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An Introduction to the Northern Lights



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The Northern Lights, aurora borealis, is the name given to certain bright displays of light which are frequently seen in the skies above the higher latitudes, i.e. towards the Arctic Circle, (Southern Lights, known as aurora australis, can be seen southern skies) and these fantastic displays of light have fascinated man for ages. These lights are most visible in those areas that most closely border the Arctic Circle so that residents of Siberia, Alaska and the Scandinavian coasts are more likely to see these awesome displays than are those who live farther south. Sometimes, however, these light displays are so pronounced that they can be seen in latitudes that lie much farther south of the Arctic but above the tropics.

As with several other natural phenomena, the earliest human observers of these lights attached any number of supernatural meaning to them. According to Norse legend, for instance, the lights were caused by the flashing armor and spears of the Valkyries, the warrior handmaidens of Odin, the chief Norse deity, as they rode out to collect the souls of the warrior dead to take them to Odin’s palace of Valhalla. As with most such natural phenomena, the truth is at once more prosaic and more wonderful.

The appearance of the Northern Lights is intimately connected with the Earth’s relationship with the Sun. Like all the other bodies that make up the Sun’s retinue as it journeys through space, the Earth is constantly bathed in what is known as a solar wind. This wind is made up of highly charged protons and electrons that have been able to break free of the Sun’s gravity field and move out into space. As these charged particles pass by the Earth, the planet’s magnetic field attracts some of them and they are drawn within the Earth's atmosphere. Traveling along the Earth’s magnetic field lines, these particles congregate above the planet’s polar regions where they come into contact with the atoms of the gases, such as nitrogen and oxygen, that make up Earth’s atmosphere. Contact with the solar visitors excite the local atoms and, as these excited atoms settle back into their normal state, they release the excess energy that they have acquired as a result of contact with the solar particles in the form of visible light. It is these lights that the earthbound observer sees as the Northern Lights.

The various colors of light that can be seen in the displays of the Northern Lights result from the fact the different gasses that make up the atmosphere emit lights of different wavelengths. Nitrogen, for instance, emits wavelengths that an earthbound observer sees as blue and/or red light whilst oxygen, which requires a bit more time than nitrogen does for it to return to a state of rest, emits wavelengths of green and/or brownish-red lights.

Consequent upon this, the lights in any display viewed by an earthbound observer will obviously depend upon the relative proportions of the various gases in the atmosphere as collisions between excited atoms of different atmospheric gases cause the colliding atoms to release stored energy more quickly and return to a state of rest.

The proportions of atmospheric gases vary by altitude so that at higher altitudes where oxygen is proportionately more in the atmosphere, there will be plenty of red lights displayed. This is because there are fewer non-oxygen atoms to collide with the oxygen atoms and the oxygen atoms have enough time (about 2 minutes) to emit light in brownish-red wavelengths. Moving to lower altitudes, oxygen becomes proportionately less in the atmosphere and as a result the numbers of collisions between oxygen and non-oxygen atoms increase. At such altitudes, green lights (oxygen emits in green wavelengths in about three quarters of a second)  will tend to be dominant in displays. The reds that are more prominent at higher altitudes become less so because increased collisions do not allow oxygen enough time to emit those wavelengths. Where there are sufficient non-oxygen molecules to prevent large scale emissions by the oxygen atoms, the blue/red emissions of nitrogen will be predominant.

Although the closer it is that one resides to the north pole the more likely it is for such a person to see these wonderful displays of nature, the lights can be seen in many latitudes that lie far south of the pole.  For instance, a paper delivered to the Royal Society on 21 November 1861 by the British scientist and director of the Kew Observatory, Balfour Stewart (1828 - 1887), told of two auroral events (28 August and 2 September 1859) of extreme brilliance which were seen , reported and recorded in places as far apart as Boston, USA, and Japan, as well as by several ships that were on the high seas. Many American newspapers, including the New York Times, reported that the display of 2 September was so brilliant that ordinary print could be read by the light it gave off at about 1 a.m. in Boston.

For folk who live in areas where the Northern Lights can be seen, the best time to attempt a viewing is between 10 p.m. and 2 a.m. on a clear moonless, midwinter night far away from any sources of light pollution, or fog. The lights are more prominent in these southern latitudes at the peak of the 11 year cycle of sunspots and for about two or three years thereafter. As it happens, 2013 is the final year for good viewing opportunities in the current sunspot cycle.  For those who live in latitudes where the Northern Lights never make a show, i.e. all tropical latitudes, the US agency, the National Oceanic and Atmospheric Administration, operates a site which visitors can visit in order to follow the current state of auroral activity over the Earth’s poles.

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