A compass needle becomes agitated when there are strong occurrences of the Northern Lights. This led to the conclusion that they have a magnetic background.
The sun is the primary source of the Northern Lights. The flow of particles coming from the sun, the solar wind, carries electrons and protons towards the earth’s atmosphere. The solar wind is no gentle breeze; the particles are discharged at a velocity of as much as 1000 kilometres an hour.
The earth’s magnetic field, the magnetosphere, provides protection from the majority of the particle bombardment. However, part of the particles, penetrate through the magnetic shield and is directed to the poles.
The particles gain added velocity at the poles of the magnetic field. When they crash into oxygen and hydrogen atoms as well as nitrogen molecules in the upper atmosphere, the ionosphere, the particles in the solar wind transfer energy to them and set off a state of excitation. Light is created when this state excitation is discharged – the result is the Northern Lights.
The Northern Lights are at their strongest when sudden changes takes place in the particle activity of the sun. Sunspots are evidence of changes in its activiy. Sunspots are magnetically active areas on the surface of the sun; because they are cooler that their surrondings, they stand out as dark spots.
The number of sunspots varies in periods 10-11 years. Immediately after the maximum and minimum years, the prevalence of Northern Lights is lower than on average.
The main colours of the Northern Lights are greenish-yellow, red and purple. Greenish-yellow and red are created by the excitation of oxygen; blue and purple originate from nitrogen. Altitude also has an effect on the colours.
Certain stages repeat themselves in the display of Northern Lights. The light show starts with greenish-yellow arcs going in an east-west direction, the direction of the lines of force of the earth’s magnetic field. At the same time, the solid coloured area settles into vertical rays and it moves towards the zenith.
According to statistics on the Northern Lights, three nights out of four are Northern Lights nights in Kilpisjarvi (Finland). On the northern coast of Norway, the Northern Lights are seen even more frequently than this. The further south one goes, the frequency of the Northern Lights drops sharply. In Rovaniemi (Finland) and Sodankyla (Finland), the heavenly lights blaze on average every other night. Also, right in the center of auroral oval, in the polar regions, there is less activity.
Ref: Arcticum Museum Rovaniemi