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Astronomy Question of the Week

Why is the sky alight?

Week 17

 An aurora over Alaska
zum Bild An aurora over Alaska

There have been accounts of what are known as auroras – the eerie reddish and greenish flickering of light in the night sky – for the past 2000 years. The Vikings interpreted the luminous effect as being the shimmer of moonlight on the armour of the ghostly Valkyries as they rode across the sky. But why do auroras occur?

 The solar wind meets the Earth's magnetic field
zum Bild The solar wind meets the Earth's magnetic field

Astronomy provides us with a scientific explanation for the 'fire in the sky': auroras are created when the solar wind meets the Earth's atmosphere. Solar wind is the name given to the stream of electrically charged particles (for example electrons and protons) that the Sun radiates into space. However, Earth is not completely defenceless against this solar bombardment – its magnetic field shields the Earth’s surface from the majority of the high-energy particles.

The solar wind causes the Earth's sky to light up

However, if the solar wind is especially strong the particles can penetrate our atmosphere, in particular in the polar regions, where the magnetic field meets the Earth's surface at right angles. The shower of particles there causes air molecules to glow: highly excited oxygen atoms at a height of around 100 kilometres emit green light and red light originates from oxygen atoms at a height of around 200 kilometres. Very large amounts of energy are necessary to excite nitrogen atoms, which then emit light that is violet to blue in colour.

In mid-latitude regions, including most of Europe, auroras are unusual events. They usually have a red colour here, as only rarely can the solar wind penetrate more deeply inside the atmosphere.

Henning Krause
German Aerospace Center

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Last update: 24/04/2009 09:27:59