Auroras: lights made by space weather — Astronomy, 11–13 years
The northern and southern lights begin with particles from the Sun. Earth’s magnetic field steers them towards the polar skies, where they make thin gases glow.
What an aurora is
An aurora is a moving glow high above Earth, often green but sometimes red, pink or violet. Fast charged particles from the Sun reach Earth’s neighbourhood, and our magnetic field guides some towards the poles. There, the particles hit oxygen and nitrogen atoms high in the atmosphere. Those atoms release the energy as coloured light.
Why auroras form near the poles
The problem was to explain strange lights that appeared in cold northern and southern skies. The answer joined two kinds of evidence: the Sun sends a stream of charged particles, and Earth acts like a giant magnet. Magnetic field lines enter and leave near the poles, so particles can travel down them there. Strong solar storms can push the glow farther from the poles.
Why an aurora can be green
Take a green aurora as a sequence. First, a solar particle reaches an oxygen atom about 100 to 300 kilometres above Earth. Next, the collision gives the atom extra energy. Then the atom loses that energy by sending out green light near 557 nanometres. Nitrogen and oxygen at different heights can instead produce red, blue or violet colours.
The reasonable mistake
It is easy to think an aurora is a cloud reflecting coloured sunlight. The colours really do look like a curtain, and sunlight does illuminate the upper atmosphere during the day. But an aurora makes its own light: energetic particles excite atoms, and the atoms emit particular colours. It can shine at night even when the Sun is below the horizon.
Auroras and space weather
Auroras are beautiful signs of space weather, but the same solar storms can disturb satellites, radio links and electrical grids. Forecasts use solar observations and measurements from spacecraft near Earth. Astronauts also watch this activity because extra charged particles can increase radiation risk. The glow is not itself dangerous on the ground; it is a visible clue to what is happening above us.
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