The Northern Lights - where, when and what.

By Truls Lynne Hansen
Tromsø Geophysical Observatory - University of Tromsø.

Where can we see the northern lights? The Northern Lights, as the name suggests, are especially related to the polar regions. They occur most frequently in a belt of radius 2500 km centered on the magnetic north pole. This so-called auroral zone extends over northern Scandinavia, Island, the southern tip of Greenland and continuing over northern Canada, Alaska and along the northern coast of Siberia. The coasts of the Norwegian counties of Troms and Finnmark lay where occurrence is greatest, making northern Norway, due to its ease of access and mild winter climate, an attractive destination for people interested in observing this atmospheric phenomenon. The Northern Lights can be seen from regions both north and south of the auroral zone, but the likelihood decreases with distance. There is a corresponding auroral zone around the southern magnetic pole, but these 'Southern Lights' are largely only seen from Antarctica and the surrounding ocean. Of the populated regions in the southern hemisphere, the Southern Lights, may only be glimpsed from Tasmania and southern New Zealand. The Northern and Southern Lights occur simultaneously and are almost mirror images of each other.

How often can you see the Northern Lights? In Troms and Finnmark, we can see the Northern Lights every other clear night, if not even more frequently. From southern Norway, sightings would be only a few times a month while in central Europe hardly more than a few times a year and they have even been seen from the Mediterranean but only a few times each century. To the north of the auroral zone, on Spitzbergen, the Northern Lights are a common sight, although they don't appear as often as in northern Norway.

When can we see the Northern Lights? We associate the Northern Lights with wintertime, although in reality they are present the year round; it's just that we can't see them when the nights are light as the background sky has to be fairly dark. In practice, in northern Norway we are restricted to the period starting at the beginning of September and extending until the middle of April. On the other hand, if the Northern Lights are strong enough, they may still be seen against a twilight sky, and it is not unusual to see them from Tromsø on an August evening. The Northern Lights are often referred to as 'night aurora' because they occur on the night side of the Earth and they commonly appear in the early evening and continue late into the night. Although this is the most usual form of aurora, during winter on Spitzbergen, where it is dark even at midday, it is possible to observe the rarer 'day aurora' which occurs on the 'day side' of the Earth. The aurora lies well above the highest clouds, so we need clear skies to be able to see it. In fact, cloudy skies are the greatest obstacle for auroral observations in northern Norway and for this reason the inland regions are better suited than near the coast. The days around full moon are not conducive to viewing the Northern Lights because the background sky becomes so light. Finally, one should avoid cities and areas with much street lighting in order to experience the Northern Lights to the full.

How high up are the Northern Lights? Most aurorae occur between 90 and 130 km above sea level, but some, particularly the ray-like forms, extend to several hundred kilometers up. In comparison, the usual altitude for a jet aircraft is around 10km and the ozone layer lies between 20 and 30km so we have to be almost up at the heights of satellites orbits to be at the same height as the aurora. A consequence of its great height is that the aurora is visible at horizontal distances of several hundred kilometers. Thus an aurora over Bear Island will be visible from both Spitzbergen and Tromsø, and one over Tromsø can be seen in the northern sky from central Norway.

What exactly are the Northern Lights? The Northern Lights stem from when large numbers of electrically charged particles (electrons) at high speed stream in towards the Earth along its magnetic field and collide with the highest air particles. The air then lights up rather like what happens in a fluorescent light tube. The resulting colours reflect which gases we find up there, the most usual yellow-green colour coming from oxygen. Red colouring is also due to oxygen with a contribution from nitrogen. The violet we often see at the lower edge of the aurora is due to nitrogen, as is most blue colouring. The charged particles originate from the sun, and it is the 'weather' conditions on the sun that decide whether or not we will see the aurora. Particles can stream out from the sun and some are captured by the Earth's magnetic field and find their way into the polar regions. On the way, they travel out into the night side of the Earth and gain extra energy - we still lack understanding of exactly what happens out there!