Northern lights near the arctic circle Finland
The northern lights are a light phenomena within the earth atmosphere.
As the name already tells, the phenomenon is normally seen in the polar regions of the earth.
This means that the lights can be seen almost exclusively north of the northern and south of
the southern arctic circle.
Northern lights near the arctic circle Finland |
The phenomenon of "northern lights" is also called "Aurora borealis"
in the northern hemisphere of the earth. Correspondingly, the names on the southern
hemisphere of the earth are "southern lights" and "Aurora australis".
The aurora occurs in a hight of 70 - 800km due to the excitation of oxygen atoms
and nitrogen molecules. Electrons, reaching the earth by the solar wind, act as exciting particles.
In addition, protons from the solar wind also can act as exciting particles.
The excited particles emit light of different wavelength depending on their hight in the atmosphere
and the nature of the excited atom/molecule.
The major part of the emission is caused by oxygen atoms in about 120km.
The green photons correspond to a wavelength of 557.7nm. In addition, red photons
corresponding to a wavelength of 630nm are emitted. The latter have their origin in oxygen atoms
at about 200km. Besides the light from the oxygen atoms also nitrogen molecules add
purple photons with a broad band of wavelength to the whole emission spectrum.
 Aurora in Germany |
The emission of the nitrogen does not occur with every auroral display, but only
during the biggest and brightest auroral storms or so called geomagnetic storms.
Storms of this kind almost always happen after strong explosions on the sun ejected huge
masses of material towards the earth. Such eruption are most frequently observed every 11 years
during the phase of maximum solar activity. In these times of solar max many and very complex
sunspots can be observed, which prove to be the origin of the mass ejections.
If a geomagnetic storm reaches extraordinary strength, sightings of auroral activity become
possible even outside the polar regions. From time to time the storms get even big enough to
bring the colorful northern lights as far south as central Europe.
 Sun with sunspots |
The "central-european aurora" is very different if compared to the
"normal aurora". This can be attributed to the earth magnetic field, which
is heavily involved into the formation of the aurora. The excitation of atoms and molecules
can only occur if the needed elctrons can penetrate deep enough into the earth atmosphere.
To do so, the electrons and protons follow the field lines of the magnetic field.
As those lines are running almost vertical into the atmosphere only near the poles,
the typical green aurora only can occur in those regions because the electrons can penetrate
the atmosphere deep enough.
Above Europe the field lines run in a more shallow angle and relatively high in the atmosphere.
Therefore, in the so called "middle latitudes" the red aurora, originating in greater
hights, dominates the phenomenon.
Only during the most intensive geomagnetic storms it is possible to observe a green auroral arc
over the northern horizon of central European locations.
During those very rare events the more typical red aurora can often be seen down in the
southern regions of the sky.
To observe the aurora, one does not need any special equipment, only the human eye. But for a good observation, a dark location far away from artificial light sources should be prefered. Only under a real dark sky it becomes possible to catch the whole range of the phenomenon in its sheer beauty and great variety. That variety and the very fine structures can never be captured by photo or video documentation. One has to see it with his very own eyes to be able to understand that.
Literature and internet links:
[1] Kristian Schlegel, Vom Regenbogen zum Polarlicht, Spektrum Akademischer
Verlag, Heidelberg, Berlin, 1999, ISBN 3-8274-0530-0 (in German)
[2] Neil Davis, The Aurora Watcher´s Handbook, University of Alaska Press,
Fairbanks, 1992, ISBN 0-912006-59-5 (in English)
[3] http://www.ulrich-rieth.de
[4] http://www.spaceweather.com
[5] http://www.spacew.com
[6] http://www.meteoros.de