What is the frequency of Aurora Borealis or Northern Lights?

Aurora Borealis or Northern Lights is the phenomenon of light seen in the sky at night in the northern hemisphere. Generally in the higher latitudes. Aurora comprises-an electrical discharge, and when observed as far south as in England it is almost always accompanied by a Magnetic Storm-but not when it is limited to higher latitudes. It assumes a number of forms.
The must stable form is the quiet arc of light, that persists sometimes with little visible change for many hours; an arc with streamers, on the other hand, is constantly changing shape. Like Aurora Australis, it takes place in the Ionosphere. Aurora is very rarely seen in southern Europe, but its frequency increases rapidly north wards; and a belt of maximum frequency running north of Norway and south of Greenland encircles the North Magnetic Pole. Its· actual frequency is not certain, for it.is obscured by daylight and by cloudy skies, and, when weak, by twilight or moon light.