The northern lights-what are they?

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

Nothing else in the sky looks like the northern lights. The sun and the moon, the stars and the planets are an eternal, regular and predictable part of human life. The northern lights, on the other hand, are transient, variable and unpredictable. Here the cosmos parades its electrical and magnetic forces, and produses colours and movements that are unique in Nature. Photograps are pale, static shadow of the real thing. The northern lights must be seen andexperienced outside, under a still and cold winter sky.

The northern lights-the aurora borealis-can be seen, on rare occasions, almost anywhere on earth. In February 1872 , for example, the north lights were seen in Bombay and in Egypt, and in September 1909 they were observed in Singapore and Jakarta! However they belong primarily to the polar regions of the world, occurring most often in a belt around the magnetic pole at a distance of 2,500 km from it. This so-called auroal zone passes over northern Skandinavia, over Iceland and the southern tip of Greenland, through northern Canada, over Alaska and along the northern coast of Siberia. The coast from Troms and Finnmark is where the northern lights appear with the greatest frequency fo all. So it is obvious that northern Norway, with its easy accessibility and mild winter climate, is attractiv to people who wants to see this phenomenon of the heavens.

There is an exactly equivalent zone around the south magnetc pole. These "southern lights"-the aurora australis-can generally be seen only in the Antarctic and the surrounding waters. Of the inhabited aeras in the southern hemisphere, it is only in Tasmania and in the southern part of New Zealand that people get a glimps of them with any frequency. Insidentally, the northern lights and "the southern lights" occur simultaneously and are almost mirror images of each other.

In the auroal zone the northern lights are an everyday phenomenon. From a scientific point of view, it can well be clamed that there is an aurora every night, but some of the occurrences are so faint that most people hardly notice them. From the tourists` point of view, however, it is hardly an exaggeration to say that we can see the northern lights at least every other clear night in the counties of Troms and Finnmark. If we go to southern Norway, they will occur a few times a month, and in central Europe they will perhaps be seen once a year. In the Mediterranean area are among the very rare events which occur perhaps a few times each country. In the polar regions, within the auroal zone itself. Thus there are fewer auroras in Svalbard than in northern Norway. We associate the northern lights with winter, but in fact they are there all the year. It is just that we cannot see them on light nights; the sun has to be belowe the horizon. In practice, in northern Norway the northern lights are limited to the period from the beginning of September to the middle of April. Intens auroras can, however, be seen against a very light sky. In northern Norway it is, for example, not uncommon to see the northern lights in an evening sky in August.

The northern lights we see in northern Norway are usually called the night-time aurora because they are on the night side of earth. The display usually begins in the late afternoon or in the evening and continues with varying intensity often far into the night. This is the common form of aurora, but on Svalbard in the polar night we can see in addition the rarer day-time aurora, which occurs on the day side of the earth.

The northern lights are far above cloud cover, so we must have clear weather in order to see them. The weather is therefore the most important obstacle to the observation of the northern lights in the northern Norway. The most stable winter weather with a clear sky undoubtedly occurs in inner fjord areas and the interior of the country. So for tourists in search of the northern lights these areas are probably to be preferred to the coast.

The days around the full moon are not the best ones for observing the northern lights. The sky is so light then that the expirience is considerably paler. Finally, one ought to get away from the towns and densely populated areas with lots of lights to get full value from an evening with northern lights. We often forget this last point. People today are surrounded by so much artificial light that to a large exent they have "lost" the darkness and forgotten what the firmament looks like.

The height of the northern lights above ground level was a controversial subject for a long time. Around 1900 there were few people who claimed that the northern lights could reach right down to the ground, but whether they were located at a height of a few kilometers or many hundered kilometers up was unclear. The problem was solved by photographing them simultaneously from two places. If the appropiate distance between the two places was selected, it was possible to discover the displacement of the northern lights in relation to the stars in each photographs and thus calculate their height. Thousands and thousands of such triangulations were preformed from 1910 up to the 1950s. They showed that the northern lights were generally between 90 and 130 km above ground level, but many auroras - particulary the rayed aurora - extend to a height of several hundred kilometers. By way of comparison, the usual flight level of a jet is ca 10 km, and the ozone layer is located 20-30km up. We have to go almost as hight as satellites to find the northern lights. A consequence of the great height is that the northern lights are visible over distances of several hundred kilometers. Thus an aurora over Bear Island could be seen in both northern Norway and Svalbard, and an aurora over Finnmark could be seen in the northern sky in the county of Troendelag.

