Aurora

In many parts of Europe, the aurora is unfortunately a rare phenomenon, but in Scandinavia, North America, and also at the South Pole it is frequently visible. It comprises luminescent bands, arcs and streamers. which sometimes have a typical curtain-like structure and periodically light up in a rhythmic fashion. The light comes from very great heights in the atmosphere (100 to 300 km). The light of an aurora is often not very intense (like that of the Milky Way), but sometimes it becomes very bright, and its beautiful colours are then visible.

The red higher part of the aurora is polarized, the lower green and purple parts of it are not. The direction of polarization is perpendicular to that of the magnetic field of the Earth, hence directed mainly west-east. The polarization is maximal when we look perpendicularly to the magnetic zenith, which is at a height of about 70º in the northern sky for central England. Theoretically, the degree of polarization of the red light can amount to as much as 60 %. The best chance to see this polarization in Western Europe is low near the northern horizon where the degree of polarization must be about 55 %. In the case of rare bright displays, which may even extend in Western Europe to the south of the firmament, the most favourable point is in the south at a height of roughly 20º

This polarization is related to the Zeeman effect, and manifests itself as a consequence of the magnetic field of the Earth. However, not every atomic transition is sensitive to it, and that is why only the red aurora has a considerable degree of polarization. The aurora is the only phenomenon of this kind that shows polarization in Nature; other electrical effects do not show it to any noticeable degree. The reason is that other discharges (lightning for example) take place at a much higher atmospheric pressure so that the influence of magnetic fields is negligible.

It seems probable that shifting of colour in the aurora can be seen with a polarizing filter: when the filter is held in such a way that the red has been maximally extinguished, the (unpolarized) green must become relatively brighter. The yellow part of the aurora (where red and green are mixed) will consequently have to shift up or down, according to the position of the filter, since the ratio between red and green light has changed. So, for a minimal transmission of the filter, the lower part of the red will have to turn yellowish and the yellow to greenish: the effect will be reverse in the case of a maximum transmission.