The aurora can come in many different colours. From stunning greens to deep reds and velvety purples, the aurora never fails to amaze. But have you ever seen a golden aurora?

A “golden aurora” captured on 16 March 2016 Andy Stables from the Isle of Skye, Scotland. All rights reserved. Used with permission.

In the photo above, captured by Andy Stables on 16th March 2016 from the Isle of Skye (Scotland), there’s a wonderful golden band of light beneath the more familiar green. From Andy’s remote location overlooking the sea, we know it’s not light pollution and Andy described the light as an “arc moving across the horizon”. All signs point to aurora.

Spotting this “golden aurora” is actually quite a rare thing – especially from here in the UK. So what caused it?

Additive mixing

Additive colour mixing (R,G,B).

Gold is not a “pure” colour, it is a combination of two colours added together. When sources of green light and red light are mixed together, they produce yellow. Varying the amount of each of the colours being mixed produces a whole range of different colours and shades. This is how LED TVs, which only have three coloured LEDs (red, green, and blue) per pixel, are able to produce an array of colours.

There are many websites, including this one for example, that let you have a go at mixing different colours at various concentrations if you want to try this process out for yourself. You’ll note that the results are different than from mixing paint. This is because paint absorbs colour rather than emitting it and so the process is known as subtractive colour mixing.

Our gold aurora is in fact the result of additive mixing of a red/magenta aurora and the green aurora. But where does that magenta aurora come from?

Molecular nitrogen

The thing that makes gold auroras quite rare is that they require a red aurora emission at the same altitude as the green aurora emission. Normally we associate red aurora with high-altitude oxygen (150-200km) but this is too high in the sky to mix with the green auroral light (100-150km).

Atomic oxygen emission spectrum (red and green). Image credit: NCAR/HAO/Tom Eklund.

During particularly strong aurora, though, the energetic particles that collide with atmospheric gases and produce the auroral light have more energy than usual. This allows some of them to reach lower into the atmosphere (150km or lower). At this altitude, the atmosphere is made up mostly of molecular nitrogen (N2) and excited molecular nitrogen emits a variety of colours including dark reds and purples.

The emission spectrum of molecular nitrogen. Image credit: NCAR/HAO

It is the additive mixing of the red molecular nitrogen and green atomic oxygen that produces the yellowy-gold colour we see in Andy’s photo. Here is another example of this mixing going on:

Additive mixing of atomic oxygen green with molecular nitrogen red producing a yellowy-gold light. Image credit: Tom Eklund

So why don’t we see the red band in Andy’s photo? Well if you look hard enough you might notice a slightly more orange tinge. It seems as though this is probably the red band but with a little green mixed in. This is possibly just down to the viewing angle of the aurora from Skye.

So there we have it: the “golden aurora”: gold, but not really!

Have you spotted the golden aurora before? If so, why not leave us a comment with a photo? 🙂