What causes the aurora borealis?

Energetic charged particles, energised by the solar wind and guided by Earth's magnetic field, collide with oxygen and nitrogen in the upper atmosphere near the poles. The gases release that energy as coloured light.

Why are the northern lights mostly green?

Green, at 557.7 nanometres, is light from oxygen atoms about 100–250 kilometres up, where most auroral collisions happen. Our eyes are also most sensitive to green.

Where is the best place to see the aurora borealis?

Under the auroral oval: Finnish Lapland, northern Norway including Tromsø and Lofoten, Abisko in Sweden, Iceland, northern Canada and Alaska, on a dark, clear night away from city lights.

When is the best time to see the northern lights?

From late August to April when nights are dark, ideally on a clear night around 10 pm to 2 am. Activity often peaks around the equinoxes and in the years around solar maximum.

What is the Kp index?

A scale from 0 to 9 describing global geomagnetic activity. The higher the Kp, the further the aurora spreads from the poles towards the equator.

Can I see the aurora in the app?

Yes. Watch it in the 3D visualisation on this page, or land anywhere far enough north on Earth, for example in Lapland, Norway or the Finnish archipelago, and switch the sky to Aurora.

The northern lights: curtains of colour where the solar wind meets Earth's atmosphere

Aurora borealis

The aurora borealis, or northern lights, is one of the most beautiful sights in nature: shimmering curtains, arcs and rays of green, red and violet light moving across the night sky. Its southern counterpart is the aurora australis. Both appear when charged particles, energised by the stream of gas flowing from the Sun, rain down into the upper atmosphere around the magnetic poles and make oxygen and nitrogen glow, much like the gas in a neon sign.

Auroras are seen most often in a ring around each magnetic pole called the auroral oval, which crosses northern Scandinavia, Finland, Iceland, Greenland, northern Canada, Alaska and Siberia. During strong geomagnetic storms the oval expands towards the equator and the lights can be seen much further south. In this app you can watch them overhead in the 3D view on this page, or switch on the aurora sky while exploring the far north in 3D.

Watch it in 3D

Key facts

Altitude: About 100–300 kmThe bright lower edge is usually near 100 km
Green light: Oxygen, 557.7 nmThe most common colour, roughly 100–250 km up
Red light: Oxygen, 630.0 nmHighest up, above about 250 km, strongest in big storms
Blue and violet: NitrogenIonised nitrogen molecules, often at the lower edges
Where: The auroral ovalsRings around both magnetic poles, near 65–70° magnetic latitude
Activity scale: Kp index 0–9Higher numbers mean the aurora reaches further from the poles
Solar cycle: About 11 yearsThe Sun reached the maximum of solar cycle 25 in 2024
Named by: Galileo Galilei, 1619After Aurora, goddess of the dawn, and Boreas, the north wind
Finnish name: Revontulet"Fox fires", from the legend of the Arctic fire fox

What causes the northern lights

The Sun constantly blows a thin wind of charged particles, mostly electrons and protons, out into space. Earth's magnetic field shields us from most of it, forming a protective bubble called the magnetosphere. But the solar wind stretches the magnetosphere into a long tail on the night side and pumps energy into it. From time to time that energy is released, accelerating electrons down along the magnetic field lines into the upper atmosphere near the poles.

When these fast electrons collide with oxygen atoms and nitrogen molecules, they knock them into excited states. As the gases settle back down, they release the extra energy as light of very specific colours. Millions of these collisions, following the shape of the magnetic field, draw the curtains and rays we see from the ground.

Why auroras are green, red and purple

Each colour comes from a particular gas at a particular height. The familiar green, at a wavelength of 557.7 nanometres, comes from atomic oxygen roughly 100 to 250 kilometres up. Higher still, above about 250 kilometres, oxygen glows a deep red at 630.0 nanometres. This red light takes longer to be released, so it only survives where the air is thin enough that the atoms are not bumped out of their excited state first, which is why red auroras are higher and appear mostly in strong displays.

Nitrogen adds blue and violet, especially from ionised molecules, and during intense displays the lower edge of a curtain can glow pink or magenta where fast particles reach deep into the atmosphere. Our eyes are most sensitive to green, so faint auroras often look pale grey-green to the naked eye, while cameras with long exposures reveal stronger colours.

