Auroras have fascinated humanity for thousands of years, but it is only in the last century that their origins have been understood in detail. It's actually pretty simple.
Did you know? The Earth's magnetic field funnels the particles in the solar wind toward the North and South Poles so that two separate auroras appear: the aurora borealis (north) and aurora australis (south).
The glow of auroras is caused by the excitation of oxygen and nitrogen atoms in the Earth’s upper atmosphere.. Up there, very fast moving particles from the Sun (the “solar wind”) get trapped in the Earth's magnetic field and collide with the oxygen and nitrogen atoms . When these collisions occur, the atoms become excited to a higher energy state, and that extra energy is released in the form of light.
Atoms of different elements emit different colours, which explains why auroras display reds, greens and sometimes even purples. For example, collisions with oxygen molecules more than 300 km above the earth’s surface result in red auroras. At lower altitudes (100-300 km), oxygen gives off a yellow-green colour. The red-purple colour of auroras comes from nitrogen.
Did you know? The solar winds that cause auroras travel at over one million kilometres per hour.
While it is not possible to predict precisely when the Sun will produce a very strong solar wind, there does seem to be more auroral activity during the spring and fall. In northern regions, winter is a good time to view the auroras since there are long periods of darkness. The biggest auroras appear during the strongest solar activity, which occurs during the peak of the 11-year sunspot cycle. The next peak is supposed to happen in 2013 (see here for more details).
Did you know? The first reported observation of an aurora was in China in 2600 B.C.
If you want to get out and see the aurora borealis, you need the darkest skies possible. A century ago people could see them regularly from major cities like Chicago and Toronto, but today this is rarely possible. So if you know an aurora might be coming (you can check for activity at this website), the best place to see them is away from the lights and out in the countryside.
The Northern Lights Space and Science Centre
Aurora; What Causes Them? (video)
Why auroras are different colours
AuroraMAX: Broadcasting the Northern Lights
(Photo of Aurora borealis above Bear Lake, Alaska via Wikimedia Commons)
Article first published February 9, 2012