Auroras

Auroras are one of the most beautiful natural sights on Earth: shimmering curtains of green, pink, red and purple light dancing across the night sky. They happen in two ring-shaped regions around the magnetic poles, where they are called the Northern Lights (Aurora Borealis) and the Southern Lights (Aurora Australis). Auroras are caused by particles streaming from the Sun colliding with gases high in Earth's atmosphere. They have been inspiring myths and stories in polar cultures for thousands of years.

  • Northern Lights nameAurora Borealis"Northern dawn" in Latin
  • Southern Lights nameAurora Australis"Southern dawn" in Latin
  • Height90 to 300 kmIn the thermosphere
  • Common colourGreenFrom excited oxygen atoms
  • Rarer coloursRed, purple, pink, blueFrom other altitudes and gases
  • Best viewingAuroral ovalsIceland, Norway, Alaska, Canada, NZ's south

What causes auroras

Auroras start with the Sun. Our star constantly throws out a stream of charged particles (mostly protons and electrons) called the solar wind. Occasionally the Sun also has bigger eruptions called coronal mass ejections that send huge clouds of particles racing towards Earth at hundreds of kilometres per second.

When these particles reach Earth, most of them are deflected by our planet's magnetic field. But near the magnetic poles, the field lines bend down towards the surface, funnelling some of the particles down into the upper atmosphere. There, they collide with atoms of oxygen and nitrogen, knocking electrons into higher energy states. When those electrons fall back, they release the extra energy as a flash of light. Billions of these tiny flashes together produce the glowing curtains of the aurora.

Why auroras have different colours

The exact colour of the aurora depends on which gas is being hit and at what altitude.

  • Green (the most common colour): excited oxygen atoms at around 100 to 240 km altitude. The classic colour of the Northern Lights.
  • Red (rare): also from oxygen, but at higher altitudes (above 240 km). Sometimes seen as a red glow above a green aurora.
  • Blue and purple: from excited nitrogen molecules, usually at lower altitudes during strong displays.
  • Pink: from oxygen and nitrogen mixed together at the lower edge of the aurora.

Where and when to see them

Auroras are most often seen in two ring-shaped regions called the auroral ovals, around 65 to 75 degrees from the equator (about the latitude of the Arctic and Antarctic Circles). The northern oval includes:

  • Iceland (easy to access, popular tourist destination)
  • Norway, Sweden, Finland (especially the far north)
  • Northern Scotland and the Shetland Islands (occasional displays during big solar storms)
  • Alaska, northern Canada
  • Greenland

The southern oval is mostly over the ocean. The southern tip of New Zealand, Tasmania and southern Argentina sometimes get good views, but the Aurora Australis is much harder to see from inhabited land than the northern aurora.

Fact Auroras have been recorded in human history for thousands of years. The earliest written description is on a Babylonian clay tablet from 567 BC. Ancient Chinese and Japanese records refer to the lights as the breath of dragons. Norse mythology saw them as light from the armour of the Valkyries. Several indigenous Arctic peoples believed the lights were the spirits of dead ancestors. Almost every northern culture has its own story for the aurora.

The solar cycle

How often you see auroras depends partly on the solar cycle: the Sun goes through an 11-year cycle of more and less active phases. Near solar maximum, the Sun produces many more solar flares and coronal mass ejections, and auroras are much brighter and more common. Near solar minimum, displays are fainter and rarer. The current cycle (cycle 25) is expected to peak around 2024 to 2025, so the next few years are an especially good time to chase the lights.

Did you know? Earth is not the only planet with auroras. Jupiter and Saturn both have spectacular auroras, hundreds of times more powerful than Earth's, photographed by the Hubble Space Telescope and the Juno and Cassini probes. Mars and Venus have weak auroras too. Auroras require three things: a magnetic field, an atmosphere, and a source of charged particles. Many planets and moons in the solar system have all three.
Deeper dive: how a big solar storm could affect modern life

Auroras are beautiful, but they are also a visible sign of something more serious. The same solar storms that produce them can also damage technology on Earth.

The most famous example is the 1859 Carrington Event: an enormous coronal mass ejection that hit Earth on 1 September 1859. The resulting solar storm caused auroras visible as far south as the Caribbean. Telegraph wires (the newest technology of the day) sparked and caught fire. Some telegraph operators were shocked unconscious. Some telegraph stations briefly carried on operating with no power supply connected at all, just using current from the storm.

A Carrington-class storm hitting today would be much more serious. Modern electrical grids, satellites, GPS systems, undersea internet cables and high-voltage transformers are all far more vulnerable than 1859 telegraph wires. Estimates suggest the damage from a really big storm today could cost trillions of pounds and knock out power to large parts of the world for weeks or months.

Scientists at NASA, ESA and several other agencies now constantly monitor the Sun for signs of large eruptions. We typically get one to three days' warning before a coronal mass ejection arrives at Earth, which gives power companies time to disconnect grids and satellite operators time to put equipment into safe mode. The next time a really big storm hits, the auroras will be amazing, but the technological consequences could be severe.

For more, see layers of the atmosphere and the International Space Station.