The Northern Lights
In winter the sky in polar regions is often lit up with green, yellowish, sometimes purple and red light. These are the northern lights, or the “aurora borealis”. Northern lights are an electric glow of the upper atmosphere at the heights of 80 to 600 km.
The glow appears under the influence of electrons and protons emitted by the sun (“solar wind”). Getting into the geomagnetic field, electrons begin to move around the magnetic lines of force. Near the poles where the lines of force thicken, the electrons are reflected by the magnetic field. Along the same lines of force, they return to the upper atmosphere and begin to descend towards the Earth surface in the other hemisphere.
Approaching the Earth near the poles the electrons run into molecules and atoms of atmospheric gases – nitrogen and oxygen – and give them their energy. It happens at the height of approximately 100 km. These are the very gases, which create visible radiation – the northern lights (similar to the glow of gaseous-discharge lamps).
Molecules of nitrogen brighten blue and purple spectral lines, atoms of oxygen-green and red ones. Rare, all-red auroras are produced by high-altitude oxygen, at heights of up to 200 miles.
Northern lights occur in every season and, like stars, can be best seen in a dark sky, but even in city light polluted environment they can be spotted, if bright enough. Green is the predominant colour.
During the period of higher solar activity, northern lights intensify and can be seen even in midlatitudes.
(all images in the Kola Peninsula)