What causes the Northern and Southern Lights?

Answer

The so-called solar wind continuously streams off the surface of the sun. It is a stream of charged particles, mostly electrons and protons. The earth's magnetic field repels most of these particles so that they flow around the earth like water past a keel. The exception to this rule is around the magnetic poles, the locations that the compass needle points to, one in the North and the other, diametrically opposite, in the South. A small amount of these particles escapes inside the earth's magnetic field in the vicinity of these poles. The points where the penetration of the particles is most, form a collar around the magnetic poles.

A picture of the Northern Lights taken in Finland.

The charged particles that enter the earth's magnetic field move at great velocity along spiral tracks around the magnetic field lines between the magnetic poles. So the Protons and electrons flow towards the magnetic poles and as they approach, the particles collide with the atmosphere, usually at altitudes between 100 and 250 km. Energy in the electrons and protons stimulate molecules and atoms in the atmosphere which emit energy in the form of the visible light we call Northern or Southern Lights depending on the pole they are seen at. The colours we usually see are green and red - purple, caused on the one hand by charged oxygen molecules and on the other by charged nitrogen.

As the effects of solar wind are greatest at a collar round the magnetic poles, the aurora (Northern and Southern Lights) are most visible at those locations. In Iceland we are lucky enough to be within the North Pole collar during the nights, in normal conditions. Changes in solar winds cause the collar to grow or shrink, so the Lights are seen at varying latitudes. An example of this kind of variation can be seen when solar flares send massive emissions of material into space. When the material reaches the earth, the northern and southern collar zones can stretch a long way towards the equator, and there are examples of aurora having been seen at the equator itself.

Translated by Paul Richardson.

Picture: Science @ Nasa