The Vivid Lights: What Causes the Colour of the Aurora?

Traditionally, Earth’s aurora are well known for appearing as a bright green glow across the night sky. Whilst green is the most common colour we see, it’s certainly not the only one! In this article, we talk about the different colours you might be able to catch if you see the aurora, as well as some of the science behind them.

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Green and purple aurora as seen from Scotland. Photo: ShinyPhotoScotland via Flickr. CC BY-NC-ND 2.0

It’s all in the Atom

Auroral origins lie in space, within what we call the solar wind. The solar wind is a continuous stream of charged particles which comes directly from the sun. The solar wind interacts with Earth’s magnetic field and, eventually, results in charged particles entering our atmosphere close to the north and south poles. Here, these charged particles interact with gases in our atmosphere and cause the lights we see.

The process of light emission happens right at the atomic level. Electrons can be thought of as orbiting around the centre of the atom (the nucleus) in what are known as energy shells. You can think of them as multiple ‘layers’, where higher energy electrons will have a larger orbit. When an electron in an atom comes into contact with a charged particle originating from the solar wind, it may ‘jump’ up to a higher excited energy level. But electrons always want to be in the lowest energy shell possible, meaning at some point it will drop back to its original energy.

A diagram of the emission of a photon from electron relaxation to a lower energy state.

Where does this extra energy go? It gets released from the atom in the form of a photon of light. The wavelength, and therefore the colour, of the photon depends on how much energy the electron gets rid of. Different gases will move electrons to different energy levels than others, meaning the wavelength and colour of the light emitted from them will also be different. This all occurs in a region at the top of the atmosphere about 100-600km high called the Ionosphere.

Let’s take a closer look at some of these and their associated colours:

Green – Oxygen

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Green aurora as seen from Ballykelly, Northern Ireland. Photo: John Purvis via Flickr. CC BY-NC-SA 2.0

Starting with the most prominent of colours, the green aurora comes from the oxygen in our atmosphere. The oxygen green is emitted with a wavelength of 557.7nm (about 0.5 millionths of a metre) and occurs mostly around 100km altitude where the emissions last a very short amount of time (about 0.7 seconds).

Red – Nitrogen & Oxygen

Red occurs due to the collisions of the incoming charged particles with nitrogen and oxygen high above the green layer. The light is emitted at wavelengths of 630nm and 658nm respectively. The lifetimes of these excited states, i.e. the time between the electron in the atom gaining energy and then releasing it as a photon, are much longer than the green oxygen emission at over 200 seconds each.

Purple – Nitrogen

At high altitudes (400km+) where there’s not as much oxygen, emissions due to ionic nitrogen (nitrogen atoms which are missing an electron) dominate at wavelengths of 427.8nm making a gentle purple colour. During extremely strong aurora, it is also possible to catch purple underneath the main green band, but this does not happen as often because that emission is from the very stable molecular nitrogen (two nitrogen atoms bonded together).

Other colours

There are lots of different energy levels in the various gases that make up our atmosphere and many of these can produce visible light. However, the ones we have listed are by far the most common. Some other colours you might spot are actually a mix of several other colours which can create a truly dazzling display.

It’s also important to remember that light pollution can create some odd colours too. For example, the aurora are generally not yellow or orange but the light emitted by sodium street lamps is!

Other Planets

Earth is not the only planet with aurora! Other planets also have magnetic field, meaning they get some lights too. Because they have different atmospheres, we expect the colours to be different, and they are!

UV false colour aurora at Jupiters north pole taken by the Hubble telescope. Image: Nasa, ESA, and J. Nichols (University of Leicester).

Jupiter’s hydrogen rich atmosphere creates a dense and highly structured ultraviolet aurora which is not visible by eye. It can also also produce infra-red (again not visible by eye) and visible red emissions.

False colour UV image of aurora on both of Saturn’s poles, captured by the Hubble telescope. Image: NASA.

Saturn also has ultraviolet aurora because of hydrogen emissions, but there has also been evidence for a visible pink emission.

For more information about auroras on other planets, take a look at this article.

Have you seen any other colours in the aurora? We would love to see them over on our Flickr group. You can also join us on Facebook and Twitter.