[Northern lights (with Table
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Many consider the aurora to be something of the most beautiful things that can be seen. The different colours in green, blue and red that swiftly move across the sky are really fascinating. If it is the first time you see a sparkling aurora it is easy to lose one's breath. Regardless of the fact that I have seen quite many beautiful auroras, I cannot just walk by when I see a new one. There are no two auroras that are alike, so if you have seen one aurora it does not mean that you have seen them all.
Auroras appear in many different shapes. The aurora mostly seen early in the evening is shaped as an arc and stretches all across the sky in an east-west direction. The arc has indistinct edges and is green.
Photo: Yamauchi Masatoshi
Sometimes arcs can become active and start to look like pieces of drapery with distinct rays that show the direction of the Earth's magnetic field. The length of an aurora arc can be quite large, maybe 1000 kilometres or more, but the width can be as small as 100 metres.
Photo: Yamauchi Masatoshi
If you stand directly beneath an aurora in the direction of the magnetic field the rays look like they are all coming from one point and radiating in all directions. This is called a corona. If you stand further north or south and look at the same aurora you will see it as an arc.
During the most active auroras, which occur during so-called sub storms, the whole sky may be filled with the most incredible colours. The shapes and colours can change from one second to the next. The most intensive phase of an aurora normally lasts only for about 10 minutes.
When the magnetosphere gets rid of most of the surplus energy related to a sub storm you can often see a different type of aurora, pulsating auroras. The sky will be filled with pale light spots that are switched on and off independently of each other and at different speed; the spots are lit for a few seconds. This type of aurora is common after midnight.
Photo: Bengt Holback
The picture shows a series of photographs taken during a pulsating aurora. They are taken with a special camera adjusted to faint light. The dot indicates the position of a sounding rocket. At 9:54:58 p.m. the rocket was surrounded by the aurora, but four seconds later it was almost gone, reappearing two seconds later.
The most commonly occurring auroras are not so easily noticed, since they have no shape. This is the indistinct aurora that lies like a faint glow across the sky.
Photo: Yamauchi Masatoshi
Very rarely red aurora (630 nm wavelength) appears quite a long way south of the ordinary green (558 nm wavelength from atomic Oxygen) and purple (428 nm wavelength from ionized Nitrogen molecule) aurora. This happens only a few times during solar maximum, i.e., large red aurora like this photo appears only a few times in a decade.