What turns on the Northern and Southern Lights - Aurora Borealis and Aurora Australis?
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 18th 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. 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.
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.
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.
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 2012 and activity is now declining. However aurorae can occur at any time and observers in northern latitudes should always look out for them.
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.