The aurora borealis, also known as the 'northern lights', is one of the most spectacular displays in the night sky. What is the science behind these curtains of light?

The sight filled the northern sky; the immensity of it was scarcely conceivable. As if from Heaven itself, great curtains of delicate light hung and trembled. Pale green and rose-pink, and as transparent as the most fragile fabric, and at the bottom edge a profound and fiery crimson like the fires of Hell, they swung and shimmered loosely with more grace than the most skilful dancer. Lyra thought she could even hear them: a vast distant whispering swish.

Phillip Pullman, His Dark Materials

What is the aurora?

The aurora can be seen near the poles of both the northern and southern hemisphere. In the north the display is known as the aurora borealis; in the south it is called the aurora australis.

These 'northern' and 'southern lights' have fascinated, frightened and inspired humans for centuries. More recently, photographers have gone to remarkable lengths to try and capture the beauty of these atmospheric events.

See spectacular aurora photography at the Astronomy Photographer of the Year exhibition

What causes the aurora borealis or 'northern lights'?

The lights we see in the night sky are in actual fact caused by activity on the surface of the Sun.

Solar storms on our star's surface give out huge clouds of electrically charged particles. These particles can travel millions of miles, and some may eventually collide with the Earth.

Most of these particles are deflected away, but some become captured in the Earth’s magnetic field, accelerating down towards the north and south poles into the atmosphere. This is why aurora activity is concentrated at the magnetic poles.

“These particles then slam into atoms and molecules in the Earth’s atmosphere and essentially heat them up,” explains Royal Observatory astronomer Tom Kerss. “We call this physical process ‘excitation’, but it’s very much like heating a gas and making it glow.”

What we are seeing therefore are atoms and molecules in our atmosphere colliding with particles from the Sun. The aurora's characteristic wavy patterns and 'curtains' of light are caused by the lines of force in the Earth’s magnetic field.

The lowest part of an aurora is typically around 80 miles above the Earth's surface. However, the top of a display may extend several thousand miles above the Earth.

What causes the different colours in the aurora?

Aurora Australis from Beerbarrel Beach by James Stone, Insight Investment Astronomy Photographer of the Year 2019
Aurora Australis from Beerbarrel Beach by James Stone, Astronomy Photographer of the Year 2019

Different gases give off different colours when they are heated. The same process is also taking place in the aurora.

The two primary gases in the Earth’s atmosphere are nitrogen and oxygen, and these elements give off different colours during an aurora display.

The green we see in the aurora is characteristic of oxygen, while hints of purple, blue or pink are caused by nitrogen.

“We sometimes see a wonderful scarlet red colour, and this is caused by very high altitude oxygen interacting with solar particles,” adds astronomer Tom. “This only occurs when the aurora is particularly energetic.”

How to photograph the aurora

Is the aurora borealis visible in the UK?

The aurora borealis can be seen in the northern hemisphere, while the aurora australis is found in the southern hemisphere.

Polar lights by Grigorii Paramonov, Astronomy Photographer of the Year 2019

While the best places to see the aurora are concentrated around the polar regions, the aurora borealis can sometimes be seen in the UK. The further north you are the more likely you are to see the display, but in the past the northern lights have been seen as far south as Cornwall and Kent.

Lancaster University's Department of Physics runs a website called AuroraWatch UK, which estimates the likelihood of an aurora being visible based on geomagnetic activity. Follow the team's Twitter account to see the latest UK alerts.

The conditions do still need to be right however. Dark and clear nights, preferably with little light pollution, offer the best chance of seeing the aurora.

Do other planets have aurorae?

Any planet with an atmosphere and magnetic field is likely to have aurorae. Scientists have captured incredible images of aurorae on Jupiter, Saturn, Uranus and Neptune.

Aurora on Uranus taken by Hubble telescope
The aurora on Uranus, captured by the Hubble Space Telescope (ESA/Hubble & NASA, L. Lamy / Observatoire de Paris)

Aurorae on Mars have also been seen, but as the 'red planet' does not have a global magnetic field, aurorae behave differently and appear to be far more widespread.

What are solar flares and how do they affect the aurora?

The aurora is a very dramatic example of the ways in which solar activity affects the Earth.

Solar flares are like enormous explosions on the surface of the Sun in which streams of charged particles are emitted into space. It typically takes two days after the flare is seen on the Sun for the particles to reach Earth. Upon their arrival, these particles can result in aurora activity.

Out on a Limb by Alastair Woodward, Astronomy Photographer of the Year 2019

What are geomagnetic storms?

Intense aurora displays are generated following massive explosions on the Sun known as 'coronal mass ejections'. These explosions release clouds of hot plasma containing billions of tons of material travelling at around two million miles per hour. When the clouds reach the Earth, they interact with the Earth's magnetic field to cause events called geomagnetic storms.

The Sun's activity fluctuates, with activity reaching a peak every 11 years. The last time solar activity peaked was in 2014, and the cycle is now reaching its minimum. However, solar activity is predicted to rise again through to the mid-2020s.

Regardless of the Sun's activity, aurorae can still occur at any time and observers in high latitudes should always look out for them.

Main image: Goðafoss Flow by Larryn Rae, Astronomy Photographer of the Year 2021