This article is a travel topic
The Northern Lights or Aurora Borealis is a natural phenomenon that can paint the night sky with unearthly, surreal color. To observers at far-northern latitudes, they're a frequent occurrence, but many who live in more temperate climates have never seen them, even though they're sometimes seen as far south as 35 degrees north latitude. This article will help you improve your chances of seeing the Lights if you journey north.
The Northern Lights usually circle the globe in a circular or elliptical "oval" centered on the earth's North Magnetic Pole, which is not at the same location as the North Geographic Pole, but rather is slightly offset in the direction of northern Canada. This oval is known as the "auroral oval". Auroral displays aren't strongest at the North Magnetic Pole; the area of greatest auroral activity is usually right under the auroral oval, which changes dynamically according to how the solar wind is flowing on any particular night.
The auroral oval, meaning the area with the highest probability of seeing the northern lights, covers most of Alaska, northern parts of Canada, the southern half of Greenland, Iceland, Northern Norway, Sweden and Finland. As well as the western half of the Russian north (with the Kola Peninsula of Murmansk Oblast being the most popular viewing spot).
Regions such as central and southern Scandinavia and the north-central United States as well as Scotland also occasionally see the Northern Lights during periods of high geomagnetic activity when the auroral oval expands further south than usual. Svalbard, which is about half way between continental Europe and the north pole, actually sees less Northern Lights than Northern Scandinavia (despite being so far north). But it is a great place to observe the fainter Day Northern Lights during its long Polar night and is a good choice during the solar minimum part of the solar cycle.
A curiosity is that the exact location of the North Magnetic Pole varies from year to year, sometimes by tens of miles. The magnetic pole has been moving for a millenia and more. But it is only the past few hundred years we (or ancient mariners to be precise) have tracked it's movement in order to correctly calibrate their ships compasses. As of 2006; it's now near Ellesmere Island in the nearly uninhabited far north of Canada. As a consequence, the advantages of being on the "right side" of the earth are not as pronounced as they were some years ago. Still, there's a slight North American bias even today in your chance of seeing the Lights.
This said, the actual latitudes of the Lights vary considerably. In times of high solar activity (more on that later), the Lights may be seen in North America at latitudes as low as 35 degrees north, the time to see the lights is 6pm to 1am meaning that all but the southernmost parts of the United States may get a display. The offset of the Pole keeps solar storms from benefiting Europe quite as strongly, but most of the countries of northern Europe will get displays during periods of solar storms.
Planning to see the Northern Lights
There is no guarantee to see the Northern Lights, even if you are in the best areas. However, a bit of planning will radically increase your chances.  In short, good periods are between late September and late March. Statistically there are more Aurora Display in proximity of the two equinoxes. Consider also that in winter there is little or no light for other activities during the day.
What time of the year
Darkness is required. Most Northern Lights locations are found at high latitudes, meaning there is no darkness from mid-April until mid-August (even more in far northern locations like Svalbard). In this period of time, no Northern Lights can be observed.
In the most intense Northern Lights area (notably Alaska, Iceland, Northern Scandinavia and Yukon), the lights are observed from late August to mid April. However, from late September to late March, it is dark after 6pm, and one enjoys maximum chances.
On a yearly basis, the Lights are at their peak in September and March. The reasons for this trend are due to the March and September equinox.
What time of day
The time between 6pm and 4am is the most intense period of the day. The highest probability within this timespan is between 10 and 11pm. However, this is a guideline, and during the Polar night auroras can be observed as early as 4pm, and all through the night. In periods of strong activity, one can expect several flares starting at around 6pm, peaking around 10pm, and going on until 1pm.
In the longer term, auroral displays are correlated with an 11-year cycle in sunspot activity and other perturbations of the sun; the more restless the sun, the more aurorae. 2006-7 corresponded to a minimum in solar activity, and the next maximum in solar activity will be around 2013, with frequent Northern Lights displays likely for another two or three years after that.
In addition to these more or less regular variations in frequency of the aurora, there are also less predictable, erratic displays resulting from solar storms. Some of these, particularly near solar-activity maximum, can lead to visible Northern Lights remarkably far south, if you're in an area with clear, transparent night skies. The "Alerts" section below will help you stay on top of solar activity and prepare for some viewing when a solar storm does occur.
Last but not least, don't forget the weather forecast — aurora occur very high up in the atmosphere, and if there are clouds in the way you will not see anything. In Northern Scandinavia, the weather is notably better towards the end of the Northern Lights season (February-March), than in the beginning. The weather is probably the most important success factor in the areas under the Northern Lights oval, where there is visible Northern Lights up to 80% of all clear nights.
Please note that most displays of the Northern Lights are quite faint phenomena, and the bright ones occur significantly more rare. Therefore for best viewing you need a location with as least "light pollution" as possible. The same display may be barely visible in a center of a big city, yet quite spectacular in an open field some 5-10km away from major sources of artificial light.
If you have the luxury of being able to travel into aurora-viewing territory on short notice, you can improve your chances of seeing something by being aware of "space weather," the things going on beyond the earth's atmosphere as a result of solar activity. A good site for this information is http://www.swpc.noaa.gov/ operated by the (US) National Oceanic and Atmospheric Administration, precisely for the purpose of keeping up on space weather. (The commercial site http://www.spaceweather.com/ presents much of the same information in digested, more accessible form.)
The University of Alaska Fairbanks maintains an Aurora Alert website at http://www.gedds.alaska.edu/AuroraForecast/, and the University of Alberta maintains Aurora Watch at http://www.aurorawatch.ca/.
If a major solar storm develops that is forecast to have a good chance of producing Northern (and Southern) Lights, your time to respond will be measured in hours to a few days, rather than either minutes or weeks. The forecasts will usually include some indication of how far from the magnetic poles the activity is expected to extend. For purposes of travel planning, it's a good idea to plan conservatively and go to a locale somewhat closer to the pole than the maximum extent of the aurora; things don't always work out as forecast, and the Lights may be relatively weak and/or confined to the northern horizon if you're at the southern edge of the activity, either limitation possibly creating difficulties for you in viewing owing to light pollution.
Into the wilds
Taking good pictures of the Northern Lights is very difficult, since they're fast-moving, often faint and against a pitch-dark background, all of which befuddles consumer point-and-shoots. Here's what you need for a sporting chance:
Northern Lights usually form 60mi/100km above the surface of the earth. This means that an eruption is visible over large tracts of land. In principle, all areas under the Northern Lights oval are good observation points. However, most of these areas are remote and inaccessible, suffering harsh climatic conditions. The mentioned locations provide some kind of infrastructure, like tours, observation points etc.
Okay, so how about the Southern Lights?
Aurorae happen in a circle/ellipse about the South Magnetic Pole just as they do about the north one, and the South Magnetic Pole is similarly offset from the geographic South Pole. Would-be observers of the Southern Lights or Aurora Australis benefit from the happy accident that the offset of the South Magnetic Pole is generally in the direction of Australia, although the Pole itself is still in Antarctica like the geographic one. Tasmania is therefore relatively favored for the Aurora Australis, and southern Australia gets more than its share of Lights relative to latitude. All of the considerations about maximizing your chances of seeing the Northern Lights apply equally to seeing the southern ones.