What Causes the Northern Lights? How the Aurora Gets Its Colours

Shimmering curtains of green light ripple across a dark winter sky, sometimes edged with pink and purple, folding and moving as if alive. For thousands of years people have told stories about the northern lights. Today we know they are a giant natural light show powered by the Sun.

Wind from the Sun

The Sun does not only send us light. It also constantly blows out a stream of electrically charged particles, mainly electrons and protons, called the solar wind. It travels at hundreds of kilometres per second and reaches Earth in a few days. During solar storms, such as coronal mass ejections, the Sun hurls out huge clouds of particles that make the flow much stronger.

Earth’s magnetic shield

Earth is surrounded by a magnetic field, generated deep in its molten iron core. This field forms a protective bubble, the magnetosphere, which deflects most of the solar wind. But the field lines curve down into the planet near the magnetic poles. When the solar wind disturbs the magnetosphere, energy and particles are funnelled along these lines towards the polar regions.

The light show

High in the atmosphere, about 100 to 300 kilometres up, the incoming particles collide with oxygen and nitrogen. The collisions give these atoms and molecules extra energy. When they settle back down, they release that energy as light, much like the gas in a neon sign glows when electricity passes through it.

Because the particles arrive along the field lines, the aurora forms an oval ring around each magnetic pole. In the north it is called the aurora borealis; in the south, the aurora australis.

Why the colours differ

• Green, the most common colour, comes from oxygen atoms at about 100 to 250 kilometres.
• Red comes from oxygen much higher up, above about 250 kilometres, where the air is very thin. It is strongest during intense storms.
• Blue and purple come from nitrogen, often visible along the lower edges of the curtains.

Cameras often show more colour than our eyes, because our night vision is poor at seeing colour in dim light.

Where and when to see it

The best places lie under the auroral oval, roughly 65 to 72 degrees north: northern Norway, Iceland, Finnish Lapland, northern Sweden, Canada and Alaska. Choose a clear, dark night between about September and March, far from city lights, and check an aurora forecast.

Auroras are more frequent around solar maximum, the peak of the Sun’s roughly 11-year activity cycle. During the exceptional geomagnetic storm of 10–11 May 2024, auroras were seen and photographed far to the south, including in southern Europe and parts of Turkey, where they are normally almost never visible.

More than a pretty sight

The same solar storms that create beautiful auroras can also disrupt satellites, GPS and radio communications, and in extreme cases damage power grids. That is why agencies such as NOAA’s Space Weather Prediction Center watch the Sun around the clock.

In short

• Auroras are caused by charged particles from the Sun, the solar wind.
• Earth’s magnetic field guides some of them towards the polar regions.
• They hit oxygen and nitrogen 100–300 km up, which then release light.
• Green and red come from oxygen; blue and purple from nitrogen.
• The best viewing is under the auroral oval, on dark, clear nights from September to March.
• Auroras are stronger near the peak of the Sun’s 11-year cycle.