Auroras - ghostly links

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aurorasAurora is the name given to ghostly lights that appear in the skies of the Earth towards the polar regions. In the north, they are called aurora borealis (northern lights). In the south, aurora australis (southern lights).
Image © J Curtis.

Solar Connection

Auroras are caused be the Sun's solar wind, a continuous stream of charged particles that heads out into the solar system. When these particles reach the Earth, they are deflected towards the North and South poles by its magnetic field. The particles then hit the atmosphere, and cause the shimmering lights to appear.

Violent Explosions

Coronal Mass EjectionThe strength of the solar wind varies over an 11 year cycle, connected to sunspot activity. Sometimes the sun violently throws off material in an event called a coronal mass ejection (CME). If these head towards the Earth as part of the solar wind, the auroras produced are particularly impressive! In fact they can even be seen as far south from the Arctic as England.

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Lucky for Life

The existence of a strong magnetic field on Earth is essential for life on the planet. The deflection of the Solar Wind to the poles dramatically reduces the damage it causes to our thin atmosphere. Mercury has no atmosphere for this very reason, and Mars has only a thin veil by comparison.

Colourful Displays

colourful auroraThe colours of auroras vary depending on the height at which most of the collisions occur. Street lamps and 'neon' signs emit different colours of light due to the types of gas inside them. The same applies to auroras. Image © J Curtis.

If the predominant gas is oxygen, as it is above 300 km altitude, auroras will be red. This is rare, and only occurs at times of maximum solar activity. The most common colour is yellow or green, again caused by oxygen, but at lower altitudes. Nitrogen at about 100 km produces a red light often seen at the lower edges of auroras.


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