Aurorae over Wiltshire in 2003 image by Richard Fleet
Aurorea (the plural, Aurora singular) are caused by energetic particles which have been ejected from the Sun, hitting the Earths atmosphere. Earth has a strong magnetic field and the particles ejected from the Sun are also magnetic. When the particles hit the Earth's magnetic field they are deflected around the magnetic field and towards the poles. As the particles move through the upper atmosphere they ionise the atoms of the atmosphere and cause them to glow much like in a florescent light.
Earth has a North and South Pole like a bar magnet
Particles from the Sun are deflected around the magnetic field
Aurorea take many shapes and colours and move in different ways. The main colours are red from Nitrogen and green from Oxygen but when the show is faint colours are hard to discern a grey glow may be all that is seen. The colours are caused by the constituent gases in the atmosphere being ionised depending on how energetic the in-coming particles are.
Different shapes may occur and appear to be moving across the sky, sometimes at considerable speed. The most common shapes are sheets, rays and curtains. Sheets are large areas of colour which have no discernable pattern or movements. Rays are long straight beams of light which move like search light beams. Curtains are the most impressive of all and do look like huge coloured curtains swirling across the sky. The Colours are much more noticeable in photographs because longer camera exposure times allows more light to be captured and recorded on the film. The eye can only record very brief pictures on its own.