Sunlights contains in all the colours of the rainbow in an unbroken series from blue through green and yellow to red. In an aurora, on the other hand, the light is collected in a selcetion of narrow bands of colour, so-called spectral lines. An aurora occurs when large quantities of electric particles (electrons) approach the earth at height speed along the magnetic field and collide with the upper layers of the atmosphere. The molecules and the atoms in the gas will then be infused with energy which they subsequently emit again as shafts of light. It is a bit like what happens in a neon tube. The spectral lines reflect which gases are found up there. It turns out that the northern lights are dominated by lines which comes from common gases like nitrogen and oxygen. The oxygen molecules, however, are split by sunlight so that we have atomic oxygen (O), and some of the nitrogen molecules (N2) have lost an electron and become a positiv ion (N2+). The northern lights are characteristically a greenish-yellow colour, but also have a considerable element of blue. The greenish-yellow is due to a strong line from O, whereas the blue comes from N2+. When the northern lights acquire an element of redish-violet on the lower border, the red lines from the N2+ are making an apperance. If they turn red in the upper parts, we are seeing the light from O again. During large-scale auroral displays, this red oxygen may be very prominent and colour large parts of the northern sky deep red when seen in southern Scandinavia and even central Europe. It was this red aurora which spread alarm and terror in central Europe in previous centuries.

The light from a very intensiv aurora can be compared to the light from a full moon, but usually it is some what fainter. Normaly, however, photographing the norhtern light is not a problem. You need a tripod-mounted camera with a high-speed lens (at least f:1,8) and fast film (400 ISO). Then you can reduce your exposure to about one second and capture the finer details which are often lost with long exposures.

There are many reports of sound in connection with the northern lights. On still nights with unusually intese auroras people say that they hear a crackling sound. Scientific instruments have not yet succeeded in recording this phenomenon, but it must nevertheless be accepted as genuine on the basis of many reliable testimonies. It is quite inconceivable that the sound reach us from a height of 100 km, but we know that the northern lights are associated with a power full electric field which can be recorded at ground level. Presumably the sound is the result, eighter in the form of electrical discharges or directly influenced by our auditory nerves.

The northern lights orginate in a complicated interplay between the so-called solar wind and the earth`s magnetic field. The solar wind is a constant stream of electric particles from the sun. It varies in intensity and therefore links the northern lights with the solar activity. The solar winds rushes along the earth`s magnetic field, compresses it on the day side, draws it out into a tail on the night side and generates electric currents and fields in the areas around the earth. A number of solar wind particles are trapped in the earth`s magnetic field and, together with particles which orginate in the earth`s atmosphere, end up in the tail on the magnetic field on the night side. As a result of mechanisms we still do not really understand, they receive extra energy there, stream toward the polar regions at great speed and give us the night-time aurora. These unpredictable showers of electric particles controlled by the earth`s magnetic field give the northern lights their forms and movements. The showers tear along like impetuous squalls, creating arcs, draperies and rays.

The magnetic field is the key to why the northern lights prefer the polar regions. The electric particles travel most easily along the magnetic field and reach farthest down into the atmosphere in the polar regions, because here in the field is almost perpendicular to the earth`s sureface. Most such particle precipitation is found in a ring around the magnetic poles, and in this ring the northern lights are situated like an unbroken halo around both poles. We usually call this snapshot of the northern lights the auroral oval. On the night side of the earth the particles from the tail of the magnetic field from the powerful and active night-time northern lights. On the day side they come more directly from the solar wind and from the fainter and more subdued day-time northern lights which we can see in Svalbard, where it is dark 24 hours a day in the middle of winter. The auroral oval is fixed in a relation to the sun, while the earth revolves below. In northern Norway we enter it in the afternoon and pass through it in course of the evening and night. During the day the oval is to the north of us, over Svalbard.

When solar activity is intens, the oval may expand towards the south so that we in Troms and Finnmark are on the inside, while we get large auroral displays over southern Norway and perhaps even farther down into Europe. During periods of modest activity it contracts and we get the northern lights to the north of us. Then they are usully also faint. The auroral oval must not be confused whit the auroal zone. The former is the aurora as it is found above the earth at the given time; the auroral zone tells us where the night side of the oval is generally to be found.

The magnetic poles move slowly and take the auroral zone and auroral oval with them. Over periods of several hundred years the changes can be great. Five hundred years ago the auroral zone was probably over southern Norway, and presumably it will move northwards from Norway in centuries to come. Thus we are now in a favourable era in norther Norway with regards to the northern lights. If the magnetic field were to disappear, we would lose the northern ligths and be left with a pale souvenir in form of a diffuse light over the whole of the night sky.

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