The auroral oval and the Kp index

Auroras are not spread evenly across the polar regions. They are concentrated in two rings, the auroral ovals, centred on the north and south magnetic poles. The northern oval usually passes over Lapland, northern Norway and Sweden, Iceland, southern Greenland, northern Canada and Alaska, which is why these places see the lights on most clear, dark nights in winter.

Space weather forecasters describe geomagnetic activity with the Kp index, a scale from 0 to 9. At low Kp the oval stays near the Arctic. As Kp rises the oval widens and slides towards the equator, so at Kp 5 the aurora may be seen overhead at around 56° geomagnetic latitude, and in the most extreme storms, near Kp 9, it can appear far further south than usual. The 3D view on this page lets you see how activity changes the display.

When and where to see them

You need darkness and clear skies. In the far north the aurora season runs roughly from late August to April, when nights are dark; in midsummer the sky never gets dark enough. The lights are often strongest in the hours around local magnetic midnight, typically between about 10 pm and 2 am, and activity tends to be higher around the spring and autumn equinoxes.

Popular places to watch include Finnish Lapland, Tromsø and the Lofoten Islands in Norway, Abisko in Sweden, Iceland, Yellowknife in Canada and Fairbanks in Alaska. Get away from city lights, give your eyes time to adapt to the dark, and check a space weather forecast for the Kp index and cloud cover.

Photographing the aurora

Use a tripod, a wide lens opened up as far as it goes and exposures of a few seconds, with the focus set to infinity. Modern phones with night modes can also capture surprisingly good aurora photos.

Solar storms and great auroras

The biggest displays follow coronal mass ejections, huge clouds of magnetised gas thrown out by the Sun. When one hits Earth it can trigger a geomagnetic storm that drives the aurora far from the poles. In September 1859 the Carrington Event produced auroras seen close to the tropics and disrupted telegraph systems. On 10–11 May 2024 the first G5, or extreme, geomagnetic storm in more than twenty years painted the sky over much of Europe and North America.

Strong storms are more common around solar maximum, the busy peak of the Sun's roughly 11-year cycle. Besides the spectacle, they can disturb satellites, radio communication, GPS and power grids, which is why space weather is monitored continuously.

Legends, sounds and auroras on other worlds

Many northern cultures have stories about the lights. In Finnish they are called revontulet, "fox fires", after a legend of an Arctic fox whose tail swept up snow and sparks into the sky as it ran across the fells. Some people also report hearing faint crackling or clapping sounds during strong displays; researchers at Aalto University in Finland have proposed that these may come from electrical discharges in the cold air much closer to the ground.

Earth is not the only planet with auroras. Jupiter and Saturn have powerful auroras around their poles, imaged by the Hubble and James Webb space telescopes, and Uranus, Neptune and even Mars have them too. Astronauts on the International Space Station often see Earth's auroras from above, rippling far below them.

Surprising facts about the aurora

The bright lower edge of an aurora is usually about 100 kilometres up, higher than most meteors burn up.
Red auroras appear only very high in the atmosphere, where the air is thin enough for oxygen's slow red glow to escape.
The Finnish name revontulet means "fox fires".
The northern and southern lights often mirror each other at the same moment.
In May 2024 the strongest geomagnetic storm in two decades brought auroras far south across Europe and North America.
Jupiter's auroras are far more powerful than Earth's and shine brightly in ultraviolet light.

Frequently asked questions

What causes the aurora borealis?: Energetic charged particles, energised by the solar wind and guided by Earth's magnetic field, collide with oxygen and nitrogen in the upper atmosphere near the poles. The gases release that energy as coloured light.
Why are the northern lights mostly green?: Green, at 557.7 nanometres, is light from oxygen atoms about 100–250 kilometres up, where most auroral collisions happen. Our eyes are also most sensitive to green.
Where is the best place to see the aurora borealis?: Under the auroral oval: Finnish Lapland, northern Norway including Tromsø and Lofoten, Abisko in Sweden, Iceland, northern Canada and Alaska, on a dark, clear night away from city lights.
When is the best time to see the northern lights?: From late August to April when nights are dark, ideally on a clear night around 10 pm to 2 am. Activity often peaks around the equinoxes and in the years around solar maximum.
What is the Kp index?: A scale from 0 to 9 describing global geomagnetic activity. The higher the Kp, the further the aurora spreads from the poles towards the equator.
Can I see the aurora in the app?: Yes. Watch it in the 3D visualisation on this page, or land anywhere far enough north on Earth, for example in Lapland, Norway or the Finnish archipelago, and switch the sky to Aurora.