The sky is dark and you're ready for bed. You switch off the lights, then notice an unusual glow in the northern sky. You know that there is no city glow there and it can't be twilight - the Sun set hours ago. What you're seeing may be one of the most astonishing of natural displays - an aurora (plural aurorae or auroras), popularly known as the northern lights.
These ghostly light shows are a familiar sight in high northern latitudes, but often they creep into the mid-latitudes too. You might get a glimpse of them if you know what to look for and when.
A ring of northern light
Why should the sky light up like this? It's all due to atomic particles from the Sun cascading onto the Earth's upper atmosphere and making it glow like a fluorescent lamp.
The atomic particles don't slam into the atmosphere all over the Earth. Our planet's magnetic field funnels the particles down onto a ring around the poles, known as the auroral oval. [Click to see a current map generated by satellite observation.] Anyone living underneath the oval, which runs from Alaska over northern Canada to Iceland, northern Norway and Siberia, could expect to see an aurora on most clear nights.
For the rest of us farther south, we have to wait for the Sun to throw out clouds of gas in large eruptions known as coronal mass ejections (CMEs). When these huge solar storm clouds reach Earth the auroral ovals strengthen and expand. Then aurorae are seen at lower latitudes, such as the southern states of the US and the British Isles. (Aurorae occur around the south pole, too, but fewer people live far enough south to see them.)
The Sun's storminess rises and falls in a cycle lasting roughly 11 years, and the number of aurorae follows in step. Early in 2011 solar activity was picking up after a particularly long minimum and with it the number of bright aurorae was still high in 2014.
Aurorae take many shapes, and that's part of their beauty. The simplest is a glow on the horizon. In large towns and cities, alas, such glows are swamped by the 'urban aurora' of sky glare from streetlights so you'll need to be out in the country to appreciate the show.
As the aurora rises higher in the sky you see what's called an arc. This can fold into a band or curtain, rippling and curling like drapery wafting in a breeze.
In addition to these basic shapes you can also get rays - vertical streaks that shoot up much higher than the rest of the display like searchlight beams. These are a sign of a really active display.
And if the display is right above you, you can see what's called a corona, a perspective effect in which the bands and rays seem to converge on a point overhead.
Click to watch a beautiful speeded-up video made by Tor Even Mathisen of aurorae in Norway.
Why the colors?
Photographs show aurorae as bright green and red, but in practice you won't see such strong colors with the eye. That's because the human eye sees faint light as shades of gray - think of how the colors in a landscape become dulled at twilight.
On photographs, the top of the aurora appears red, while lower down the color is green. It's the green part that's usually the brightest visually. These colors are caused by oxygen and nitrogen gas in the upper atmosphere at altitudes of 100 to 250 kilometers (60 to 150 miles) glowing ethereally as it's bombarded by electrons from the Sun.
The most energetic electrons of all can penetrate lower into the atmosphere to excite molecules of nitrogen, producing a fringe of purple at the bottom of the aurora.
When can I see an aurora?
Aurorae can flare up at almost any time but are most common in the spring and fall. You can receive advance warning of solar storms via a website called Spaceweather.com. This publishes pictures of the latest aurorae, and includes a link to a NASA site showing the current auroral oval.
A good auroral display should be on anyone's must-see list. Many enthusiasts now go on specialized aurora-spotting tours to far northern latitudes by land and sea for the best views. Google 'northern lights tours' for a selection.
If you're a keen photographer, click for some expert advice from Dennis Mammana.