What is the aurora?
Light given off high in the atmosphere when particles guided in by Earth's magnetic field strike oxygen and nitrogen.
The aurora is light from the upper atmosphere. Electrons, and to a lesser extent protons, are guided down Earth's magnetic field lines and collide with oxygen atoms and nitrogen molecules about 100 to 400 km up. Each collision lifts the atom or molecule into an excited state. When it settles back, it gives off light of a particular colour. Millions of these collisions every second make the glow you see.
Northern and southern lights
The aurora happens around both magnetic poles at once: the aurora borealis in the north and the aurora australis in the south. The two are nearly mirror images, because the same field lines reach down into both hemispheres. Polar Lights forecasts both.
Where the energy comes from
The particles that make the aurora don't fly straight in from the Sun. The Sun's energy arrives as the solar wind, a constant stream of charged gas carrying the Sun's magnetic field. When that wind couples to Earth's magnetic field, energy is stored in the field on Earth's night side and then released, accelerating particles down into the atmosphere. See From the Sun to the sky.
Why it moves
Aurora often appears first as a quiet arc. When stored energy is released suddenly (a substorm), the arc brightens, folds into curtains and races across the sky, sometimes for 10 to 30 minutes. Fainter patches can switch on and off every few seconds; see Rare sights.