What are the northern lights?

What are the northern lights?

The indigenous Inuit people living in and around the Arctic believed that the northern lights were the souls of the dead, playing games with a walrus skull. And in Finnish mythology they were the sparks from the flames of foxes whose tails twinkled with fire as they ran across the Arctic.

But the northern lights, also known as the aurora borealis, are a natural light display caused by the interaction of the Sun’s activity with the Earth’s atmosphere. They also appear around the south pole which are known as the aurora australis.

Aurorae appear as colourful pillars of light or sometimes as a more diffuse glow in the night sky and can be seen with the naked eye. Their intensity and appearance can vary even throughout the span of a night - sometimes barely moving and at other times shimmering like a curtain, before disappearing altogether.

Whilst they’ve been observed by humans throughout history, in modern times they’ve also been viewed from space by astronauts on the International Space Station.

What causes an aurora?

The Sun, being a huge ball of supercharged gas, has a dynamic magnetic field. When its magnetic field lines become twisted on the solar surface (a bit like an elastic band), the strain causes them to suddenly snap and then reconnect.

As this occurs, a large burst of charged particles and radiation is released in a solar flare or as a more powerful coronal mass ejection. When this solar storm reaches the Earth, the charged particles are funnelled by our planet's magnetic field towards the poles of the Earth.

Here the charged particles collide with the molecules in our atmosphere ‘exciting’ them. The molecules of nitrogen and oxygen that primarily make up our atmosphere then release this energy in the form of light – giving us aurorae.

And since aurorae usually occur at higher altitude where the density of molecules is lower, aurorae can be faint. But stronger solar storms that generate a greater amount of charged particles can produce more vibrant aurorae, as there's more energy available to excite the atoms in Earth's atmosphere. 

Can they be predicted?

Alike to earthquakes or volcanic eruptions on Earth, we can’t say exactly when aurorae will appear in the future. But just like tectonic activity, there are signs we can monitor and use to update predictions closer to the occasion.

Because there is a continuous stream of particles ejected form the Sun in the form of the solar wind, aurorae happen every day – but they’re not always easy to see. 

But the Sun goes through a cycle of heightened activity every 11 years, so we know we can expect more aurorae then. And because some active regions on the Sun can last for months, we can build predictions every 27 days as the same part of the Sun rotates to face the Earth again.

Astronomers also monitor the Sun for solar activity giving us up to a few days warning of when an ejected solar storm might hit the Earth. And real-time data from satellites between our planet and our star can measure the solar wind as it approaches, providing an alert for the location and likelihood of aurorae down to 30 minutes.

When and where can I see the northern lights?

Aurorae are most visible in the auroral oval - a ring-shaped region around the Earth's magnetic poles. As a result, many of us around the world might struggle to ever see an aurora unless we travel to these regions.

Facing the northern horizon, with clear skies, the best time to see the northern lights is around the midnight hours when skies are at their darkest. So even though they occur year-round, the winter season (extending from late August to mid-April) is the best time to see them.

Many smartphones today have cameras more sensitive than our eyes that automatically adjust to the correct settings to take beautiful astrophotography images.

Increased solar activity can ‘push’ aurorae to later lower latitudes so that they become visible further south. And twice a year around the equinoxes in March and September, northern lights tend to be more intense. This is because the tilt of the Earth’s magnetic field at this time acts to attract the solar magnetic fields carried to Earth via the solar wind, just like bar magnets attract each other. 

How to photograph aurorae

Bigger aperture – the pupils of our eyes are small so can only capture a limited amount of light. Find a camera with a larger aperture - the size of the opening where the light enters. The larger the aperture, the more light that can be collected, and the brighter your image will be.

Higher ISO – this controls how sensitive the camera's sensor is to light. The higher the ISO, the fainter the light that can be detected, but it will also increase the graininess of your photos, so you’ll need to strike a balance. Somewhere between 800 and 3,200 ISO is ideal.

Exposure – Because aurorae move, taking a long exposure photo can lead to a blurry image. But the longer you leave your camera shutter open, the brighter your image will be as you’ll be collecting more light. Use a short exposure time (a few seconds) for fast moving or bright aurorae, and a longer (up to 30 seconds) for slower or fainter aurorae.

Tripod – use a tripod to balance your camera where possible to produce steady and focused images.

Why are aurorae different colours?

The different molecules in our atmosphere are responsible for the different coloured aurorae we see. Green is the colour given off by excited oxygen up to around 240km in our atmosphere. And because our eyes are most sensitive to green light, this is the colour we most commonly see.

Red light is produced by oxygen that lies above 240km. To produce this colour, the oxygen is excited to a higher energy level and has to fall to its normal energy state without being disrupted, which wouldn’t be possible at lower altitudes where the air is denser. For enough red light to be emitted such that it becomes noticeable, intense solar activity is needed.

Purple light is caused by nitrogen above 100km. Again, strong solar activity is necessary for purple aurorae to be seen, as the charged particles from the Sun have to reach the lower atmosphere where the nitrogen is present.

Blue light can be produced by nitrogen under 100km but is difficult to see against the dark sky, so this colour is best spotted in the twilight hours.