Swirling rivers of greenish-blue light against a clear sky, dancing seemingly with a will of their own, sometimes almost static, the Northern Lights (Aurora borealis) are one of nature’s most spectacular displays. For all their beauty, though, they are the product of a violent event high above us, the clash of charged particles from the Sun with the Earth’s magnetic field.
Solar winds send energised particles from the Sun’s surface, hitting the Earth’s upper atmosphere at speeds of around 45 million mph. Acting as a defensive shield, the Earth’s magnetic field forces the charged particles to move in spirals along the magnetic field lines towards its magnetic poles. Upon hitting the gas atoms and molecules in the Earth’s atmosphere, they transfer their energy which is transformed into photons.
The colour of the Northern Lights is dependent upon three factors at the time of the collision: the amount of energy held in the solar wind’s electrons, the type of gas atoms and molecules they collide into, and the altitude at which it occurs. A red light is produced when high-energy electrons interact with oxygen at an altitude in excess of 290 kilometres in the ionosphere while the more familiar green light is the result of the impact of low-energy electrons and oxygen at lower altitudes. A collision with nitrogen can produce a blue or red hue and other colours such as pink or purple are created when there is a mix of gases.
As the solar wind particles are funnelled towards the Earth’s magnetic poles, aurorae are most likely to be seen in a circular area around them, in the northern hemisphere, principally around the northern coast of Siberia, Scandinavia, Iceland, the southern tip of Greenland, northern Canada, and Alaska. When solar activity is particularly intense, as at the end of February 2023, they are visible much further south. The southern hemisphere has its own lights, the zone passing mostly over Antarctica and the Southern Ocean and are most likely to be seen from land in Tasmania with occasional sightings in southern Argentina and the Falklands.
Aurorae are not intermittent events but happen all the time, a fact brought vividly to life by The Space Weather Prediction Centre’s fascinating interactive forecast of the location and intensity of an aurora in the world over the next thirty to 90 minutes[1]. Whether we see them or not is dependent upon sky conditions and the level of light pollution.
Named Aurora borealis by Galileo in 1619 and explained scientifically by Norwegian physicist, Kristian Birkelan in 1902/3, the Northern Lights have long fascinated mankind, featuring in cave paintings found in South-western France dating to 30,000 BC, and first recorded by an astronomer in the court of the Babylonian king, Nebuchadnezzar II on a tablet from 567 BC. Absent a rational explanation for their cause, they inspired many myths and superstitions.
For the Vikings, they were the shimmering reflections of the armour of the Valkyries sent by Odin to collect the bodies of the warriors slain in battle, while in Finland fire foxes travelled through the sky so quickly that their tails produced sparks as they brushed the mountains. In northern Sweden the lights, created by shoals of herrings, were a harbinger of a plentiful catch.
Elsewhere they represented the souls of the dead, in Greenland of children who had died in childbirth and in Norway of old maids, while for the Sámi they were to be feared and respected. Provoking them by waving or whistling in their presence ran the risk of being snatched away. In Scotland, the lights, known as “Merry” or “Pretty Dancers, were created by fallen angels and warriors who battled it out in the skies, their drops of blood creating the distinctive red specks on the green heliotrope known as bloodstone.
[1] https://www.swpc.noaa.gov/products/aurora-30-minute-forecast
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