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The Northern Lights - what are they?

The Finnish name for the lights (revontulet) comes from an old Finnish legend whereby the tail of a fox running along snow- covered fells strikes the snow drifts, sending a trail of sparks into the sky. Revontulet literally means "foxfire".

The scientific studies of the Northern Lights have revealed that they are shining at a height of 100 km as a result of charged particles (electrons and protons) accelerating in the Earth's magnetic field and colliding with air molecules. These give up part of their surplus energy in the form of light. The phenomenon is rather similar to the formation of an image in a television tube, or to the emission of light by a halogen lamp. The usual greenish-yellow, and occasionally reddish, appearance of the aurora is produced by the oxygen in the atmosphere, whereas the blue and violet light is due to nitrogen. The particles causing the Northern Lights come ultimately from the sun, from which they are ejected out into space at speeds of up to 1000 km a second forming the so-called solar wind.

The amount of solar wind particles varies during an 11-year cycle in accordance with sunspots. However, the incidence of the lights does not quite coincide with fluctuations in the manifestation of sunspots. The occurrence of auroras is frequent close to the sunspot maximum year and a couple (3-4) years after the maximum. Immediately after maximum year of sunspots, the auroras are seen less than on average. The most recent sunspot maxima and minima were in 1989 and 2000, 1996 and 2006 (expected), respectively. Magnificent auroras were seen in Mid- and North-Europe in April 2000 and October 2003.

Studying Northern Lights with cameras

The Finnish Meteorological Institute (FMI) began photographing the Northern Lights in the International Geophysical Year (IGY) of 1957-58 at four locations in Lapland. Space research being carried out by the FMI includes the use of large numbers of pictures of the Northern Lights taken for space physics studies.

Today, the FMI maintains a modern digital all-sky camera system that takes the auroral pictures without film directly to the memory of a computer. At the moment there are four cameras in operation in Finland. The pictures are transmitted, principally in real-time, through Internet to be seen everywhere, mostly in the scientific community. Due to light nights in Lapland during spring and summer, the auroral cameras have been switched off.

Studies and observations of the Northern Lights are done also in the University of Oulu and its Sodankylä geophysical observatory in Lapland.

Records of the Northern Lights dating from the 1700s

Systematic observations on the Northern Lights began in Finland in the mid 1700s at the academy in Turku. Later, Professor G.G.Hällström (1775-1844) of the University of Helsinki compiled records of the aurora borealis spanning almost a hundred years. The series that Hällström worked on contained around 1000 observations and descriptions of appearances of the Northern Lights. This appears to be one of Finland's earliest sets of geophysical observations.

Even in Hällström's time it was known that there are distinct seasonal variations in the appearance of the auroras. The results from the last century are in excellent harmony with the monthly activity of the Northern Lights as known to us at the present day. The highest incidence of the auroral phenomenon occurs in February-March and September-October, when 3/5ths of the year's total lights are seen. The lights are least common during the summer and around Christmas. The scarcity of the auroras in summer is, of course, partially due to the light summer nights. On the other hand, the statistical probability of collisions between solar wind particles thrown off by the sun and the Earth's magnetic field is minimal at mid-summer and Christmas, due to the unfavourable orientation of the Earth's magnetic field in relation to the sun.

Where and when are the Northern Lights most seen?

Statistics reveal that most Northern Lights are observed in the Kilpisjärvi region, in Lapland. With a clear sky during the dark period of the year on average the auroras is seen there on three nights out of four. Further north, at Utsjoki, this ratio is 10 % lower. The 100 % (every night) level is attained in northern Norway, along the shore of the Arctic Ocean. Further north still their occurrence again decreases. As one travels south, the incidence rapidly diminishes. In the Sodankylä region the Northern Lights are seen every second night, and in the Oulu-Kuusamo area every fourth night. In Finland's most southerly region, in the vicinity of Helsinki, it is statistically possible to observe the lights only on one night a month. On cloudy nights, of course, it is not possible to see them at all.

The general appearance of the Northern Lights varies according to the clock. At 9 pm in the evening, Finnish time, the average probability of occurrence in Lapland exceeds 50 %. It reaches a peak at approx. 11.30 pm, when there is a magnetic midnight and the disturbances in the Earth's magnetic field that the lights depend on are at a maximum. After midnight the incidence of the lights dwindles, so that by 4 am to 5 am the probability of occurrence falls below 50 %.

Why study the Northern Lights?

The study of the Northern Lights as part of the electric and magnetic phenomena taking place in space close to the Earth provides valuable information on the characteristics and behaviour of the magnetosphere. Alongside traditional optical observation methods, equipment based on sophisticated research principles has been developed, and the FMI has also played an important role in interpreting the measurements obtained with these as part of international co-operation in this field. The Northern Lights are photographed and satellites observe magnetospheric processes associated with them. Long-term observations continuing from one decade to the next, together with registration of the Earth's magnetic field, provide indirect information on fluctuations in the radiative activity of the sun. Exceptional states in regard to the sun's radiation are also interesting because they have been suspected of causing disturbances in the world's climate. Long series of observations by the FMI and its predecessors make a noteworthy contribution to this research topic.

©Finnish Meteorological Institute Feb 6, 2004

For further information contact:

Heikki Nevanlinna Finnish Meteorological Institute POB 503 FI-00101 Helsinki Finland

Tel: (09) 1929 4649 , + 358 9 1929 4649 (Intl) Fax: (09) 19294603, + 358 9 19294603 (Intl) E-mail: heikki.nevanlinna@fmi.fi

More info: http://www.fmi.fi

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The Northern Lights - what are they?

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