aurora

Coloured light in the night sky near the Earth's magnetic poles, called aurora borealis (‘northern lights’) in the northern hemisphere and aurora australis (‘southern lights’) in the southern hemisphere. Although auroras are usually restricted to the polar skies, fluctuations in the solar wind occasionally cause them to be visible at lower latitudes. An aurora is usually in the form of a luminous arch with its apex towards the magnetic pole, followed by arcs, bands, rays, curtains, and coronae, usually green but often showing shades of blue and red, and sometimes yellow or white. Auroras are caused at heights of over 100 km/60 mi by a fast stream of charged particles from solar flares and low-density ‘holes’ in the Sun's corona. These are guided by the Earth's magnetic field towards the north and south magnetic poles, where they enter the upper atmosphere and bombard the gases in the atmosphere, causing them to emit visible light.

The French philosopher Pierre Gassendi coined the term ‘northern dawn’ (the literal meaning of ‘aurora borealis’) in 1621.