Northern lights - Aurora Borealis

Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the sun.  The Earth´s magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth.
Collisions between these ions and atmospheric atoms and molecules causes energy releases in the form of auroras appearing in large circles around the poles.  Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.  Seen from space, these fiery curtains form a thin ring in the shape of a monk´s tonsure.
The Northern Lights occur high above the surface of the earth where the atmosphere has become extremely thin, in an altitude of 100-250km

In the past theories have been proposed to explain the phenomenon. These theories are now obsolete.

Benjamin Franklin theorized that the mystery of the Northern Lights was caused by a concentration of electrical charges in the polar regions intensified by the snow and other moisture.

The first Old Norse account of norðrljós is found in the Norwegian chronicle Konungs Skuggsjá from AD 1230.  The chronicler has heard about this phenomenon from compatriots returning from Greenland and he gives tree possible explanations: that the ocean was surrounded by vast fires, that the sun flares could reach around the world to its night side, or the glaciers could store energy so that they eventually became fluorescent.

Usually from late September until the beginning of April, the Northern Lights can be seen in Iceland in most places. In dark areas away from city lights these are best seen. Important one chooses the right evening with  clear skies.