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Aurora over Northamptonshire
Aurora over Northamptonshire(Image: Kevin Smith)

The Aurora Borealis and Aurora Australis are seen in the northern and southern hemispheres respectively. Aurorae are generally confined to higher latitudes but large displays associated with violent magnetic disturbances can be seen from as far south as southern Britain.

Both are due to the interaction of a stream of particles from the Sun with the Earth's atmosphere.They have been associated with magnetic activity since the eighteenth century. Their association with sunspot activity, and later with solar flares, gave the key to understanding their cause.

Solar flares are like enormous explosions on the surface of the Sun in which streams of charged particles are emitted into space. Depending on the relative geometry of the locations of the emitting region, the Earth, the particle stream and its velocity, the particle stream from a solar flare can impinge on the Earth's upper atmosphere. The charged particles in the stream interact with the Earth's magnetic field and enter the atmosphere, primarily at high latitudes where the magnetic field emerges nearly vertically from the Earth's surface.

The arrival of the particles is typically two days after the flare is seen on the Sun. The particles interact with the atoms in the atmosphere giving up their energy and exciting the atmospheric atoms. The atoms then emit light while returning to their unexcited state.

Aurorae are often bright enough to be seen with the naked-eye. They often take the form of curtains of light in which the folds rapidly move and vary in brightness. Often the curtain rests on an arc of light. Colours are seen in the brightest displays and can be seen to be red and green.

Aurora animation
Aurora animation(Courtesy of Kevin Smith)

The lowest part of an aurora is at a height of about 100km above the Earth's surface while the top of a display may extend to several thousand kilometres.

AuroraWatch UK

While the northern lights (Aurora Borealis) are a common feature of the Arctic and far north, viewers in Britain must be looking at exactly the right time to be lucky enough to see them. To help out, scientists in the Physics Department at the University of York have created AuroraWatch UK, a web site which will tell you exactly when to look.

Launched on 19th September 2000, AuroraWatch UK offers unique real-time monitoring of the magnetic variations which accompany aurorae. During active times, an 'aurora-alert' e-mail will be sent, which can be forwarded as a text message to mobile phones.

Intense aurora displays are generated following massive explosions on the sun, known as coronal mass ejections. These explosions release clouds of hot plasma, containing billions of tons of material travelling at around 2 million miles per hour, into the solar wind. When the clouds reach the Earth, they can interact with the Earth's magnetic field to cause events called geomagnetic storms.

The sun's activity varies with a peak every 11 years. The cycle last peaked in 2001 and activity is now declining. It should reach its next peak in 2011–2012 when sunspot numbers will once again reach a maximum. However aurorae can occur at any time and observers in northern latitudes should always look out for them.

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