Aurora Polaris, or 'polar lights', occurs most often above 60° north or south of the equator. Generally, if an auroral band has an easily discernible lower border, this will be at around 60 to 70 miles in altitude. Auroral rays may extend above the lower border for hundreds of miles. If the lower border has a pinkish edge to it (resulting from an emission of molecular Nitrogen), the altitude may be around 50 to 60 miles. A diffuse red aurora occurs above 150 miles.
Most aurorae occur between 90 and 130 km above sea level, but some, particularly the ray-like forms, extend to several hundred kilometers up. In comparison, the usual altitude for a jet aircraft is around 10km and the ozone layer lies between 20 and 30km so we have to be almost up at the heights of satellites’ orbits to be at the same height as the aurora. A consequence of its great height is that the aurora is visible at horizontal distances of several hundred kilometers. Thus an aurora over Bear Island will be visible from both Spitsbergen and Tromsø, and one over Tromsø can be seen in the northern sky from central Norway.
The Northern Lights, as the name suggests, are especially related to the Polar Regions. They occur most frequently in a belt of radius 2500 km centred on the magnetic north pole. This so-called auroral zone extends over northern Scandinavia, Island, the southern tip of Greenland and continuing over northern Canada, Alaska and along the northern coast of Siberia. The coasts of the Norwegian counties of Troms and Finn-mark lay where occurrence is greatest, making northern Norway, due to its ease of access and mild winter climate, an attractive destination for people interested in observing this atmospheric phenomenon.
The Northern Lights can be seen from regions both north and south of the auroral zone, but the likelihood decreases with distance. There is a corresponding auroral zone around the southern magnetic pole, but these “Southern Lights” are largely only seen from Antarctica and the surrounding ocean. Of the populated regions in the southern hemisphere, the Southern Lights may only be glimpsed from Tasmania and southern New Zealand. The Northern and Southern Lights occur simultaneously and are almost mirror images of each other.
The above image is taken by NASA. Aurora Australis as taken by the space shuttle Discovery from low Earth orbit. The variation of the colour of the glow of the aurora with height may be clearly seen with the red aurora occurring at greater heights than the green aurora.
