Every occurrence of the Aurora Borealis is different. While luminous green and white coruscations are most common, great bands of other colours can appear, such as red, blue, violet and even delicate pinks.
Sometimes the effect is as if wind caught a light net curtain that covers the heavens and disappointingly sometimes the lights look just like small balls of rolling smoke.
If there's a lot of sunspot activity the lights will explode into coronas, which, depending on luck and conditions, may last scant seconds or a period of hours. They're caused by huge quantities of electrically charged particles blown away from the sun and attracted by the Earth's magnetic poles.
While seeing them is never guaranteed, you can maximise your chance of seeing them by travelling at times of predicted high sunspot activity but know that it's never guaranteed. Cloud, mist or dense snow might obscure them or they may simply fail to appear. The longer you stay (and the more of the evening you're prepared to put aside) the more likely it is.
Where are they?
Northern Lights can be seen over Greenland, the Arctic regions of Norway, Sweden and Finland, Siberia, Alaska, Manitoba, the Yukon and Northwest Territories of Canada but this changes according to solar activity so that they are sometimes clear in Scotland, Newcastle and very occasionally the Mediterranean. The same phenomenon can also be seen over Antarctica.
What is it?
The visible effect of the results of high-speed electrons hitting the Earth's magnetic field.
Best time to see it?
While they happen all year round, you need dark, clear and preferably dry conditions as well as a large number of solar activity to see them best. Early autumn and spring are considered the best time, but the lucky traveller can catch them mid-summer in Manitoba. For the best chance avoid times around the full moon and go for the best combination of long nights and dry season as you can.
And wherever you go, dress for warmth, as you'll be moving very little while waiting for the best shot.
If the lights are good enough and you have a choice, go to a place where the local scenery will contrast with the lights such as a tall stand of snow clad trees, statue or landmark. While they're not necessary for your viewing, such places make for much more interesting photographs.