enjoyed the sight of unusually vivid aurora in both hemispheres.

While the aurora is normally only visible close to the poles, this weekend it was spotted as far south as Hawaii in the northern hemisphere, and as far north as Mackay in the south.

This spectacular spike in auroral activity appears to have ended, but don’t worry if you missed out. The Sun is approaching the peak of its 11-year sunspot cycle, and periods of intense aurora are likely to return over the next year or so.

If you saw the aurora, or any of the photos, you might be wondering what exactly was going on. What makes the glow, and the different colours? The answer is all about atoms, how they get excited – and how they relax.

When electrons meet the atmosphere

The auroras are caused by charged subatomic particles (mostly electrons) smashing into Earth’s atmosphere. These are emitted from the Sun all the time, but there are more during times of greater solar activity.

Most of our atmosphere is protected from the iNFLux of charged particles by Earth’s magnetic field. But near the poles, they can sneak in and wreak havoc.

Earth’s atmosphere is about 20% oxygen and 80% nitrogen, with some trace amounts of other things like water, carbon dioxide (0.04%) and argon.