Northern Lights – the light organ of the polar sky
Auroras – often called northern lights are nature’s most magnificent display. These fantastic, glorious lights appeal to our imagination. During large displays the aurora changes constantly. It dances back and forth over the sky, in many different forms and combinations of sparkling colours. |
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| Introduction |
| Many people living in the polar region, have stopped in their daily routine - even if it is bitterly cold, to watch the flaming diversity of lights fill the sky. The fluttering drapery of the aurora suggests dancing spirits and dynamic hordes to some. |
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| Auroral-particles precipitation |
| The auroral radiation is emitted by atmospheric constituents that are excited by precipitating particles. The primary auroral particles, populations of electrons and ions with energy from ~ 10 eV up to small multiples of 100 keV, can be measured directly by the use of instrumented rockets and satellites. |
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| Altitude and Intensity of Auroras |
| The aurora appear as a luminous cloud, having an apparent surface brightness. As absorption within the visible spectrum is negligible, the apparent brightness is proportional to the integrated emission per unit volume along the line of sight. The surface brightness is used to define the intensity of an aurora. |
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| Auroral structures and forms |
| Those who have only seen auroras a few times, may have the impression that it is a chaotic phenomenon – the variation in colour, shape and movement is immense. |
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| The colours of the aurora – the auroral spectrum |
| In order to explain the colours it is necessary to use a model of the atom. This is explained in the listed quotation from Egeland et al. (1999) paper. |
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| The Auroral Zone |
| The aurora is related to the Earth's magnetism. An important clue was found by keeping tabs on how often aurora was seen in various locations. It turned out that the important factor was distances from the magnetic poles. |
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| The auroral oval – based on ground and satellite |
| The topic on where and when the auroras can best be seen is called auroral morphology. A new auroral word was introduced into the language in the 1960s: the auroral oval. |
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| Artificial Aurora and Magnetic Field |
| Between 1895 and 1912 the Norwegian physicist Kristian Birkeland (1867-1917) studied the behaviour of auroras in his famous terrella laboratory (terrella is Latin for little Earth). |
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| Dayside Cusp/Cleft Auroras |
| It was only in the 1960s that it was clearly documented that auroras also occur during daytime. Because the auroras during daytime are less bright than at night and are located nearly 1000 km farther north than night-time auroras, studies of dayside auroras were neglected until about 1980. |
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| Polar cap auroras – the sundial of space |
| All auroras on the polar side of the oval – deep within the polar cap (poleward of 75 – 80 degrees Λ), are called polar cap auroras. In this section we will discuss auroras oriented along the Sun-Earth direction – i. e. they point toward the Sun. |
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| How are Aurora ignited? |
| We have already stated that charged particles from the solar corona – mainly electrons and protons, can get inside the Earth’s magnetosphere, and then be guided by the geomagnetic fields into the polar ionosphere. The question we now will try to answer is how the aurora is ignited. What starts and keeps the "fires" going? |
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| Auroral variations with time |
| The dynamics of auroras are of special interest. The auroras can move more than 500 kilometres north or south in one hour, depending on conditions in the solar wind and the interplanetary magnetic field. |
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| The auroral substorm |
| The most common auroral forms are arcs, bands and rays. At times they change shape rapidly, advance, retreat or bulge out in a violent fashion, and they also get quite bright. |
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| Auroras on other planets |
| Our Earth is not the only planet with auroras, because the solar wind blows even beyond Pluto. To have auroras, an atmosphere and magnetic fields are needed. |
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| Auroral research as a tool to study the Sun and nearby space |
| Because auroras are so closely connected to solar activity and the physical processes in space between the Sun and the Earth, as well as to disturbances in the geomagnetic field, auroras can be used to study properties of the Sun, the solar wind, and the geomagnetic field as well as composition and height of the upper atmosphere. |
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