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 climes 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 are similar to a sunset in the sky at night, but appear occasionally in arcs or spirals usually following the earth's magnetic field. They are most often light green in color but sometimes a variety of other colors. The Aurora Borealis is caused by charged particles ejected from the sun. When these particles reach the earth, they collide with gas atoms in the earth's atmosphere causing them to energise which results in a spectacular multi-coloured light show.
Contrary to intuition, seeing the Northern Lights isn't just a matter of heading "north." The Lights usually circle the globe in a circular or elliptical band centered on the earth's North Magnetic Pole, which is not at the same location as the North Geographic Pole, but rather is offset in the direction of northern Canada. Furthermore, auroral displays aren't strongest at the North Magnetic Pole; the band of greatest auroral activity is usually offset from the Magnetic Pole by 20 degrees or so.
This quirk is actually fairly convenient for would-be aurora watchers. It means that locations in the north-central United States, for example Minnesota and North Dakota, and also southern Canada see Northern Lights much more frequently than they would if the Lights were centered on the North Geographic Pole. Alaska and Lapland (the northern part of Finland, Norway and Sweden) also fall in the region of greatest probability, while the far-north territory of Siberia that misses out on some of the Lights (because the Magnetic Pole is displaced away from that region) tends to be more inaccessible to the traveler.
A curiosity is that the exact location of the North Magnetic Pole varies from year to year, sometimes by tens of miles. The Pole has been moving north for a few years now 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, 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.
There are both seasonal and long-term variations in the likelihood of a Northern Lights display. On a yearly basis, the Lights are at their peak in September and March, but also in October and April. The reasons for this trend aren't fully known, but it's definitely real, not just an artifact of the weather or other viewing conditions. The Lights can best be seen at around midnight.
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. Unfortunately, 2006-7 corresponds to a minimum in solar activity, and therefore the number of Northern Lights-lit nights. However, at the most favorable latitudes, the Lights are still likely to be seen even at solar minimum; it's mainly at lower latitudes that they get scarce during the inactive times. The next maximum in solar activity will be in about 2012, with frequent Northern Lights displays likely for another two or three years after that, so you have plenty of time to plan a trip.
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.
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.sec.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.)
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.
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:
A camera that supports manual exposure (5 to 40 seconds).
A fast lens (aperture f/2.8 or better)
Fast film (800 ASA or better), or equivalent ISO setting on a digital camera
A tripod to hold the long exposure
Cable release or self-timer to trigger shots without stirring the camera